Form switching during human locomotion: traversing wedges in a single step

We examined the neural control strategies used to accommodate discrete alterations in walking surface inclination. Normal subjects were tested walking on a level surface and on different wedges (10 degrees, 15 degrees, 20 degrees, and 30 degrees ) presented in the context of level walking. On a given trial, a subject walked on a level surface in approach to a wedge, took a single step on the wedge, and continued walking on an elevated level surface beyond the wedge. As wedge inclination increased, subjects linearly increased peak joint angles. Changes in timing of peak joint angles and electromyograms were not linear. Subjects used two distinct temporal strategies, or forms, to traverse the wedges. One form was used for walking on a level surface and on the 10 degrees wedge, another form for walking on the 20 degrees and 30 degrees wedges. In the level/10 degrees form, peak hip flexion occurred well before heel strike (HS) and peak dorsiflexion occurred in late stance. In the 20 degrees /30 degrees form, peak hip flexion was delayed by 12% of the stride cycle and peak dorsiflexion was reached 12% earlier. For the level/10 degrees form, onsets of the rectus femoris, gluteus maximus, and vastus lateralis muscles were well before HS and offset of the anterior tibialis was at HS. For the 20 degrees /30 degrees form, onsets of the rectus femoris, gluteus maximus, and vastus lateralis and offset of the anterior tibialis were all delayed by 12% of the stride cycle. Muscles shifted as a group, rather than individually, between the forms. Subjects traversing a 15 degrees wedge switched back and forth between the two forms in consecutive trials, suggesting the presence of a transition zone. Differences between the forms can be explained by the differing biomechanical constraints imposed by the wedges. Steeper wedges necessitate changes in limb orientation to accommodate the surface, altering limb orientation with respect to gravity and making it necessary to pull the body forward over the foot. The use of different forms of behavior is a common theme in neural control and represents an efficient means of coordinating and adapting movement to meet changing environmental demands. The forms of locomotion reported here are likely used on a regular basis in real-world settings.     

Coordination of multi-joint arm movements in cerebellar ataxia: Analysis of hand and angular kinematics

Kinematic abnormalities of fast multijoint movements in cerebellar ataxia include abnormally increased curvature of hand trajectories and an increased hand path and are thought to originate from an impairment in generating appropriate levels of muscle torques to support normal coordination between shoulder and elbow joints. Such a mechanism predicts that kinematic abnormalities are pronounced when fast movements are performed and large muscular torques are required. Experimental evidence that systematically explores the effects of increasing movement velocities on movement kinematics in cerebellar multijoint movements is limited and to some extent contradictory. We, therefore, investigated angular and hand kinematics of natural multijoint pointing movements in patients with cerebellar degenerative disorders and healthy controls. Subjects performed self-paced vertical pointing movements with their right arms at three different target velocities. Limb movements were recorded in three-dimensional space using a two-camera infrared tracking system. Differences between patients and healthy subjects were most prominent when the subjects performed fast movements. Peak hand acceleration and deceleration were similar to normals during slow and moderate velocity movements but were smaller for fast movements. While altering movement velocities had little or no effect on the length of the hand path and angular motion of elbow and shoulder joints in normal subjects, the patients exhibited overshooting motions (hypermetria) of the hand and at both joints as movement velocity increased. Hypermetria at one joint always accompanied hypermetria at the neighboring joint. Peak elbow angular deceleration was markedly delayed in patients compared with normals. Other temporal movement variables such as the relative timing of shoulder and elbow joint motion onsets were normal in patients. Kinematic abnormalities of multijoint arm movements in cerebellar ataxia include hypermetria at both the elbow and the shoulder joint and, as a consequence, irregular and enlarged paths of the hand, and they are marked with fast but not with slow movements. Our findings suggest that kinematic movement abnormalities that characterize cerebellar limb ataxia are related to an impairment in scaling movement variables such as joint acceleration and deceleration normally with movement speed. Most likely, increased hand paths and decomposition of movement during slow movements, as described earlier, result from compensatory mechanisms the patients may employ if maximum movement accuracy is required.     

Multijoint arm movements in cerebellar ataxia: Abnormal control of movement dynamics

In cerebellar ataxia, kinematic aberrations of multijoint movements are thought to originate from deficiencies in generating muscular torques that are adequate to control the mechanical consequences of dynamic interaction forces. At this point the exact mechanisms that lead to an abnormal control of interaction torques are not known. In principle, the generation of inadequate muscular torques may result from an impairment in generating sufficient levels of torques or from an inaccurate assessment and prediction of the mechanical consequences of movements of one limb segment on adjacent joints. We sought to differentiate the relative contribution of these two mechanisms and, therefore, analyzed intersegmental dynamics of multijoint pointing movements in healthy subjects and in patients with cerebellar degeneration. Unrestrained vertical arm movements were performed at three different target movement velocities and recorded using an optoelectronic tracking system. An inverse dynamics approach was employed to compute net joint torques, muscular torques, dynamic interaction torques and gravitational torques acting at the elbow and shoulder joint. In both groups, peak dynamic interaction forces and peak muscular forces were largest during fast movements. In contrast to normal subjects, patients produced hypermetric movements when executing fast movements. Hypermetric movements were associated with smaller peak muscular torques and smaller rates of torque change at elbow and shoulder joints. The patients’ deficit in generating appropriate levels of muscular force were prominent during two different phases of the pointing movement. Peak muscular forces at the elbow were reduced during the initial phase of the movement when simultaneous shoulder joint flexion generated an extensor influence upon the elbow joint. When attempting to terminate the movement, gravitational and dynamic interaction forces caused overshooting extension at the elbow joint. In normal subjects, muscular torque patterns at shoulder and elbow joint were synchronized in that peak flexor and extensor muscular torques occurred simultaneously at both joints. This temporal pattern of muscular torque generation at shoulder and elbow joint was preserved in patients. Our data suggest that an impairment in generating sufficient levels of phasic muscular torques significantly contributes to the patients’ difficulties in controlling the mechanical consequences of dynamic interaction forces during multijoint movements. Received: 28 October 1996 / Accepted: 30 September 1997     

A kinematic comparison of single and multijoint pointing movements

Summary Rapid pointing movements (no accuracy or reaction time requirements) were performed under three conditions which limited motion to the shoulder, elbow or a combination of these two joints. Velocity profiles of the hand's trajectory differed during single and multijoint movements. For the same magnitude of displacement, the hand always had a higher peak velocity, shorter rise time (time to peak velocity) and shorter movement time during single joint movements. However, when the profiles were normalized with respect to amplitude and movement time, no significant differences were observed between these three movement conditions. The velocity profiles of the elbow and/or shoulder were also compared under single and multijoint movement conditions. Analysis of these profiles revealed that the relationships between peak velocity and displacement and between movement time and displacement remained the same at the shoulder joint during single and multijoint movements. In contrast, the elbow joint velocity profiles were significantly affected by movement conditions. These relationships (peak velocity/ displacement and movement time/displacement) changed during multijoint movements and became the same as those observed at the shoulder joint. The shape of the hand velocity profile and its invariance across movement conditions can best be explained by dynamic optimization theory and supports the notion that movement of the hand is of primary importance during rapid pointing. However, the consistency of the shoulder velocity profile and the highly significant relationships between the movement of the elbow and shoulder joints indicates that a subordinate joint planning strategy is also used. The purpose of this strategy is to functionally decrease the available degrees of freedom and to simplify coordination between the moving joints. Thus, the organization of arm movements is hierarchically structured with important, but different contributions being made on both the hand planning and joint planning levels.     

Cerebellar dysmetria at the elbow, wrist, and fingers

1. The objective was to investigate in cerebellar patients with dysmetria the kinematic and electromyographic (EMG) characteristics of large and small movements at the elbow, wrist, and finger and thereby to determine the nature of cerebellar dysmetria at distal as well as proximal joints. Flexions were made as fast as possible by moving relatively heavy manipulanda for each joint to the same end position through 5, 30, and 60 degrees. 2. In normal subjects flexions at all joints were accompanied by similar triphasic EMG activity. Movements of increasing amplitude were made with increasing movement durations and increasing durations and magnitudes of initial agonist EMG activity. Antagonist activity often appeared to have two components: one coactive with the initial agonist burst but starting later, the other reaching its peak at about peak velocity. 3. Cerebellar patients with dysmetria showed hypermetria followed by tremor at all three joints when movements were made with the manipulanda. Hypermetria was most marked for aimed movements of small amplitude (5 degrees) at all joints. 4. A characteristic of cerebellar disordered movements, which could be present at all amplitudes and all joints, was an asymmetry with decreased peak accelerations and increased peak decelerations compared to normal movements. Both the asymmetry and the hypermetria for small amplitude movements could be used clinically as sensitive indicators of cerebellar dysfunction. 5. The EMG abnormalities accompanying hypermetria and asymmetry were a more gradual buildup and a prolongation of agonist activity and delayed onset of antagonist activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of movement speed on limb segment motions for reaching from a standing position

The performance of a standing reaching task that necessitates some forward bending requires: (1) the coordination of multiple joints (i.e., the trunk and limb segments) to reach the target, and (2) the preservation of postural stability. It has been proposed that the neural control of multijoint reaching tasks can be simplified by time scaling of joint motions while keeping joint excursions the same. To determine if time scaling of joint motions was used in this more complex reaching task, we had 20 healthy subjects (10 male and 10 female) reach for two targets located in a parasagittal plane while standing on a force platform. Subjects reached for the targets at a comfortable speed and a fast paced speed. Sagittal plane motions of the right shank, thigh, pelvis, trunk, humerus, and forearm were measured. At the fast paced movement speeds subjects had significantly larger excursions of the thigh, pelvis, humerus, and forearm compared to the comfortable speed. Thus, segment motions are not simply time scaled for standing multijoint reaches. We explored three possible reasons for not obeying time scaling: (1) to reduce scaling of peak kinetic energy, (2) to reduce scaling of peak horizontal ground reaction force, and (3) a convergence of movement strategies at faster speeds. While subjects modified their movement strategy in relationship to movement speed, these changes had no significant effect on the expected scaling of peak kinetic energy, or peak horizontal ground reaction forces. Given the intersubject differences in movement strategies used to perform these reaching tasks at the fast speeds, a convergence of movement strategies was ruled out. We propose that the increase in segment motions with speed may be a consequence of rules underlying motor output, the increases being greater for segments in which viscoelastic resistance to movement is more significant compared to inertial resistance. Electronic Publication     

Increased variability in finger position occurs throughout overarm throws made by cerebellar and unskilled subjects.

We investigated the ability of cerebellar patients and unskilled subjects to control finger grip position and the amplitude of finger opening during a multijoint overarm throw. This situation is of interest because the appropriate finger control requires predicting the magnitude of back forces from the ball on the finger throughout the throw and generating the appropriate level and rate of change of finger flexor torque to oppose the back force. Cerebellar patients, matched controls, and unskilled subjects threw tennis balls and tennis-sized balls of different weights. In all cases angular positions of five arm segments in three dimension were recorded at 1,000 Hz with the search-coil technique as subjects threw from a seated position. When the hand was stationary, cerebellar patients showed a normal ability to grip the ball and open the fingers and drop the ball. In contrast, in overarm throws where a back force occurred on the fingers, cerebellar patients showed an abnormally large variability in amplitude of the change in finger position when gripping, in amplitude of finger opening, and in amplitude of the change in finger position 10 ms after ball release. This was not due to more trial-to-trial variation in throwing speed. When throwing balls of increasing weights, both controls and cerebellar patients had increasing finger flexions after ball release that indicated that, on average, both scaled finger force in proportion to ball weight during the throw. Unlike skilled controls, cerebellar patients showed a small (<20 degrees ) increase in the amplitude of finger opening with balls of increasing weight. However, neither the increase in variability of finger position nor the increase in finger amplitude with balls of increasing weight were unique cerebellar signs because both were observed to various degrees in unskilled throwers. It is concluded that in the absence of either normal cerebellar function or skill, the central neural activity that controls finger opening in throwing can increase finger flexor force to oppose an increase in back force from heavier balls and can open the fingers but cannot control finger force or finger opening precisely and consistently from throw to throw. These results fit with the idea that cerebellar disorders are greater in multijoint than single-joint movements because control of force is more complicated. They are also consistent with the hypothesis that the cerebellum produces skill in movement by reducing variability in the timing and force of muscle contractions.     

Causes of left-right ball inaccuracy in overarm throws made by cerebellar patients

Cerebellar patients throw inaccurately in the left-right direction but the cause of this multijoint ataxia is unclear. We tested whether it was due, as originally proposed, to variable left-right directions of the hand path, or, alternatively, to variable timing of ball release occurring on a right to left curved hand path. We also examined the cause of the variability in hand path direction per se. Six right-handed cerebellar patients and six control subjects were instructed to throw tennis balls at a slow, medium and fast speed from a seated position while angular positions in 3D of five arm segments were recorded at 1000 Hz with the search-coil technique. Compared to controls, cerebellar patients threw slower and less accurately, had more variable timing of ball release occurring on a right to left curved hand path and had more variable left-right directions of hand paths at a fixed point in front of the sternum. In all cerebellar patients, ball left-right inaccuracy was related both to timing of ball release and to hand path direction at the fixed point. The cause of the increased variability in hand path direction varied between patients and could not be explained by disorder in a single joint rotation. No evidence was found that it resulted from variable stabilization at the shoulder during elbow extension. Instead, the more variable left-right direction of the hand path was related to the initial pattern of joint rotations occurring early in the throw before the onset of elbow extension, and to the amplitudes of radioulnar pronation and wrist abduction occurring late in the throw. The results emphasize that in the presence of a cerebellar lesion, ball left-right inaccuracy in overarm throws cannot be explained by a single disorder. Rather ball inaccuracy was likely due to disorders in central commands to proximal joint rotations that produced the hand path and in central commands to distal joints that controlled the timing of finger opening.     

Central programming of postural movements: adaptation to altered support-surface configurations

We studied the extent to which automatic postural actions in standing human subjects are organized by a limited repertoire of central motor programs. Subjects stood on support surfaces of various lengths, which forced them to adopt different postural movement strategies to compensate for the same external perturbations. We assessed whether a continuum or a limited set of muscle activation patterns was used to produce different movement patterns and the extent to which movement patterns were influenced by prior experience. Exposing subjects standing on a normal support surface to brief forward and backward horizontal surface perturbations elicited relatively stereotyped patterns of leg and trunk muscle activation with 73- to 110-ms latencies. Activity began in the ankle joint muscles and then radiated in sequence to thigh and then trunk muscles on the same dorsal or ventral aspect of the body. This activation pattern exerted compensatory torques about the ankle joints, which restored equilibrium by moving the body center of mass forward or backward. This pattern has been termed the ankle strategy because it restores equilibrium by moving the body primarily around the ankle joints. To successfully maintain balance while standing on a support surface short in relation to foot length, subjects activated leg and trunk muscles at similar latencies but organized the activity differently. The trunk and thigh muscles antagonistic to those used in the ankle strategy were activated in the opposite proximal-to-distal sequence, whereas the ankle muscles were generally unresponsive. This activation pattern produced a compensatory horizontal shear force against the support surface but little, if any, ankle torque. This pattern has been termed the hip strategy, because the resulting motion is focused primarily about the hip joints. Exposing subjects to horizontal surface perturbations while standing on support surfaces intermediate in length between the shortest and longest elicited more complex postural movements and associated muscle activation patterns that resembled ankle and hip strategies combined in different temporal relations. These complex postural movements were executed with combinations of torque and horizontal shear forces and motions of ankle and hip joints. During the first 5-20 practice trials immediately following changes from one support surface length to another, response latencies were unchanged. The activation patterns, however, were complex and resembled the patterns observed during well-practiced stance on surfaces of intermediate lengths.(ABSTRACT TRUNCATED AT 400 WORDS)     

Failure of cerebellar patients to time finger opening precisely causes ball high-low inaccuracy in overarm throws.

We investigated the idea that the cerebellum is required for precise timing of fast skilled arm movements by studying one situation where timing precision is required, namely finger opening in overarm throwing. Specifically, we tested the hypothesis that in overarm throws made by cerebellar patients, ball high-low inaccuracy is due to disordered timing of finger opening. Six cerebellar patients and six matched control subjects were instructed to throw tennis balls at three different speeds from a seated position while angular positions in three dimensions of five arm segments were recorded at 1,000 Hz with the search-coil technique. Cerebellar patients threw more slowly than controls, were markedly less accurate, had more variable hand trajectories, and showed increased variability in the timing, amplitude, and velocity of finger opening. Ball high-low inaccuracy was not related to variability in the height or direction of the hand trajectory or to variability in finger amplitude or velocity. Instead, the cause was variable timing of finger opening and thereby ball release occurring on a flattened arc hand trajectory. The ranges of finger opening times and ball release times (timing windows) for 95% of the throws were on average four to five times longer for cerebellar patients; e.g., across subjects mean ball release timing windows for throws made under the medium-speed instruction were 11 ms for controls and 55 ms for cerebellar patients. This increased timing variability could not be explained by disorder in control of force at the fingers. Because finger opening in throwing is likely controlled by a central command, the results implicate the cerebellum in timing the central command that initiates finger opening in this fast skilled multijoint arm movement.     

搂膝拗步膝踝关节预激活与共收缩的表面肌电分析

目的 分析长期太极拳练习者进行搂膝拗步和正常行走时下肢膝、踝关节肌群预激活与共收缩的表面肌电（surface electromyography, sEMG）特征,探讨太极预防跌倒的神经肌肉控制策略。方法 采用Vicon运动捕捉系统、Kistler测力板和Noraxon表面肌电图系统同步采集搂膝拗步和正常行走时股直肌、股二头肌、胫骨前肌、外侧腓肠肌的sEMG信号和体位信息。通过股直肌和股二头肌、胫骨前肌和外侧腓肠肌两对肌肉的积分肌电分别计算膝、踝关节预激活和共收缩。结果 与正常行走相比,搂膝拗步在4个阶段的平均用时显著增加;搂膝拗步在4个阶段内时间百分比存在显著性差异;搂膝拗步膝关节共收缩水平和预激活水平降低,踝关节共收缩水平和预激活水平升高。结论 长期的太极拳练习可能使膝关节周围肌肉的激活水平提高,增强肌肉群之间的协同作用,以帮助稳定关节。研究结果为神经肌肉控制障碍疾病的康复评估和训练提供参考。     

Modulation of coordinated muscle activity during imposed sinusoidal hip movements in human spinal cord injury

Individuals with chronic spinal cord injury (SCI) often demonstrate multijoint reflex activity that is clinically classified as an extensor spasm. These responses are commonly observed in conjunction with an imposed extension movement of the hips, such as movement from a sit to a supine position. Coincidentally, afferent feedback from hip proprioceptors has also been implicated in the control of locomotion in the spinalized cat. Because of this concurrence, we postulated that extensor spasms that are triggered by hip extension might involve activation of organized interneuronal circuits that also have a role in locomotion. If true, imposed oscillations of the hip would be expected to produce activity of the leg musculature in a locomotor pattern. Furthermore, this muscle activity would be entrained to the hip movement. The right hip joints of 10 individuals with chronic SCI, consisting of both complete [American Spinal Injury Association (ASIA) A] and incomplete (ASIA B,C) injuries, were subjected to ramp and hold (10 s) movements at 60 degrees /s and sinusoidal oscillations at 1.2, 1.88, and 2.2 rad/s over ranges from 40 to -15 degrees (+/-5 degrees ) using a custom servomotor system. Surface EMG from seven lower extremity muscles and sagittal-plane joint torques were recorded to characterize the response. Ramp and hold perturbations produced coactivation at the hip, knee, and ankle joints, with a long duration (5-10 s). Sinusoidal perturbations yielded consistent muscle timing patterns that resulted in alternating flexor and extensor joint torques. EMG and joint torques were commonly entrained to the frequency of movement, with rectus femoris, vastus medialis, and soleus activity coinciding with hip extension and medial hamstrings activity occurring during hip flexion. Individual muscle timing patterns were consistent with hip position during normal gait, except for the vastus medialis. These results suggest that reflexes associated with extensor spasms may occur through organized interneuronal pathways, such as spinal centers for locomotion.     

Axial synergies during human upper trunk bending

Upper trunk bending movements were accompanied by opposite movements of the lower body segments. These axial kinematic synergies maintained equilibrium during the movement performance by stabilizing the center of gravity (CG), which shifted on average across all the subjects by 1±4 cm in the anteroposterior direction and thus always remained within the support area. The aim of the present investigation was to provide an insight into the central control responsible for the performance of these synergies. The kinematic analysis was performed by the method of principal components (PC) analysis applied to the covariation between ankle, knee and hip joint angles and compared with CG shifts during upper trunk bending. Subjects were asked to perform backward or forward upper trunk bending in response to a tone. They were instructed to move as fast as possible or slowly (2 s), with high or low movement amplitudes. PC analysis showed a strong correlation between hip, knee and ankle joint changes. The first principal component (PC1) representing a multijoint movement with fixed ratios between joint angular changes, accounted, on average, for 99.7%±0.2% of the total angular variance in the forward trunk movements and for 98.4%±1.4% in the backward movements. The instructed voluntary regulation of the amplitude and velocity of the movement was achieved by adapting the bell-shaped profile of the velocity time course without changes in interjoint angular relations. Fixed ratios between changes in joint angles, represented by PC1, ensured localization of the CG within the support area during trunk bending. The ratios given by PC1 showed highly significant dependence on subjects, suggesting the adaptability of the central control to each subject’s biomechanical peculiarities. Subject’s intertrial variability of PC1 ratios was small, suggesting a stereotyped automatic interjoint coordination. When changing velocity and amplitude of the movement, the ratios remained the same in about half the subjects while in others slight variations were observed. A weak second principal component (PC2) was shown only for fast movements. In forward movements PC2 reflected the early knee flexion that seems related to the disturbances caused by the passive interaction between body segments, rather than to the effect of a central command. In fast backward movements, PC2 reflected the delay in hip extension relative to the movement onset in the ankle and knee that mirrors intersubject differences in the initiation process of the axial synergy. The results suggest that PC1 reflects the centrally controlled multijoint movement, defining the time course and amplitude of the movement and fixing the ratios between changes in joint angles. They support the hypothesis that the axial kinematic synergies result from a central automatic control that stabilizes the CG shift in the anteroposterior direction while performing the upper trunk bending. Received: 8 August 1996 / Accepted: 7 July 1997     

Accuracy of classifying the movement strategy in the functional reach test using a markerless motion capture system

The purpose of this study was to examine the accuracy of classifying the movement strategy in the functional reach test (FRT) using a markerless motion capture system (MLS) on the basis of values acquired with a marker-based motion capture system (MBS). Sixty young, injury-free individuals participated in this study. The task action involved reaching forward in the standing position. Using the Microsoft Kinect v2 as an MLS and Vicon as a MBS, the coordinates of the hip joints, knee joints and ankle joints were measured. The hip and ankle joint angles during the task were calculated from the coordinate data. These angles between MLS and MBS were compared using a paired t-test. The accuracy of movement strategy defined using MLS was examined based on the MBS. A t-test showed a significant difference in both the hip and ankle joint angles between systems (p?<?.01). However, in case of using data of left ankle joint, indices of the classification accuracy of MLS were 0.825 for sensitivity, 1.000 for specificity, infinity for positive likelihood ratio and 0.175 for negative likelihood ratio. The results for the right joint angle were similar to those of the left joint angle. Although the absolute measures in the hip and joint angles obtained using MLS differ from MBS, the MLS may be useful for accurately classifying the movement strategy adopted in the FRT.     

基于自适应CPG的人体节律运动协同特征刻画

人体柔顺自然的节律运动是全身各关节协同有序转动的展现,关节间的耦合时序与动态转动特性内蕴着人体节律运动中各肢体间的协同关系。招募20名年轻健康被试(男女各半)进行行走与跳绳实验,采集运动时主要关节的角度数据;引入自适应Hopf振荡器参数辨识模型并结合关节协同相位分布,针对人体节律运动协同特征的刻画问题进行研究。通过设立关节相位基准点,解算人体关节间耦合转动时序,基于自适应Hopf振荡器,建立关节单元参数辨识模型,获取复杂关节动态转动规律的刻画参数,进而利用中枢模式发生器(CPG)网络的相位耦合特性,重构完整的人体节律协同运动,并对重构结果的准确性进行量化分析。结果表明,基于刻画参数还原的人体节律运动姿态规范,关节重构轨迹与实际数据变化规律高度一致,两者间的相关系数高于0.99,最大平均误差低于0.01 rad,最大误差小于0.03 rad,阈值绝对偏差在4%以下。所提出的关节转动时序计算准则和自适应关节单元参数辨识方法,可准确刻画人体节律运动中的关节耦合特性与协同规律。     

Asymmetric knee loading at heel contact during walking in patients with unilateral knee replacement

Increased load on the knee joint by excessive levels of impact forces during initial contact has been suggested to lead to knee osteoarthritis (OA). Asymmetric loading after knee replacement may also relate to the development of OA in the contralateral limb, therefore this study investigated the heel strike transient vertical force and subsequent lower extremity kinematic, kinetic and spatio-temporal parameters during level walking between the operated and the contralateral limbs in patients 12 months following unilateral knee replacement. A six camera motion analysis system with a force plate was used to investigate the differences between limbs in the heel strike transient vertical GRF and its relative timing, and hip, knee and ankle angles and moments at initial contact, as well as spatio-temporal parameters during the stance phase of walking in 19 subjects with unilateral knee replacement. Paired t tests showed a significant difference in the contralateral limb relative to the operated limb in the heel strike transient magnitude (p=0.03), hip moment (p=0.01), knee moment (p=0.02) and ankle moment (p=0.03). No significant differences were found for the joint angles at heel contact or the spatio-temporal parameters (p>0.05). The heel strike transient magnitude was lower for the operated limb with no differences in the spatio-temporal parameters or the joint angles at initial contact between the limbs. Differences in the hip, knee and ankle moments were also found indicating an asymmetric loading of the impact force at initial contact on the lower extremity. The current findings may indicate an asymmetric loading on the knee joint and therefore may be clinically relevant in patients undergoing unilateral knee replacement.     

Dosimetric characteristics of wedges mounted beyond the blocking tray.

In a beam accessory configuration for a linear accelerator using a prototype multileaf collimator, newly designed wedges were mounted beyond the blocking tray. The isodose curves, depth of maximum dose, surface dose, and wedge transmission factors were measured for the wedges designed for this unique configuration. The same set of wedges was used for both 6- and 18-MV x rays. The shape of the wedged isodose curves was essentially unchanged from those produced by the conventional wedges located above the blocking tray. The isodose curves exhibited the desired wedge angles over the range of field sizes from 5 x 5 to 15 x 40 cm. In the 10 x 10-cm field, the average difference between the observed wedge angle and the desired wedge angle was 3.8 degrees. The surface doses ranged from 18% to 35% for the wedged 10 x 10-cm fields as compared with about 15% for the same open field. Dosimetrically the wedges were acceptable for clinical use.     

Compensation for interaction torques during single- and multijoint limb movement

During multijoint limb movements such as reaching, rotational forces arise at one joint due to the motions of limb segments about other joints. We report the results of three experiments in which we assessed the extent to which control signals to muscles are adjusted to counteract these "interaction torques." Human subjects performed single- and multijoint pointing movements involving shoulder and elbow motion, and movement parameters related to the magnitude and direction of interaction torques were manipulated systematically. We examined electromyographic (EMG) activity of shoulder and elbow muscles and, specifically, the relationship between EMG activity and joint interaction torque. A first set of experiments examined single-joint movements. During both single-joint elbow (experiment 1) and shoulder (experiment 2) movements, phasic EMG activity was observed in muscles spanning the stationary joint (shoulder muscles in experiment 1 and elbow muscles in experiment 2). This muscle activity preceded movement and varied in amplitude with the magnitude of upcoming interaction torque (the load resulting from motion of the nonstationary limb segment). In a third experiment, subjects performed multijoint movements involving simultaneous motion at the shoulder and elbow. Movement amplitude and velocity at one joint were held constant, while the direction of movement about the other joint was varied. When the direction of elbow motion was varied (flexion vs. extension) and shoulder kinematics were held constant, EMG activity in shoulder muscles varied depending on the direction of elbow motion (and hence the sign of the interaction torque arising at the shoulder). Similarly, EMG activity in elbow muscles varied depending on the direction of shoulder motion for movements in which elbow kinematics were held constant. The results from all three experiments support the idea that central control signals to muscles are adjusted, in a predictive manner, to compensate for interaction torques-loads arising at one joint that depend on motion about other joints.     

Adaptation of the walking pattern to uphill walking in normal and spinal-cord injured subjects

 Lower-limb movements and muscle-activity patterns were assessed from seven normal and seven ambulatory subjects with incomplete spinal-cord injury (SCI) during level and uphill treadmill walking (5, 10 and 15°). Increasing the treadmill grade from 0° to 15° induced an increasingly flexed posture of the hip, knee and ankle during initial contact in all normal subjects, resulting in a larger excursion throughout stance. This adaptation process actually began in mid-swing with a graded increase in hip flexion and ankle dorsiflexion as well as a gradual decrease in knee extension. In SCI subjects, a similar trend was found at the hip joint for both swing and stance phases, whereas the knee angle showed very limited changes and the ankle angle showed large variations with grade throughout the walking cycle. A distinct coordination pattern between the hip and knee was observed in normal subjects, but not in SCI subjects during level walking. The same coordination pattern was preserved in all normal subjects and in five of seven SCI subjects during uphill walking. The duration of electromyographic (EMG) activity of thigh muscles was progressively increased during uphill walking, whereas no significant changes occurred in leg muscles. In SCI subjects, EMG durations of both thigh and leg muscles, which were already active throughout stance during level walking, were not significantly affected by uphill walking. The peak amplitude of EMG activity of the vastus lateralis, medial hamstrings, soleus, medial gastrocnemius and tibialis anterior was progressively increased during uphill walking in normal subjects. In SCI subjects, the peak amplitude of EMG activity of the medial hamstrings was adapted in a similar fashion, whereas the vastus lateralis, soleus and medial gastrocnemius showed very limited adaptation during uphill walking. We conclude that SCI subjects can adapt to uphill treadmill walking within certain limits, but they use different strategies to adapt to the changing locomotor demands. Received: 10 March 1998 / Accepted: 29 December 1998     

老年人下楼梯下肢关节做功模式分析

目的 探究下楼梯行走过程中老龄化对老年人下肢关节做功模式的影响,丰富楼梯行走的防跌倒理论。 方法 采用 Vicon 红外运动捕捉系统和 Kistler 三维测力台同步采集青年人和老年人下楼梯行走的运动学和动力学数据,利用下肢关节角度、力矩、功率、做功贡献度指标对下肢关节做功模式进行量化评定。 结果 下楼梯过程中,青 年组和老年组下肢三关节角度、力矩、功率的变化趋势一致。 在优势腿的 1 个支撑相内,老年人的屈髋力矩峰值、伸膝力矩第 1 峰值、第 2 峰值、跖屈力矩第 1 峰值、膝负功率第 1 峰值、第 2 峰值、踝负功率峰值以及髋、膝、踝关节净功均显著降低(P<0. 05);伸髋力矩峰值、髋负功率峰值、踝关节做功贡献度显著增加(P<0. 05),髋、膝关节做功贡献度并未出现显著性差异(P>0. 05)。 结论 在下楼梯过程中,老年人下肢关节力学特征显著降低。 老年人采取不同于青年人的下肢关节做功模式。 老年人通过较大的伸髋姿势抵制躯干的过度前倾,同时采取踝关节做功的代偿模式,提高下楼梯行走的身体稳定性。 建议老年人在锻炼时应以增加膝、踝关节肌肉力量的项目为主,以维持下楼梯的姿势控制能力。