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Conceptual Biomechanics and Kinesiology pp 49–52 Cite as

Principles of Kinetics and Kinematics on Human Body

  • Animesh Hazari 4 ,
  • Arun G. Maiya 5 &
  • Taral V. Nagda 6  
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The human biomechanics is a complex subject and governed by certain principles pertaining to kinematics and kinetics apart from the physical laws that we have learnt in the previous chapters. It is the ability of the human body to dissociate one segment with other and apply the principles of biomechanics over the target body segment. For example, complete shoulder kinematics and kinetics is required to deliver a fast ball, while only the wrist and elbow need to be targeted for playing dart. Thus, the body mechanics strictly follows the principles of kinematics and kinetics which we shall deal with in this chapter.

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Knudson D. Fundamentals of biomechanics. Berlin: Springer Science & Business Media; 2007.

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Montgomery J, Knudson D. A method to determine stride length for baseball pitching. Appl Res Coach Athl Ann. 2002;17:75–84.

Alexander RM. The human machine. New York: Columbia University Press; 1992.

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Animesh Hazari

Dean, Manipal College of Health Professions, Manipal Academy of Higher Education, Manipal, India

Arun G. Maiya

Pediatric Orthopaedic Surgeon, SRCC Children Hospital, Director Jupiter Gait Lab, Mumbai, India

Taral V. Nagda

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Hazari, A., Maiya, A.G., Nagda, T.V. (2021). Principles of Kinetics and Kinematics on Human Body. In: Conceptual Biomechanics and Kinesiology. Springer, Singapore. https://doi.org/10.1007/978-981-16-4991-2_5

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Knee Joint Biomechanics in Physiological Conditions and How Pathologies Can Affect It: A Systematic Review

1 Shaanxi Engineering Laboratory for Transmissions and Controls, Northwestern Polytechnical University, Xi'an 710072, China

Jianbing Ma

2 Hong-Hui hospital, Xi'an Jiaotong University College of Medicine, Xi'an 710054, China

Pingping Wei

3 State Key Laboratory for Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi'an 710054, China

The knee joint, as the main lower limb motor joint, is the most vulnerable and susceptible joint. The knee injuries considerably impact the normal living ability and mental health of patients. Understanding the biomechanics of a normal and diseased knee joint is in urgent need for designing knee assistive devices and optimizing a rehabilitation exercise program. In this paper, we systematically searched electronic databases (from 2000 to November 2019) including ScienceDirect, Web of Science, PubMed, Google Scholar, and IEEE/IET Electronic Library for potentially relevant articles. After duplicates were removed and inclusion criteria applied to the titles, abstracts, and full text, 138 articles remained for review. The selected articles were divided into two groups to be analyzed. Firstly, the real movement of a normal knee joint and the normal knee biomechanics of four kinds of daily motions in the sagittal and coronal planes, which include normal walking, running, stair climbing, and sit-to-stand, were discussed and analyzed. Secondly, an overview of the current knowledge on the movement biomechanical effects of common knee musculoskeletal disorders and knee neurological disorders were provided. Finally, a discussion of the existing problems in the current studies and some recommendation for future research were presented. In general, this review reveals that there is no clear assessment about the biomechanics of normal and diseased knee joints at the current state of the art. The biomechanics properties could be significantly affected by knee musculoskeletal or neurological disorders. Deeper understanding of the biomechanics of the normal and diseased knee joint will still be an urgent need in the future.

1. Introduction

Since the number of the old and obese worldwide has been increasing yearly, the research on human motion dysfunction is getting more and more attention. The knee joint, as the main lower limb motor joint, is the most vulnerable and susceptible joint [ 1 ]. Knee impairments are the common physical problems which impact the normal living ability and mental health of these patients [ 2 ]. The influences mainly contain the supporting body weight, the assisting lower limb swing, and the absorbing strike shock [ 3 ]. The movement biomechanics, as an important branch of biomechanics, studies the coordination of the bones, muscles, ligament, and tendons in various human movements [ 4 ]. The complex interaction of these structures allows the knee to withstand tremendous forces during various normal movements [ 1 ]. Therefore, it is an urgent need to study the movement biomechanics of the normal and diseased knee joint for the assistance or rehabilitation of human locomotor function.

In the last decade, several related review papers appeared and could be divided into two aspects, normal knee biomechanics and diseased knee biomechanics. For the former, Masouros et al. [ 5 ] analyzed the knee kinematics and mechanic and surrounding soft tissue in detail. The research pointed out that the knowledge of these structures was very useful for the diagnosis and evaluations of treatment. Wang et al. [ 6 ] reviewed the modeling and simulation methods of human musculoskeletal systems. The knee kinematics and kinetics in six common motions including walking, jogging, stair ascent, stair descent, squatting, and kneeling were discussed. Chhabra et al. [ 1 ] reported the anatomic structures and their relationships in the uninjured knee joint, which provided the critical guidance for the reconstruction of the multiple ligament injured knee joint. Madeti et al. [ 4 ] discussed various model formulations of the knee joint, including mathematical, two-dimensional, and three-dimensional models. And the forces acting on the knee joint had also been compared. For the latter, Flandry et al. [ 7 ] provided an overview of the surgical anatomy of the knee joint and emphasized connective tissue structures and common injury patterns. Woo et al. [ 8 ] reviewed the biological and biomechanical knowledge of normal knee ligaments, as well as the anatomical, biological, and functional perspectives of the current reconstruction knowledge following knee ligament injuries. The research also provided guidance for improving the treatment of knee ligament injuries. Louw et al. [ 9 ] assessed the effects of the occluded vision on the knee kinematics and kinetics during functional activities, such as squatting, stepping down, drop landing, hopping, and cutting movements in healthy individuals and the individuals with anterior cruciate ligament injury or reconstruction. Sosdian et al. [ 10 ] discussed the effects of knee arthroplasty on the kinematics and kinetic properties of the frontal plane and sagittal plane during the stance phase of normal walking. The results showed that the peak knee adduction angle and moment were decreased, but the peak knee flexion moment was increased after knee arthroplasty. However, to our knowledge, there is no review that synthesized the literature discussing the movement biomechanics of both the normal and the diseased knee joint.

Understanding the knee biomechanics is a prerequisite for designing knee assistive devices and optimizing rehabilitation exercises. This paper provides an overview of the current biomechanical knowledge on normal and injured knee joints. For better assessment of the function of the knee joint, the biomechanical parameters including angle, moment, power, and stiffness from various researchers in different daily motions are reviewed and compared. For better understanding the kinematics and kinetics of real knee movement, the polycentric rotation in the sagittal plane and biomechanics in the coronal plane are also discussed. Further, the common knee disorders including musculoskeletal and neurological disorders and their influences on the knee biomechanics are also reviewed and discussed. We hypothesized that the comprehensive understanding of the knee joint biomechanics in physiological and pathological conditions could significantly improve the design of knee assistive devices and rehabilitation exercise programs.

The rest of this paper is organized as follows. In Section 2 , the search strategies adopted for the literature review are provided. In Section 3 , the selected literatures including the biomechanical properties of normal knee joint and the knee diseased effects on the biomechanics are summarized. In Section 4 , the limitations of the current studies are briefly discussed and the recommendations for future research are provided.

This review was conducted in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [ 11 ]. We systematically searched electronic databases including ScienceDirect, Web of Science, PubMed, Google Scholar, and IEEE/IET Electronic Library for potentially relevant articles. The following terms were used as keywords (identical for all databases): “knee joint,” “gait,” “knee biomechanics,” “knee disease,” and “sports biomechanics.” Given the fast advancement in acquisition equipment and theoretical research of knee biomechanics, the search time range was set from 2000 to November 2019. A total of 1787 articles were retrieved initially. The daily life activities were mainly considered in this review, the articles about more complex activities, such as squats, hops, cut manoeuvres, were excluded. After 679 articles were excluded, 1108 articles about daily life activities were selected. In addition, review of all references cited by the selected articles and more insight into other relevant authors' studies yielded an additional 35 articles for possible inclusion. Then, all selected articles were input into Excel to eliminate duplicates. After 511 duplicates were removed, 632 articles were assessed for inclusion.

Studies were considered eligible if they met the following inclusion criteria: normal knee kinematics related, normal knee dynamics related, diseased knee kinematics related, diseased knee dynamics related, English, and full-text articles. Two reviewers (LZ and ZW) independently assessed the title and abstracts of the potential studies. After an initial decision, the full text of the studies that potentially met the inclusion criteria were assessed before a final decision was made. A senior reviewer (GL) was consulted in cases involving disagreement. After exclusion of irrelevant titles and screening of abstracts, 203 articles remained. Subsequently, detailed full-text screening based on the inclusion criteria was carried out, and 65 articles were excluded. Finally, 138 full-text articles were examined for full review. The search process is demonstrated using the following diagram shown in Figure 1 .

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The PRISMA flow diagram of study selection process.

We divided the 138 selected articles, which fulfilled the literature search inclusion criteria, into two groups: biomechanical properties of normal knee joint and biomechanical properties of diseased knee joint. For the former, the real movement of a normal knee joint and the normal knee biomechanics of four kinds of daily motions in the sagittal and coronal planes, which include normal walking, running, stair climbing, and sit-to-stand, were discussed and analyzed. For the latter, an overview of the current knowledge on the movement biomechanical effects of common knee musculoskeletal disorders (KOA) and knee neurological disorders (SCI, stroke, and CP) were provided.

3.1. Biomechanical Properties of Normal Knee Joint

3.1.1. knee biomechanics of daily motions in sagittal plane.

Walking, running, stair climbing, and sit-to-stand are very frequent motions in human's daily life. In all of the motions, the main functions of the knee joint include supporting the body weight (BW), absorbing shock of heel strikes, and assisting lower limbs swing [ 3 ]. According to the previous researches, the passive knee flexion could reach 160 deg in the sagittal plane [ 1 , 5 , 12 ]. The peak load through the knee joint is 2-3 BW during walking, 2-5 BW during sit-stand-sit, 4-6 BW during stair climbing, and 7-12 BW during running [ 12 – 14 ]. In this section, the ROM, maximum moment, maximum power, and stiffness of the knee joint are mainly discussed because they are the key indexes for the design of knee assistive device and optimization of rehabilitation exercises.

As shown in Figure 2(a) , the walking gait can be divided into two main phases: stance (about 0-65% of gait) and swing phases (about 65-100% of gait) [ 15 , 16 ]. The stance phase consists of three subphases: initial (heel strike to foot flat), middle (foot flat to opposite heel strike), and terminal stance (opposite heel strike to toe off) [ 16 , 17 ]. The knee joint in the stance phase is regarded as a shock damping mechanism to accept the BW [ 18 ]. The swing phase consists of two subphases: initial (toe off to knee maximum flexion) and terminal swing (knee maximum flexion to heel strike) [ 16 , 17 ]. The main function of the knee in the swing phase is assisting flexion-extension for toe clearance, foot placement, and taking over the load in the next step [ 19 , 20 ]. Zheng [ 21 ] reported that the knee biomechanics is affected mainly by walking speed. With the speed increased, the ROM, maximum extension moment, and maximum absorption power would increase. Figure 2(b) shows the typical knee angle-time curve. There are two peak flexion (A and C) and extension (B and D) angles. Points A and B occur in the stance phase, and C and D occur in the swing phase. Comparing the two peak flexion angles, the value in the swing phase is always greater than that in the stance phase. Table 1 gives the values of these points from 18 studies. The ranges of points A, B, C, and D are from 6 to 28 deg, -2 to 5 deg, 53 to 78 deg, and -5 to 16 deg, respectively. In general, the ROM is around 53 to 75 deg for normal walking. Figure 2(c) shows the typical knee moment-time curve. There are two peak extension (E and G) and flexion (F and H) moments. Point H occurs in the swing phase and the others occur in the stance phase. Table 2 gives the values of these points from 11 studies. The values of these points vary considerably between different studies. The ranges of points E, F, G, and H are from 0.129 to 0.945 Nm/kg, -0.675 to 0.067 Nm/kg, 0.101 to 0.466 Nm/kg, and -0.420 to 0.086 Nm/kg, respectively. The first peak extension moment is always greater than the second. But it is hard to determine who is bigger between the two peak flexion moments. In general, the range of moment is about 0.458 to 1.265 Nm/kg for normal walking. Figure 2(d) shows the typical knee power-time curve. It includes one peak generation power (J) and three peak absorption powers (I, K, and L). Point L occurs in the swing phase and the others occur in the stance phase. For the knee joint, there is only absorption power in the swing phase. And in the whole gait cycle, knee absorption powers are much larger than generation powers. Mooney and Herr [ 22 ] found that the mean net knee power is about -18 W (mean generation and absorption power is about 18 W and -36 W, respectively). Table 3 gives the values of these points from 10 studies. The ranges of points I, J, K, and L are from -1.736 to -0.116 W/kg, 0.286 to 0.834 W/kg, -1.935 to -0.403 W/kg, and -2.712 to -0.321 W/kg, respectively. In general, the range of power is about 1.035 to 3.214 W/kg for normal walking.

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A sketch map of motion and the typical curves of knee angle, moment, and power in the sagittal plane for a walking gait cycle. (a) Sketch map of walking motion [ 14 ]. (b) Knee angle-time curve ((A) first peak knee flexion angle, (B) first peak knee extension angle, (C) second peak knee flexion angle, and (D) second peak knee extension angle). (c) Knee moment-time curve ((E) first peak knee extension moment, (F) first peak knee flexion moment, (G) second peak knee extension moment, and (H) second peak knee flexion moment). (d) Knee power-time curve ((I) first peak knee absorption power, (J) first peak knee generation power, (K) second peak knee absorption power, and (L) third peak knee absorption power) [ 16 , 17 ].

Overview over the experimental results of knee angle for normal walking.

A: first peak knee flexion angle; B: first peak knee extension angle; C: second peak knee flexion angle; D: second peak knee extension angle.

Overview over the experimental results of knee moment for normal walking.

E: first peak knee extension moment; F: first peak knee flexion moment; G: second peak knee extension moment; H: second peak knee flexion moment.

Overview over the experimental results of knee power for normal walking.

I: first peak knee absorption power; J: first peak knee generation power; K: second peak knee absorption power; L: third peak knee absorption power.

As shown in Figure 3(a) , the running cycle can be divided into four main phases: stance (heel strike to toe off), first float (toe off to opposite heel strike), swing (opposite heel strike to opposite toe off), and second float phases (opposite toe off to heel strike) [ 14 ]. The knee main function in running is similar to that in walking. Comparing Figures ​ Figures2 2 and ​ and3, 3 , it can be observed that the curves of angle, moment, and power in running are also similar to that in walking. Hamner and Delp [ 23 ] reported that the knee biomechanics is mainly affected by running speed. With increasing speed, the ROM, maximum extension moment, and maximum absorption power would increase. Figure 3(b) shows the typical knee angle-time curve in a gait cycle and Table 4 gives the angles of points A, B, C, and D from 7 studies. The ranges of points A, B, C, and D are from 36 to 60 deg, 13 to 29 deg, 80 to 129 deg, and 10 to 21 deg, respectively. In general, the ROM of the knee joint is around 60 to 115 deg for running. Figure 3(c) shows the typical knee moment-time curve in a running cycle and Table 5 gives the moments of points E, F, G, and H from 5 studies. The ranges of points E, F, G, and H are from 1.157 to 2.574 Nm/kg, -0.259 to 0.320 Nm/kg, 0.135 to 0.585 Nm/kg, and -1.474 to -0.277 Nm/kg, respectively. The range of moment is about 1.434 to 3.904 Nm/kg for running. Figure 3(d) shows the typical knee power-time curve in a running cycle and Table 6 gives the powers of points I, J, K, and L from 6 studies. The ranges of points I, J, K, and L are from -1.706 to -12.567 W/kg, 2.739 to 9.405 W/kg, -3.456 to -1.525 W/kg, and -3.456 to -6.732 W/kg, respectively. The range of power is about 8.724 to 21.972 W/kg. This emphasizes that the ranges of knee angle, moment, and power in running are far more than those in normal walking.

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A sketch map of motion and the typical curves of knee angle, moment, and power in the sagittal plane for a running cycle. (a) Sketch map of running motion [ 14 ]. (b) Knee angle-time curve ((A) first peak knee flexion angle, (B) first peak knee extension angle, (C) second peak knee flexion angle, and (D) second peak knee extension angle). (c) Knee moment-time curve ((E) first peak knee extension moment, (F) first peak knee flexion moment, (G) second peak knee extension moment, and (H) second peak knee flexion moment). (d) Knee power-time curve ((I) first peak knee absorption power, (J) first peak knee generation power, (K) second peak knee absorption power, and (L) third peak knee absorption power) [ 23 , 122 , 123 ].

Overview over the experimental results of knee angle for running.

Overview over the experimental results of knee moment for running.

Overview over the experimental results of knee power for running.

As shown in Figure 4(a) , the stair climbing cycle (including stair ascent and descent) can be divided into two main phases: stance phase (about 0-62% of the cycle) and swing phase (about 62-100% of the cycle) [ 24 , 25 ]. The stance phase consists of three subphases: initial (foot contact to opposite toe off), middle (opposite toe off to opposite foot contact), and terminal stance (opposite foot contact to toe off) [ 24 , 26 , 27 ]. Riener et al. [ 24 ] indicated that the knee biomechanics is mainly affected by the rate of leg length and stair height. Figure 4(b) shows the typical knee angle-time curves in a stair ascent and stair descent cycle. They all include one peak flexion (A) and extension (B) angle. For stair ascent, point A occurs in the swing phase and B occurs in the terminal stance phase. And for stair descent, point A occurs in the terminal stance phase and B occurs in the swing phase. Table 7 gives the values of these points from 7 studies. The ranges of points A and B are from 83 to 102 deg and 0 to 11 deg for stair ascent and from 83 to 105 deg and 1 to 19 deg for stair descent, respectively. In general, the ROM of the knee joint is around 78 to 94 deg for stair ascent and 76 to 90 deg for stair descent. Figure 4(c) shows the typical knee moment-time curves in a stair ascent and descent cycle. They all include two peak extension (E and G) and flexion (F and H) moments. For stair ascent, points E and F occur in the stance phase and G and H occur in the swing phase. And for stair descent, points E, F, and G occur in the stance phase and H occurs in the swing phase. Table 8 gives the values of these points from 7 studies. The ranges of points E, F, G, and H are from 0.454 to 1.409 Nm/kg, -0.556 to -0.145 Nm/kg, 0.027 to 0.144 Nm/kg, and -0.314 to -0.121 Nm/kg for stair ascent and from 0.007 to 1.512 Nm/kg, -0.070 to 0.662 Nm/kg, 0.365 to 1.620 Nm/kg, and -0.266 to 0.040 Nm/kg for stair descent, respectively. In general, the range of moment is about 1.010 to 1.815 Nm/kg for stair ascent and 0.435 to 1.815 Nm/kg for stair descent. Figure 4(d) shows the typical knee power-time curves in a stair ascent and descent cycle. They all include two peak generation (I and K) and absorption (J and L) powers. For stair ascent, the whole curve lies in the generation area mostly. And for stair descent, the whole curve lies in the absorption area mostly. Table 9 gives the values of these points from 4 studies. The ranges of points I, J, K, and L are from -1.044 to 2.887 W/kg, -0.228 to 0.071 W/kg, 0.447 to 1.020 W/kg, and -0.739 to -0.265 W/kg for stair ascent and from -0.212 to 0.569 Nm/kg, -3.621 to -0.248 Nm/kg, -1.326 to -0.429, and -5.485 to -2.077 Nm/kg for stair descent, respectively. In general, the range of power is about 1.309 to 3.481 W/kg for stair ascent and 2.114 to 6.054 W/kg for stair descent.

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A sketch map of motion and the typical curves of knee angle, moment, and power in sagittal plane for stair ascent and stair descent. (a) Sketch map of the stair ascent and stair descent motion. (b) Knee angle-time curve ((A) peak knee flexion angle and (B) peak knee extension angle. (c) Knee moment-time curve ((E) first peak knee extension moment, (F) first peak knee flexion moment, (G) second peak knee extension moment, and (H) second peak knee flexion moment). (d) Knee power-time curve ((I) first peak knee generation power, (J) first peak knee absorption power, (K) second peak knee generation power, and (L) second peak knee absorption power) [ 24 , 25 ].

Overview over the experimental results of knee angle for stair ascent and stair descent.

A: peak knee flexion angle; B: peak knee extension angle.

Overview over the experimental results of knee moment for stair ascent and stair descent.

Overview over the experimental results of knee power for stair ascent and stair descent.

I: first peak knee generation power; J: first peak knee absorption power; K: second peak knee generation power; L: second peak knee absorption power.

As shown in Figure 5(a) , the sit-to-stand begins in a sit posture and ends in a stand posture. Figures 5(b) – 5(d) show the typical knee angle-time, moment-time, and power-time curves in sit-to-stand cycle, respectively. For the knee joint, there are only extension angle, extension moment, and generation power in the whole sit-to-stand movement. The maximum angle, moment, and power occur in nearly the same time that the buttocks leave the chair. Hurley et al. [ 28 ] represented that the biomechanics of knee joint is mainly affected by the rate of leg length and chair height. Table 10 gives the experimental results of knee angle from 6 studies. The ranges of points A and B are from 82 to 96 deg and -3 to 22 deg, respectively. In general, the ROM of the knee joint is around 60 to 87 deg for sit-to-stand cycle. Table 11 gives the experimental results of knee moment from 9 studies. The ranges of points E and F are from 0.619 to 2.187 Nm/kg and -0.198 to 0.609 Nm/kg, respectively. In general, the range of moment is about 0.619 to 1.578 Nm/kg for sit-to-stand cycle. The researchers about knee power in sit-to-stand is rare, and only two researchers have been found. Spyropoulos et al. [ 29 ] reported that the knee power was about 1.973 W/kg for sit-to-stand. But Kamali et al. [ 30 ] pointed out that the value was about 0.560 W/kg for sit-to-stand.

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Object name is ABB2020-7451683.005.jpg

A sketch map of motion and the typical curves of knee angle, moment, and power in the sagittal plane for sit-to-stand. (a) Sketch map of sit-to-stand cycle [ 28 ]. (b) Knee angle-time curve ((A) peak knee flexion angle and (B) peak knee extension angle). (c) Knee moment-time curve ((E) peak knee extension moment and (F) peak knee flexion moment). (d) knee power-time curve ((I) peak knee generation power) [ 37 , 124 ].

Overview over the experimental results of knee angle for sit-to-stand.

E: peak knee extension moment; F: peak knee flexion moment.

Because of the complicated interaction of the underlying biological mechanisms, the knee joint demonstrates a spring-like behavior in common motions [ 31 – 33 ]. Figure 6 shows the typical knee moment-angle curves in the sagittal plane. A linear relationship can be seen during the sit-to-stand, and the weight acceptance and swing phase of walking, running, and stair climbing. Quasistiffness refers to the slope of the linear fit to the knee moment-angle curve [ 33 ]. During walking, running, and stair climbing, a high stiffness in the weight acceptance phase and a low stiffness in the swing phase can be observed. For walking, Zhu et al. [ 20 ] and Wang [ 12 ] found that the knee quasistiffness was around 3.0 and 2.27 Nm/deg in the stand phase. Sridar et al. [ 34 ] indicated that the knee quasistiffness was around 1.07 Nm/deg in the swing phase. For running, Elliott et al. [ 35 , 36 ] found that the knee quasistiffness was around 0.38 Nm/deg in the swing phase and 6.6 Nm/deg in the stand phase. For stair climbing, Riener et al. [ 24 ] reported that the knee quasistiffness was around 2.37 Nm/deg and 2.42 Nm/deg in the weight acceptance phase of stair ascent and stair descent and 0.19 Nm/deg and 0.04 Nm/deg in the swing phase of stair ascent and stair descent, respectively. For sit-to-stand, Wu et al. [ 37 ] reported that the knee quasistiffness was around 1.1 Nm/deg.

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The moment-angle (stiffness) curves of the knee joint for normal walking, running, stair climbing, and sit-to-stand. (a) Normal walking [ 20 , 34 ]. (b) Running [ 35 , 36 ]. (c) Stair ascent and stair descent [ 24 ]. (d) Sit-to-stand [ 37 ].

3.1.2. The Real Motion and Coronal Plane Biomechanics of Knee Joint

Since the nonuniform shape of the knee articular surface and the complicated physical structure of the femur and tibia, the knee motion cannot be modeled as simple as a perfect hinge [ 38 – 40 ]. The real knee joint moves with a polycentric motion, whereby the center of rotation changes during the rotation [ 41 ]. The femur and tibia can be approximated as a bielliptical structure, so the tibia rolls on the femur resulting in anterior-posterior (A-P) translation during the flexion-extension motion [ 40 ]. When the rotation angle is less than 20 deg, there would be a small A-P translation. Thus, the movement of a real knee joint can be approximated as pure rolling around the fixed center. But when the rotation angle is more than 20 deg, the A-P translation begins to increase, the amplitude of which can exceed 19 mm. Thus, the knee motion can be approximated as a gradual transition from pure rolling, the coupled motion of rolling, and sliding to pure sliding [ 38 , 40 , 42 ]. Smidt [ 43 ] reported that the trajectory of the center of knee seems to be a J-shaped curve in the sagittal plane.

In addition to the motion in the sagittal plane, the knee joint also has internal-external rotation in the horizontal plane [ 44 ]. During the last 10-15 deg before complete extension, the medial femoral condyle is internally rotated and the tibia is externally rotated. At the same time, the lateral meniscus is anteriorly translated and the medial meniscus is posteriorly translated. Because of the larger contact surface of the medial tibiofemoral joint, the length of the medial femoral condyle is longer than that of the lateral, and because of the limitations of cruciate-collateral ligaments and quadriceps femoris on knee motion, the knee joint is self-locking as an eccentric wheel to maintain the stability of the joint during the knee extension motion [ 44 , 45 ]. Blankevoort et al. [ 46 ], Churchill et al. [ 47 ], and Hollister et al. [ 48 ] found that the flexion-extension and internal-external rotation cause the trajectory of the knee center seem to be a spiral curve.

In the coronal plane, the knee adduction moment and the loads of knee medial and lateral compartments are key parameters of biomechanics. For the former, Gaasbeek et al. [ 49 ], Russell [ 50 ], and Briggs et al. [ 51 ] found that the maximum adduction moment is about 0.31, 0.36, and 0.26 Nm/(kg·m) in walking, respectively. Ferber et al. [ 52 ], Sinclair [ 53 ], and Gehring et al. [ 54 ] reported that the maximum adduction moment is about 0.52, 0.53, and 0.58 Nm/(kg·m) in running, respectively. Law [ 26 ] and Musselman [ 27 ] represented that the maximum adduction moment is about 0.44 and 0.34 Nm/kg in stair climbing, respectively. Trepczynski et al. [ 55 ] reported that the maximum adduction moment is about 0.45 Nm/kg in sit-to-stand. For the latter, Russell [ 50 ] found that the normal knee joint always had a little varus, in other words, the medial compartment bears more load than the lateral compartment. Specogna et al. [ 56 ] reported that the weight-bearing line (WBL) was different in each phase of the gait. Cao [ 57 ] reported that the medial compartment bears 60-80% of the load. Pagani et al. [ 58 ] found that about 70% joint force pass through the medial compartment to the ground.

3.2. Biomechanical Properties of Diseased Knee Joint

According to the pathogeny, the knee disorders can be mainly divided into musculoskeletal and neurological disorders. For the former, the pathogeny is inside the knee joint, but the neural control system of these patients is normal. Knee osteoarthritis (KOA), knee ligament injury, and meniscus injury are the most common forms of these disorders and will be mainly discussed in this section. Some evidences showed that the partial assistance from an external mechanism can alleviate the symptoms [ 59 ]. For the latter, the actuator of the knee is normal, but the knee control system or more advanced control system is injured. Although it is not considered a knee joint disease in the medical field, the neurological disorders can influence the knee movement biomechanics. Spinal cord injury (SCI), stroke, and cerebral palsy (CP) are the most common forms of these disorders and will be mainly discussed in this section. Some researchers pointed out that the partial or entire assistance from an external mechanism and rehabilitation training can recover the ambulatory ability of this patients [ 60 , 61 ].

3.2.1. Knee Musculoskeletal Disorders and Its Biomechanical Effects

KOA, one of the major health problems, affects 7-17% of individuals especially for the elder, obese, and previous limb injury people [ 62 – 65 ]. Nearly 46% of adults will develop painful KOA in at least one knee joint over their lifetime [ 66 ]. By 2020, the KOA is predicted to become the fourth leading cause of disability globally [ 67 ]. The etiology and progression of KOA are multifactorial, which includes the increasing tibiofemoral force, the femoral shaft curvature changes, enlarging bone marrow lesions, compartment cartilage loss, joint space narrowing, and tibial plateau compression. [ 63 , 68 ]. From the biomechanical view, these causes will change the tibiofemoral alignment and influence the load distribution, and then result in the deterioration of KOA [ 69 ]. Due to the medial compartment bearing about 70% of the total force, KOA is more commonly observed in the medial compartment (MKOA) than the lateral compartment with a ratio of up to 4 times [ 58 , 59 ].

Medical radiological assessment, kinematics analysis, kinetics analysis, and knee muscle analysis are the common biomechanical methods for KOA, as shown in Table 12 . In the medical radiological assessment aspect, the hip-knee-ankle angle (HKAA) on the full-0limb radiograph is regarded as the gold standard of alignment measurement, as shown in Figure 7(a) [ 63 , 69 ]. Chao et al. [ 70 ] reported that the normal HKAA was about 178.8 deg and the angle is less than the value represented by genu varum. Russell [ 50 ] found that the HKAA of normal and MKOA were about 177.7 deg and 174.2 deg, respectively. As shown in Figure 7(a) , mechanical-lateral-distal-femoral angle (mLDFA), medial-proximal-tibial angle (MPTA), and joint-line-convergence angle (JLCA) are also commonly used as the measurement parameters [ 68 ]. The normal values of these angles are 85-90 deg, 85-90 deg, and 0-2 deg, respectively. The mLDFA greater than 90 deg, MPTA less than 85 deg, or JLCA greater than 2 deg represent genu varum [ 71 ]. The mechanical axis deviation (MAD) is another measurement method. The normal MAD is about 8 mm in the medial, and the value greater than the normal MAD represents genu varum [ 71 ]. Besides, the WBL ratio and medial or lateral joint space also used to characterize the KOA. Russell [ 50 ] pointed out that the WBL ratio, medial joint space, and lateral joint space were about 41.4%, 4.5 mm, and 5.5 mm for normal individuals and 24.2%, 2.8 mm and 7.9 mm for MKOA, respectively. In the knee kinematics aspect, Russell [ 50 ] reported that the knee flexion pattern was similar, but the magnitude was lower for MKOA patients compared to that for normal subjects, as shown in Figure 7(b) . Zhu et al. [ 72 ] found that the KOA patients presented a longer gait time, a smaller stride length and ROM, a greater knee flexion angle at heel strike, and an unobvious fluctuation of knee flexion angle in the stand phase of walking. Alzahrani [ 73 ] indicated that the MKOA patients presented slower walking speeds, shorter step lengths, longer stance and double support time, and smaller cadence, stride length, and knee ROM. In the knee kinetics aspect, Russell [ 50 ] described that the knee adduction moment pattern was similar, but the magnitude was higher for MKOA patients compared to that for normal subjects in walking, as shown in Figure 7(c) . Astephen et al. [ 74 ] observed that the knee adduction moment in MKOA patients was greater than that in the normal in mid-stance. Guo et al. [ 75 ] found that the MKOA patients possessed a greater peak adduction moment during stair climbing. Rudolph et al. [ 76 ] and Schmitt and Rudolph [ 77 ] pointed out that the peak knee flexion moment in KOA patients was smaller than that in the normal during early and late stance phases. Fitzgerald et al. [ 78 ] reported that a 4-6 deg increase in varus alignment could increase around 70-90% medial compartment load during single limb bearing. Lim et al. [ 79 ] indicated that genu varum exceeding 5 deg at baseline was associated with greater functional deterioration over 18 months than the value of 5 deg or less. Kemp et al. [ 80 ] observed that a 20% increase in the peak adduction moment could increase the KOA progression risk. In the knee muscle aspect, Slemenda et al. [ 81 ], Hurley et al. [ 82 ], and Oreilly et al. [ 83 ] found that the KOA patients had smaller quadriceps strength and muscle activation. Lim et al. [ 79 ] indicated that there was no significant relationship between the varus malalignment and the EMG ratio of VM and VL. Russell [ 50 ] reported that the medial muscle (VM-ST and VM-MG) and lateral muscle (VL-BF and VL-LG) cocontraction indices were not significantly different between MKOA patients and normal person, but the quadriceps strength was significantly lower for MKOA patients. Alzahrani [ 73 ] and Hubley-Kozey et al. [ 84 ] represented that the medial and lateral muscle cocontraction was increased for the KOA patients.

An external file that holds a picture, illustration, etc.
Object name is ABB2020-7451683.007.jpg

The knee alignment measurement methods and the effect of KOA on flexion angle and adduction moment. (a) Sketch map of HKAA, mLDFA, MPTA, and MAD [ 61 ]. (b) Knee flexion angles of health and KOA individuals [ 48 ]. (c) Knee adduction moments of health and KOA subjects [ 48 ].

Overview over the biomechanical effects of KOA.

Knee ligament injury is a common and serious disease in sport injuries and can significantly change the biomechanics. According to where the injury hits, the knee ligament injury can be divided into the ACL, PCL, TCL, FCL, and PL injuries. Many researchers pointed out that the secondary injuries, e.g., cartilage injury, meniscus injury, and KOA, can occur if not treated in time. And the ligament reconstruction, as a recognized effective treatment, can dramatically recover the knee biomechanics [ 85 – 88 ]. In the five types of injures, nearly half of ligament injuries are isolated injuries to the ACL [ 89 ]. So, ACL injury will be mainly discussed in this section.

The biomechanical effects of ACL were shown in Table 13 . In the knee kinematics aspect, Zhao et al. [ 90 ] reported that the knee ROM was lower for ACL-injured patients in stair climbing. Slater et al. [ 91 ] pointed out that the peak knee flexion angle was smaller and the peak knee adduction angle was greater for the ACL injury patients in walking. Cronstrom et al. [ 92 ] represented that the knee adduction degree during weight-bearing activities for ACL-injured patients was greater in walking. Gao and Zheng [ 93 ] indicated that the ACL-injured patients had slower speed and smaller stride length during walking. In the knee kinetics aspect, Alexander and Schwameder [ 94 ] observed a 430% and 475% increase in the patella-femur contact force for ACL-injured patients during upslope and downslope, respectively. Goerger et al. [ 95 ] found that the peak knee adduction moment during weight-bearing activities was greater in patients after ACL than before injury. Slater et al. [ 91 ] reported that a smaller peak external knee flexion and adduction moment can be found in the ACL-injured patients during walking. Thomas and Palmieri-Smith [ 96 ] illustrated no difference in the external knee adduction moment among individuals with ACL injury and those who are healthy. Norcross et al. [ 85 ] demonstrated that the ACL-injured patients had a greater knee energy adsorption during landing.

Overview over the biomechanical effects of ACL and meniscus injury.

Meniscus injury, as a sport-induced injury, is common among athletes and general population [ 86 , 89 ]. The meniscus-injured patients are often coupled with traumatic ACL injury and can increase the stress and reduce the stability of the knee joint during extension and flexion motions [ 89 ]. Many studies described that the secondary diseases, e.g., cartilage wear and KOA, can occur if not treated in time [ 87 , 88 , 97 ]. According to the injured degree, different treatments including conservative treatment, meniscus suture, and meniscectomy, can be selected.

To our knowledge, there are rare research that study the biomechanical effects of meniscus injury, as shown in Table 13 . Magyar et al. [ 87 ] represented that the walking speed and knee ROM of meniscus-injured patients were significantly smaller, and the cadence, step length, duration of support, and double support phase of meniscus-injured patients were remarkably larger in walking. Zhou [ 86 ] indicated that the maximum flexion angle and maximum abduction-adduction angle between meniscus injury patients and healthy subjects have no apparent difference. The meniscus-injured patients had a larger minimum flexion angle and a smaller maximum internal-external rotation angles in walking. And the knee stressed area was smaller and the knee pressure was larger for the meniscus-injured patients in walking.

3.2.2. Knee Neurological Disorders and Its Biomechanical Effects

SCI, one of the main causes of mobility disorders, affects around 0.25-0.5 million people every year around the world especially the young [ 98 ]. Approximately 43% of SCI patients turn out to have paraplegia and the number is increasing year by year [ 99 ]. The SCI patients are at an increasing risk of many secondary medical complications, including muscle atrophy, pressure ulcer, bone density reduction, and osteoporosis [ 100 , 101 ]. Standing and walking, as the most prevalent desires of these patients, can stimulate blood circulation, ease muscle spasm, and increase the bone mineral density [ 98 , 102 ]. Some evidences showed that the SCI patients can reduce the secondary medical complications risk and recover motion capabilities by standing or walking for several hours per day [ 98 , 99 , 102 , 103 ]. The biomechanical effects of SCI were shown in Table 14 . Barbeau et al. [ 104 ] pointed out that the knee ROM and peak knee-swing-flexion angle were lower, and peak knee moment was larger for SCI patients in walking. Desrosiers et al. [ 105 ] found that the knee power was lower for SCI patients in uphill and downhill walking. Pepin et al. [ 106 ] indicated that the SCI patients presented a longer flexed knee at good contact and maintain the longer flexion throughout the stance phase of walking.

Overview over the biomechanical effects of SCI, stroke, and CP.

Stroke, a common cerebrovascular disease, has a high mortality and disability rate [ 107 , 108 ]. There are about 7.0 million stroke survivals in China and 6.6 million in the United States [ 109 , 110 ]. Stroke is known as the cause of paralysis, loss of motor function, paresis-weakness of muscle, plegia-complete loss of muscle action, and muscle atrophy [ 34 , 108 , 109 ]. Impaired walking and sit-stand transition are the main reason that poststroke patients cannot live independently [ 107 , 108 ]. And about 30% of poststroke patients have difficulty in ambulation without assistance [ 109 ]. Some evidences showed that 70% of poststroke patients can recover their walking capabilities by rehabilitation [ 108 , 111 ]. The biomechanical effects of stroke were shown in Table 14 . Sridar et al. [ 109 ] indicated that the kinematic and kinetic performance of the poststroke patients will degrade, such as reduced walking speed, quadriceps muscle moment, and quadriceps muscle power. Chen et al. [ 112 ] revealed that the poststroke patients had lower knee flexion in the swing phase of walking. Stanhope et al. [ 113 ] found that the poststroke patients can compensate their poor knee flexion in walking through faster speed. Marrocco et al. [ 114 ] reported a greater dynamic medical knee joint loading in stroke subjects in walking. However, the external knee adduction and flexion moments in walking were not significantly different between the stroke patients and healthy subjects. Novak et al. [ 115 ] observed that less energy was transferred concentrically via knee extensor muscles of stroke patients in mid-stance of walking. And the stroke patients presented lower energy absorption by the knee extensors in the late stance of walking.

CP, the most common pediatric neuromotor disorder, affects around 0.2-0.3% live births [ 19 , 116 ]. The injury in the central nervous system of the developing fetus or infant is the pathogenesis of CP, which effects the control of movement, balance, and posture [ 116 , 117 ]. The person with CP always has a variety of characteristics including rigidity, spasticity, abnormal aerobic and anaerobic capacity, decreased muscle strength and endurance, abnormal muscle tone, deformities, and muscle weakness [ 19 , 118 , 119 ]. The biomechanical effects of CP were shown in Table 14 . Crouch gait, characterized by excessive knee flexion in stance phase, is a frequent gait deviation in CP patients [ 19 , 117 , 118 ]. Hicks et al. [ 120 ] reported the minimum knee flexion angle during the stance phase exceed 40 deg for the CP patients. Compared with the normal gait, crouch gait is inefficient and consumes much more energy [ 19 , 116 ]. For maintaining the excessive knee flexion posture in walking, the stress of the knee and surrounding muscles are increasing, which can lead to bony deformities, degenerative arthritis, joint pain, and patellar stress fractures and then result in the severity of crouch gait [ 19 , 118 , 121 ]. Some evidences showed that the mobility function can be preserved and the complications can be reduced by limiting excessive knee flexion in walking [ 118 ].

4. Discussion and Conclusions

Knee disorders, including musculoskeletal and neurological disorders, have serious influences on knee biomechanics. A number of researches related with the biomechanics of normal and diseased knee joint have been done during the last decades. Many advances have been made to understand the kinematics and kinetics of normal and diseased knee during different common motions. In the aspect of normal knee biomechanics, there is no clear assessment at the current state-of-the-art. The difference between the results of different researches is significant. In the aspect of diseased knee biomechanics, a lower knee flexion angle, walking speed, muscles strength, and a higher knee contact pressure were always observed. Understanding how pathologies affect the knee joint biomechanics is important for designing knee assistive devices and optimizing rehabilitation exercise program. However, the current understanding still has not met the requirement of a designer and rehabilitative physician. And it is hard to find a research that can systematic study all aspects of knee biomechanics completely. Thus, deeper understanding of the biomechanics of normal and diseased knee joint will still be an urgent need in the future.

Some limitations of the current studies must be noted. First, the current understanding on the knee biomechanics is not enough. Many research about the theoretical analysis of knee biomechanics are based on the mathematical modeling. Whether a link model or a simulation model, there is a difference between the model and the reality. And some simplification should always be made, such as the mechanical property, geometry, and relative motion of the bone, muscle, cartilage, etc. Thus, the current computational knee biomechanics cannot describe the real knee biomechanics completely. Second, the kinematics and kinetics results from different research are vastly different. The results are hard to apply in the designing knee assistive devices and optimizing rehabilitation exercise program directly. Therefore, the kinematics and kinetics analyses must be redone in actual use. Third, the studies about the biomechanical influences of knee disorders are mainly concentrated in walking. Little research has been done on other daily life activities, such as running, stair climbing, and sit-to-stand. Fourth, there is an insufficient recognition of the influence of disorders on the knee biomechanics. The influence will always be obtained by patients-normal comparative experiments. And there is severe shortage of deeper rational analyses of the influence.

There are several limitations of our review. First, only articles published in English were included posing a language bias to article selection. Second, the review findings are limited to the articles identified by the set search strategy. Third, the quality of evidence for each study was very low because of the study designs and high heterogeneity.

In the future, the biomechanics of the normal and diseased knee joint will constitute a key research direction. More realistic biomechanical models and computing methods will be further developed for a deeper understanding of the kinematics and kinetics of the knee joint. And more rational analyses about the biomechanical influences of knee disorders will be further established to design better assistive mechanisms.

Acknowledgments

The research is supported by the Fundamental Research Funds for the Central Universities (Grant No. 31020190503004) and the 111 Project (Grant No. B13044).

Conflicts of Interest

The authors have no conflicts of interest to declare.

A new tool for assessing human movement: the Kinematic Assessment Tool

Affiliation.

  • 1 School of Mechanical Engineering, University of Leeds, Leeds, UK.
  • PMID: 19646475
  • DOI: 10.1016/j.jneumeth.2009.07.025

The study of human behaviour ultimately requires the documentation of human movement. In some instances movements can be recorded through a simple button press on a computer input device. In other situations responses can be captured through questionnaire surveys. Nevertheless, there is a need within many neuroscience settings to capture how complex movements unfold over time (human kinematics). Current methods of measuring human kinematics range from accurate but multifarious laboratory configurations to portable but simplistic and time-consuming paper and pen methods. We describe a new system for recording the end-point of human movement that has the power of laboratory measures but the advantages of pen-and-paper tests: the Kinematic Assessment Tool. KAT provides a highly portable system capable of measuring human movement in configurable visual-spatial tasks. The usefulness of the system is shown in a study where 12 participants undertook a tracing and copying task using their preferred and non-preferred hand. The results show that it is possible to capture behaviour within complex tasks and quantify performance using objective measures automatically generated by the KAT system. The utility of these measures was indexed by our ability to distinguish the performance of the preferred and non-preferred hand using a single variable.

Publication types

  • Research Support, Non-U.S. Gov't
  • Validation Study
  • Biomechanical Phenomena*
  • Motor Activity*
  • Signal Processing, Computer-Assisted*
  • Task Performance and Analysis
  • Time Factors
  • Young Adult

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Design and Development of kinematic linkages Variable Speed Drive

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This paper presents the synthesis, analysis, and review of a variable displacement four-bar crank-rocker-slider mechanism that uses low friction pin joints instead of planar joints. The synthesis technique develops the range of motion for the base four-bar crank-rocker and creates a method of synthesizing the output slider. Kinematic Linkage Transmission is a mechanical adjustable speed drive .The speed range is infinitely adjustable from ½ to ¼ of input speed under full rated load. Speed Adjustments are easily made by moving a lever control through an arc.

China conducts first nationwide review of retractions and research misconduct

  • Smriti Mallapaty

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Technicians wearing full PPE work in a lab

The reputation of Chinese science has been "adversely affected" by the number of retractions in recent years, according to a government notice. Credit: Qilai Shen/Bloomberg/Getty

Chinese universities are days away from the deadline to complete a nationwide audit of retracted research papers and probe of research misconduct. By 15 February, universities must submit to the government a comprehensive list of all academic articles retracted from English- and Chinese-language journals in the past three years. They need to clarify why the papers were retracted and investigate cases involving misconduct, according to a 20 November notice from the Ministry of Education’s Department of Science, Technology and Informatization.

The government launched the nationwide self-review in response to Hindawi, a London-based subsidiary of the publisher Wiley, retracting a large number of papers by Chinese authors. These retractions, along with those from other publishers, “have adversely affected our country’s academic reputation and academic environment”, the notice states.

A Nature analysis shows that last year, Hindawi issued more than 9,600 retractions, of which the vast majority — about 8,200 — had a co-author in China. Nearly 14,000 retraction notices, of which some three-quarters involved a Chinese co-author, were issued by all publishers in 2023.

This is “the first time we’ve seen such a national operation on retraction investigations”, says Xiaotian Chen, a library and information scientist at Bradley University in Peoria, Illinois, who has studied retractions and research misconduct in China. Previous investigations have largely been carried out on a case-by-case basis — but this time, all institutions have to conduct their investigations simultaneously, says Chen.

Tight deadline

The ministry’s notice set off a chain of alerts, cascading to individual university departments. Bulletins posted on university websites required researchers to submit their retractions by a range of dates, mostly in January — leaving time for universities to collate and present the data.

Although the alerts included lists of retractions that the ministry or the universities were aware of, they also called for unlisted retractions to be added.

research paper of kinematics

More than 10,000 research papers were retracted in 2023 — a new record

According to Nature ’s analysis, which includes only English-language journals, more than 17,000 retraction notices for papers published by Chinese co-authors have been issued since 1 January 2021, which is the start of the period of review specified in the notice. The analysis, an update of one conducted in December , used the Retraction Watch database, augmented with retraction notices collated from the Dimensions database, and involved assistance from Guillaume Cabanac, a computer scientist at the University of Toulouse in France. It is unclear whether the official lists contain the same number of retracted papers.

Regardless, the timing to submit the information will be tight, says Shu Fei, a bibliometrics scientist at Hangzhou Dianzi University in China. The ministry gave universities less than three months to complete their self-review — and this was cut shorter by the academic winter break, which typically starts in mid-January and concludes after the Chinese New Year, which fell this year on 10 February.

“The timing is not good,” he says. Shu expects that universities are most likely to submit only a preliminary report of their researchers’ retracted papers included on the official lists.

But Wang Fei, who studies research-integrity policy at Dalian University of Technology in China, says that because the ministry has set a deadline, universities will work hard to submit their findings on time.

Researchers with retracted papers will have to explain whether the retraction was owing to misconduct, such as image manipulation, or an honest mistake, such as authors identifying errors in their own work, says Chen: “In other words, they may have to defend themselves.” Universities then must investigate and penalize misconduct. If a researcher fails to declare their retracted paper and it is later uncovered, they will be punished, according to the ministry notice. The cost of not reporting is high, says Chen. “This is a very serious measure.”

It is not known what form punishment might take, but in 2021, China’s National Health Commission posted the results of its investigations into a batch of retracted papers. Punishments included salary cuts, withdrawal of bonuses, demotions and timed suspensions from applying for research grants and rewards.

The notice states explicitly that the first corresponding author of a paper is responsible for submitting the response. This requirement will largely address the problem of researchers shirking responsibility for collaborative work, says Li Tang, a science- and innovation-policy researcher at Fudan University in Shanghai, China. The notice also emphasizes due process, says Tang. Researchers alleged to have committed misconduct have a right to appeal during the investigation.

The notice is a good approach for addressing misconduct, says Wang. Previous efforts by the Chinese government have stopped at issuing new research-integrity guidelines that were poorly implemented, she says. And when government bodies did launch self-investigations of published literature, they were narrower in scope and lacked clear objectives. This time, the target is clear — retractions — and the scope is broad, involving the entire university research community, she says.

“Cultivating research integrity takes time, but China is on the right track,” says Tang.

It is not clear what the ministry will do with the flurry of submissions. Wang says that, because the retraction notices are already freely available, publicizing the collated lists and underlying reasons for retraction could be useful. She hopes that a similar review will be conducted every year “to put more pressure” on authors and universities to monitor research integrity.

What happens next will reveal how seriously the ministry regards research misconduct, says Shu. He suggests that, if the ministry does not take further action after the Chinese New Year, the notice could be an attempt to respond to the reputational damage caused by the mass retractions last year.

The ministry did not respond to Nature ’s questions about the misconduct investigation.

Chen says that, regardless of what the ministry does with the information, the reporting process itself will help to curb misconduct because it is “embarrassing to the people in the report”.

But it might primarily affect researchers publishing in English-language journals. Retraction notices in Chinese-language journals are rare.

Nature 626 , 700-701 (2024)

doi: https://doi.org/10.1038/d41586-024-00397-x

Data analysis by Richard Van Noorden.

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Reproductive rights in America

Research at the heart of a federal case against the abortion pill has been retracted.

Selena Simmons-Duffin

Selena Simmons-Duffin

research paper of kinematics

The Supreme Court will hear the case against the abortion pill mifepristone on March 26. It's part of a two-drug regimen with misoprostol for abortions in the first 10 weeks of pregnancy. Anna Moneymaker/Getty Images hide caption

The Supreme Court will hear the case against the abortion pill mifepristone on March 26. It's part of a two-drug regimen with misoprostol for abortions in the first 10 weeks of pregnancy.

A scientific paper that raised concerns about the safety of the abortion pill mifepristone was retracted by its publisher this week. The study was cited three times by a federal judge who ruled against mifepristone last spring. That case, which could limit access to mifepristone throughout the country, will soon be heard in the Supreme Court.

The now retracted study used Medicaid claims data to track E.R. visits by patients in the month after having an abortion. The study found a much higher rate of complications than similar studies that have examined abortion safety.

Sage, the publisher of the journal, retracted the study on Monday along with two other papers, explaining in a statement that "expert reviewers found that the studies demonstrate a lack of scientific rigor that invalidates or renders unreliable the authors' conclusions."

It also noted that most of the authors on the paper worked for the Charlotte Lozier Institute, the research arm of anti-abortion lobbying group Susan B. Anthony Pro-Life America, and that one of the original peer reviewers had also worked for the Lozier Institute.

The Sage journal, Health Services Research and Managerial Epidemiology , published all three research articles, which are still available online along with the retraction notice. In an email to NPR, a spokesperson for Sage wrote that the process leading to the retractions "was thorough, fair, and careful."

The lead author on the paper, James Studnicki, fiercely defends his work. "Sage is targeting us because we have been successful for a long period of time," he says on a video posted online this week . He asserts that the retraction has "nothing to do with real science and has everything to do with a political assassination of science."

He says that because the study's findings have been cited in legal cases like the one challenging the abortion pill, "we have become visible – people are quoting us. And for that reason, we are dangerous, and for that reason, they want to cancel our work," Studnicki says in the video.

In an email to NPR, a spokesperson for the Charlotte Lozier Institute said that they "will be taking appropriate legal action."

Role in abortion pill legal case

Anti-abortion rights groups, including a group of doctors, sued the federal Food and Drug Administration in 2022 over the approval of mifepristone, which is part of a two-drug regimen used in most medication abortions. The pill has been on the market for over 20 years, and is used in more than half abortions nationally. The FDA stands by its research that finds adverse events from mifepristone are extremely rare.

Judge Matthew Kacsmaryk, the district court judge who initially ruled on the case, pointed to the now-retracted study to support the idea that the anti-abortion rights physicians suing the FDA had the right to do so. "The associations' members have standing because they allege adverse events from chemical abortion drugs can overwhelm the medical system and place 'enormous pressure and stress' on doctors during emergencies and complications," he wrote in his decision, citing Studnicki. He ruled that mifepristone should be pulled from the market nationwide, although his decision never took effect.

research paper of kinematics

Matthew Kacsmaryk at his confirmation hearing for the federal bench in 2017. AP hide caption

Matthew Kacsmaryk at his confirmation hearing for the federal bench in 2017.

Kacsmaryk is a Trump appointee who was a vocal abortion opponent before becoming a federal judge.

"I don't think he would view the retraction as delegitimizing the research," says Mary Ziegler , a law professor and expert on the legal history of abortion at U.C. Davis. "There's been so much polarization about what the reality of abortion is on the right that I'm not sure how much a retraction would affect his reasoning."

Ziegler also doubts the retractions will alter much in the Supreme Court case, given its conservative majority. "We've already seen, when it comes to abortion, that the court has a propensity to look at the views of experts that support the results it wants," she says. The decision that overturned Roe v. Wade is an example, she says. "The majority [opinion] relied pretty much exclusively on scholars with some ties to pro-life activism and didn't really cite anybody else even or really even acknowledge that there was a majority scholarly position or even that there was meaningful disagreement on the subject."

In the mifepristone case, "there's a lot of supposition and speculation" in the argument about who has standing to sue, she explains. "There's a probability that people will take mifepristone and then there's a probability that they'll get complications and then there's a probability that they'll get treatment in the E.R. and then there's a probability that they'll encounter physicians with certain objections to mifepristone. So the question is, if this [retraction] knocks out one leg of the stool, does that somehow affect how the court is going to view standing? I imagine not."

It's impossible to know who will win the Supreme Court case, but Ziegler thinks that this retraction probably won't sway the outcome either way. "If the court is skeptical of standing because of all these aforementioned weaknesses, this is just more fuel to that fire," she says. "It's not as if this were an airtight case for standing and this was a potentially game-changing development."

Oral arguments for the case, Alliance for Hippocratic Medicine v. FDA , are scheduled for March 26 at the Supreme Court. A decision is expected by summer. Mifepristone remains available while the legal process continues.

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Astrophysics > Astrophysics of Galaxies

Title: probing the nature of rotation in the pleiades, alpha persei, and hyades clusters.

Abstract: Unraveling the internal kinematics of open clusters is crucial for understanding their formation and evolution. However, there is a dearth of research on this topic, primarily due to the lack of high-quality kinematic data. Using the exquisite-precision astrometric parameters and radial velocities provided by Gaia data release 3, we investigate the internal rotation in three of the most nearby and best-studied open clusters, namely the Pleiades, Alpha Persei, and Hyades clusters. Statistical analyses of the residual motions of the member stars clearly indicate the presence of three-dimensional rotation in the three clusters. The mean rotation velocities of the Pleiades, Alpha Persei, and Hyades clusters within their tidal radii are estimated to be 0.24 (0.04), 0.43 (0.08), and 0.09 (0.03) km s-1, respectively. Similar to the Praesepe cluster that we have studied before, the rotation of the member stars within the tidal radii of these three open clusters can be well interpreted by Newton's theorem. No expansion or contraction is detected in the three clusters either. Furthermore, we find that the mean rotation velocity of open clusters may be positively correlated with the cluster mass, and the rotation is likely to diminish as open clusters age.

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Science | Eureka! How a Stanford study revealed the…

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Science | for bay area, a couple of days of mostly dry weather ahead of possible weekend rain, science | eureka how a stanford study revealed the success of research failures, faced with experiments that can’t be reproduced, academia seeks to improve and “future proof” research.

Stanford neuroscientist Dr. Thomas Südhof, winner of the 2013 Nobel Prize in Physiology or Medicine, co-authored a paper with postdoc Kif Liakath-Ali that revealed flaws in previous research. Faced with a disturbing flurry of experiments that can't be reproduced, academia is ramping up efforts to “future proof” experiments.

An important paper recently published by an esteemed Stanford research team reported an unusual result: An experiment went wrong.

Usually, scientists seek to burnish their reputations by announcing positive news of a discovery that solves a problem or transforms how we view the world.

But this negative news — which revealed that earlier neuroscience research was flawed — can also be positive. It builds a stronger scientific foundation and helps restore public trust, according to a growing consensus of scientists and journal editors.

Faced with a disturbing flurry of experiments that can’t be reproduced, academia is ramping up efforts to “future proof” its research.

While headlines are dominated by fraud or research misconduct cases, including a scandal that led to the resignation of Stanford University President Dr. Marc Tessier-Lavigne , these instances are relatively rare. A bigger problem is experimentation that lacks robust design, methodology, analysis and interpretation of results — so arrives at the wrong conclusions.

“Our efforts highlight the importance of experimental rigor,” said Stanford postdoctoral neuroscientist Kif Liakath-Ali , who conducted the work with Nobel Laureate Thomas Südhof.

His revelation — that sometimes a negative can be a positive — came while he was trying to reproduce and build upon a 2017 study about the behavior of brain cells. He wanted to understand the regulation of brain cells, with major implications for memory, behavior and neurological disease. He discovered that the previous approach in the lab had killed cells, leading to “a skewing of results and biased conclusions,” he said.

I t was a professional setback for Liakath-Ali, who had aimed to build on this research to make a new and meaningful discovery.

A junior scientist, he worried that his insight, based on almost two years of work, would not advance his career. Instead, Liakath-Ali has been honored by the School of Medicine ’s new Program on Research Rigor and Reproducibili ty with an award for his integrity.

His finding has emboldened other research teams to come forward to describe their own failed attempts, he said. Although those teams had stayed silent, “they had seen the same thing.”

“That’s what good science is about,” said Dr. Steven Goodman , who leads Stanford’s Program on Research Rigor and Reproducibility. “It detected that some really important findings … were just wrong.”

“We want to reward how people do science,” he said, “and if they do it better than the last person.”

Science is famed for its “Eureka” moments. We love the tale of Scottish microbiologist Alexander Fleming, who came home from vacation to discover a mold producing penicillin, the world’s first antibiotic, growing inside a neglected Petri dish..

Real experimentation takes many twists and turns and doesn’t always deliver the expected outcome.

“I have not failed,” inventor Thomas Edison famously said. “I’ve just found 10,000 ways that won’t work.”

But modern science is competitive. In today’s “publish or perish” academic culture, careers are advanced by insights, so scientists are incentivized to announce only positive findings.

In the worst cases, this can foster fraudulent or sloppy practices. Tessier-Lavigne resigned his post after an independent review found multiple errors in five papers he had overseen , concluding that “multiple members of (his) labs over the years appear to have manipulated research data and/or fallen short of accepted scientific practices.”

Journals also favor papers that are “hot,” with impact that will be widely cited and elevate a journal’s reputation.

“A challenge for scientists has been that (experimental) repetitions are difficult to publish, no matter whether they are positive or negative,” said Südhof, who won the 2013 Nobel Prize in Physiology or Medicine. “This is bad because it creates a disincentive for repeating experiments.”

Journals are trying to change, said Holden Thorp, editor of the prominent journal Science, at last week’s Stanford conference on research integrity. “It’s a very, very challenging problem, because all of the emphasis is on novelty and ‘being first…We don’t take a lot of papers that say, ‘We tested this hypothesis and we found that it’s still correct.’”

But repetition may reveal problems, and “ensures that people have the full story,” said Emily Chenette, editor-in-chief of PLOS ONE , published by the Public Library of Science, which evaluates research on scientific validity, methodology and ethical standards — not perceived significance. PLOS ONE publishes a collection called “Missing Pieces,” which lists studies that present inconclusive, null findings or demonstrate failed replications of other published work.

A negative finding can suggest promising new directions, approaches and hypotheses. It may warn other investigators to steer clear, saving time and money, Chenette said.

“It has real life implications for people,” she said.

Early in the COVID-19 pandemic, use of the malaria drug hydroxychloroquine was spurred by anecdotal reports from China and France of patients who seemed to improve and laboratory findings of a possible antiviral effect. But a rigorous study found that the drug didn’t work — a discovery that saved many lives.

In the prestigious British journal Lancet, a doctor linked vaccines to autism, a claim that has led to clusters of resistance to inoculation. It was refuted by multiple studies, and a subsequent investigation showed his work to be bunk. 

This week, in a surprise announcement, a precious 280-million-year-old fossilized lizard turned out to be mostly … black paint. An Italian team had hoped to make history by using high-tech tools — electron microscopy, spectroscopy and micro x-rays — to reveal the cellular structure of one of the world’s oldest reptiles. Instead, they found forgery. But their revelation could lead to a rethinking of ancient taxonomy.

Stanford’s Liakath-Ali sought to better understand how brain cells, called neurons, communicate via trillions of synapses — and how things go wrong. Synapses connect using a vast network of molecules, governed by genes whose function may change if subjected to stress, causing devastating ailments like schizophrenia, autism and other neurological disorders.

He based his work on a 2017 report by scientists from China’s Tsinghua University published in the journal Nature Neuroscience. They had found that when we learn something, brain cells change in a way that helps us remember it better. But brain cells’ regulatory mechanism can be altered by stress.

He took a closer look at the Chinese research and found fault with their technique, which caused the cells to be so stressed — “hammered,” he said — that they died. This skewed their results.

“Liakath-Ali did what no one else had done: He took the care to look at the cells,” said Goodman.

Nobel Laureate Südhof commended his perseverance.

“Science operates by a trial-and-error process in which scientists, like all other humans, also make mistakes,” he said. “To distinguish valid results from erroneous ones, it is necessary to repeat experiments independently.”

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Those with normal cognition who test positive for elevated levels of certain proteins tied to Alzheimer's would be diagnosed as having Stage 1.

Health | Inside the plan to diagnose Alzheimer’s in people with no memory problems — and who stands to benefit

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AI Writes Scientific Papers That Sound Great—but Aren’t Accurate

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F irst came the students, who wanted help with their homework and essays. Now, ChatGPT is luring scientists, who are under pressure to publish papers in reputable scientific journals.

AI is already disrupting the archaic world of scientific publishing. When Melissa Kacena, vice chair of orthopaedic surgery at Indiana University School of Medicine, reviews articles submitted for publication in journals, she now knows to look out for ones that might have been written by the AI program. “I have a rule of thumb now that if I pull up 10 random references cited in the paper, and if more than one isn’t accurate, then I reject the paper,” she says.

But despite the pitfalls, there is also promise. Writing review articles, for example, is a task well suited to AI: it involves sifting through the existing research on a subject, analyzing the results, reaching a conclusion about the state of the science on the topic, and providing some new insight. ChatGPT can do all of those things well.

Kacena decided to see who is better at writing review articles: people or ChatGPT. For her study published in Current Osteoporosis Reports , she sorted nine students and the AI program into three groups and asked each group to write a review article on a different topic. For one group, she asked the students to write review articles on the topics; for another, she instructed ChatGPT to write articles on the same topics; and for the last group, she gave each of the students their own ChatGPT account and told them to work together with the AI program to write articles. That allowed her to compare articles written by people, by AI, and a combination of people and AI. She asked faculty member colleagues and the students to fact check each of the articles, and compared the three types of articles on measures like accuracy, ease of reading, and use of appropriate language.

Read More : To Make a Real Difference in Health Care, AI Will Need to Learn Like We Do

The results were eye-opening. The articles written by ChatGPT were easy to read and were even better written than the students'. But up to 70% of the cited references were inaccurate: they were either incoherently merged from several different studies or completely fictitious. The AI versions were also more likely to be plagiarized.

“ChatGPT was pretty convincing with some of the phony statements it made, to be honest,” says Kacena. “It used the proper syntax and integrated them with proper statements in a paragraph, so sometimes there were no warning bells. It was only because the faculty members had a good understanding of the data, or because the students fact checked everything, that they were detected.”

There were some advantages to the AI-generated articles. The algorithm was faster and more efficient in processing all the required data, and in general, ChatGPT used better grammar than the students. But it couldn't always read the room: AI tended to use more flowery language that wasn’t always appropriate for scientific journals (unless the students had told ChatGPT to write it from the perspective of a graduate-level science student.)

Read More : The 100 Most Influential People in AI

That reflects a truth about the use of AI: it's only as good as the information it receives. While ChatGPT isn’t quite ready to author scientific journal articles, with the proper programming and training, it could improve and become a useful tool for researchers. “Right now it’s not great by itself, but it can be made to work,” says Kacena. For example, if queried, the algorithm was good at recommending ways to summarize data in figures and graphical depictions. “The advice it gave on those were spot on, and exactly what I would have done,” she says.

The more feedback the students provided on ChatGPT's work, the better it learned—and that represents its greatest promise. In the study, some students found that when they worked together with ChatGPT to write the article, the program continued to improve and provide better results if they told it what things it was doing right, and what was less helpful. That means that addressing problems like questionable references and plagiarism could potentially be fixed. ChatGPT could be programmed, for example, to not merge references and to treat each scientific journal article as its own separate reference, and to limit copying consecutive words to avoid plagiarism.

With more input and some fixes, Kacena believes that AI could help researchers smooth out the writing process and even gain scientific insights. "I think ChatGPT is here to stay, and figuring out how to make it better, and how to use it in an ethical and conscientious and scientifically sound manner, is going to be really important,” she says.

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