Flexion Phase Research Articles

Finger inter-dependence: Linking the kinetic and kinematic variables

We studied the dependence between voluntary motion of a finger and pressing forces produced by the tips of other fingers of the hand. Participants moved one of the fingers (task finger) of the right hand trying to follow a cyclic, ramp-like flexion-extension template at different frequencies. The other fingers (slave fingers) were restricted from moving; their flexion forces were recorded and analyzed. Index finger motion caused the smallest force production by the slave fingers. Larger forces were produced by the neighbors of the task finger; these forces showed strong modulation over the range of motion of the task finger. The enslaved forces were higher during the flexion phase of the movement cycle as compared to the extension phase. The index of enslaving expressed in N/rad was higher when the task finger moved through the more flexed postures. The dependence of enslaving on both range and direction of task finger motion poses problems for methods of analysis of finger coordination based on an assumption of universal matrices of finger interdependence.

B18 足関節の機械的パワー・下肢筋出力併用計測による陸上短距離用スパイクシューズの評価(陸上競技)

The purpose of this study was to evaluate the spike shoes for the sprint race by the parallel measurement of mechanical power of foot joint and the lower limbs muscular strength. To perform this purpose, the system for the parallel measurement of mechanical power of foot joint and the lower limbs muscular strength was developed. Furthermore, the mechanical power of subject's foot joint in wearing the high performance spike, the general competition spike and the running shoes was measured in the conditions of the plantar flexion jump (PFJ ), the counter movement jump (CMJ) and the drop jump (DJ) using the sliding seat. The experiment led us conclusions as follows; the mechanical power of the high performance spike in DJ was the highest of experimental shoes. The increasing ratio from CMJ to DJ of the high performance spike was the highest of these, too. The iEMG in wearing the high performance shoes in DJ was the lowest. The high performance spike could be output the high mechanical power in the phase of the plantar flexion.

Helical axes of skeletal knee joint motion during running

The purpose of this study was to determine the changes in the axis of rotation of the knee that occur during the stance phase of running. Using intracortical pins, the three-dimensional skeletal kinematics of three subjects were measured during the stance phase of five running trials. The stance phase was divided into equal motion increments for which the position and orientation of the finite helical axes (FHA) were calculated relative to a tibial reference frame. Results were consistent within and between subjects. At the beginning of stance, the FHA was located at the midepicondylar point and during the flexion phase moved 20mm posteriorly and 10mm distally. At the time of peak flexion, the FHA shifted rapidly by about 10–20mm in proximal and posterior direction. The angle between the FHA and the tibial transverse plane increased gradually during flexion, to about 15° of medial inclination, and then returned to zero at the start of the extension phase. These changes in position and orientation of FHA in the knee should be considered in analyses of muscle function during human movement, which require moment arms to be defined relative to a functional rotation axis. The finding that substantial changes in axis of rotation occurred independent of flexion angle suggests that musculoskeletal models must have more than one kinematic degree-of-freedom at the knee. The same applies to the design of knee prostheses, if the goal is to restore normal muscle function.

Effects of transcutaneous electrical stimulation combined with locomotion-like movement in the treatment of post-stroke gait disorder: a single-case study. Short report

Purpose. This study was designed to examine the effects of electrical stimulation combined with locomotion-like movement (ES/LM) for improving gait disorder in a stroke patient.Method. A four-phase ABAB single-subject design with five therapy sessions per phase was employed. In the intervention phases, transcutaneous electrical stimulation was applied to the tibialis anterior (at the end of the hip extension phase and in the initial hip flexion phase) and the soleus (in the initial hip extension phase) during passive hip flexion and extension. To assess improvement, the soleus H-reflex and the ambulatory function were measured (gait velocity and step length).Results. Application of ES/LM resulted in a decrease of the soleus H-reflex and significant increase of gait velocity and step length.Conclusion. These findings suggest that ES/LM is a feasible treatment method for impaired ambulatory function in stroke patients at the subacute stage after the event.

Differences in pedal forces during recumbent cycling in adolescents with and without cerebral palsy

Background We showed that subjects with cerebral palsy had greater transverse and frontal plane hip and knee motion, increased duration of muscle activity, increased cocontraction, and decreased efficiency during recumbent cycling than subjects with typical development. However, it is also important to understand the forces exerted on the pedals. The purpose of this report was to compare pedal forces during cycling between adolescents with and without cerebral palsy. Methods Ten subjects (3 male, 7 female) with spastic diplegic or quadriplegic cerebral palsy (15.6 years, SD 1.8) and 10 subjects (3 male, 7 female) with typical development (14.9 years, SD 1.4) cycled on a stationary recumbent cycle at 30 and 60 revolutions per minute if able. Three-dimensional piezoelectric force transducers measured pedal forces. Data were analyzed using two-way ANOVAs. Findings Subjects with cerebral palsy spent a smaller percentage ( P < .001, r 2 = .09, power = 1.0) of the revolution applying positive force (pushing into the pedal during the extension phase) and a greater percentage ( P < .001, r 2 = .09, power = 1.0) of the revolution applying negative force (pulling away from the pedal during the flexion phase). There was no effect of cadence and no interaction effect. Interpretation These findings compliment our earlier findings of altered joint kinematics and muscle activity indicating that subjects with cerebral palsy and typical development have different cycling strategies. Methods to increase the duration of the positive force may allow subjects with CP to cycle more successfully and cycle vigorously enough to reach a heart rate necessary for improving fitness.

Transformation of the kinematic characteristics of a precise movement after a change in a spatial task

The central mechanism of motor programming was studied using a model of precise horizontal flexion of the arm at the elbow joint. Training was performed in the dark to ensure that formation of the motor program was based predominantly on the use of proprioceptive afferentation. The target was not demonstrated before training: subjects determined the angle of arm flexion during training, the moment at which the target position was reached being identified by a brief LED flash. Subjects had to perform the movement as quickly and accurately as possible. The amplitude, speed, and accuracy of the movement were measured in real time. The ten subjects were divided into two groups for initial training to precise movements, using two different protocols: flexion of the elbow to 70 degrees and to 55 degrees . At the second stage of the experiment, each subject's initial target position was suddenly changed (from 70 degrees to 55 degrees and vice versa). Training was continued until a stable accuracy in the new conditions was achieved (with errors of no more than 5% of the specified amplitude). The nature of the transformation in the kinematics of the precise movement in response to the change in the single task parameter illuminated the fundamental principle of organization of the supraspinal motor command for movements of this type. For both specified flexion amplitudes, the ratio between the acceleration and deceleration phases of the movement were identical during the period of skill fixation. On average, 70% of the total amplitude of flexion was accounted for by the acceleration phase and 30% by the deceleration phase. Adaptation of the precise movement to changes in the specified horizontal elbow flexion angle (i.e., re-achievement of the required movement accuracy in the changed conditions) during rearrangement was completed by inversion of these values. According to the results of previous studies, the most informative measure for analysis of the dynamics of the controlling central command was the acceleration of the movement. In terms of current concepts of the mechanism of motor control, the acceleration plateau can be regarded as mirroring long-term depression--the voltage plateau in Purkinje cells and motoneurons. Data processing involved calculation of the integral acceleration in both phases of the movement in relation to the angle of flexion (phase plots). These data underlie our understanding of the mechanism of transformation of movement kinematics responsible for the formation of a new central command.

오래 달리기 시 하지 근지구력에 따른 몸통의 3차원 각 변화

The purpose of this study was to investigate the influence of the muscular endurance on the kinematic factors during a prolonged run. Subjects, 12 males, who were divided into three groups(lower group, general group, and in higher group) after measuring the lower limb's muscular endurance previously. They were asked toe run on the treadmill at 7.4km/h of speed. To analyze the kinematics parameters of the trunk during running, the ProReflex MCU Camera(Qualisys, Sweden) were used. All parameters were sampled from 5 minute, 40 minute, and 60 minute moments during running. An ANOVA with Repeated Measure was used to test the statistic significance between and within groups for all parameters determined with SPSS 11.0. Significance was defined as p<.05. The conclusions were as follows; There was significantly difference within(lapse of running time) groups in the take-off and minimum knee angle event of swing phase of the trunk flexion and extension. In conclusion, the muscular endurance affected on movement of the trunk during a prolonged run. In addition, it showed that there was significant difference in the energy consumption by lapse of running time. Therefore, it seems to be relationships between the muscular endurance and running efficiency.

Fatigue-Induced Changes in Phasic Muscle Activation Patterns During Dynamic Trunk Extension Exercise

To investigate the influence of fatigue on phasic muscle-activation patterns during dynamic trunk extension exercise. Fifteen healthy volunteers performed dynamic trunk-extension exercise through a 30-degree range-of-motion (ROM) exercise to task failure at an intensity of 50% of maximum. Electromyography (EMG) signals were recorded unilaterally from the lumbar extensor, gluteus maximus, and biceps femoris muscles, and signal amplitude was analyzed in 10-degree increments during the unfatigued and fatigued states (0-10 degrees from torso horizontal to the ground was considered extension, and 11-20 and 21-30 degrees of flexion relative to this were considered midphase and flexion, respectively). Lumbar extensor EMG was approximately 75% of maximum EMG, with no differences being observed with respect to ROM or fatigue state. The gluteus maximus demonstrated an altered phasic activation pattern with fatigue, with an increased recruitment during the extension phase (fatigued-state extension-phase EMG: 89.1 +/- 8.3% > flexion phase EMG: 37.8% +/- 9.1%). The biceps femoris demonstrated a similar response during both the fatigued and unfatigued states (fatigued-state extension EMG: 77.8 +/- 5.4% > midphase EMG: 65.8 +/- 5.7% > flexion EMG: 46.8 +/- 4.0%; unfatigued-state extension EMG: 46.1 +/- 3.7% > flexion EMG: 27.1 +/- 2.6%). During this exercise, as one moves from flexion to extension, hip extensor muscle activity increases, whereas lumbar extensor activity does not. Additionally, fatigue results in an altered recruitment pattern, with the hip extensors being activated to a greater extent in the extension phase. These findings suggest that when this exercise is performed in the prone position, it can be used to stimulate the lumbar and hip extensor muscles, but the specific exercise protocol in terms of set/repetition number and ROM will influence which muscles are primarily targeted.

Lumbar–pelvic range and coordination during lifting tasks

Spine motion has been described to have two regions, a neutral zone where lumbar rotation can occur with little resistance and an elastic zone where structures such as ligaments, facet joints and intervertebral disks resist rotation. In vivo, the passive musculature can contribute to further limiting the functional neutral range of lumbar motion. Movement out of this functional neutral range could potentially put greater loads on these structures. In this study, the range of lumbar curvature rotation was examined in twelve healthy, untrained volunteers at four torso inclination angles. The lumbar curvature during straight-leg lifting tasks was then defined as a percentage of this range of possible lumbar curvatures. Subjects were found to remain neutrally oriented during the flexion phase of a lifting task. During the extension phase of the lifting task, however, subjects were found to assume a more kyphotic posture, approaching the edge of the functional range of motion. This was found to be most pronounced for heavy lifting tasks. By allowing the lumbar curvature to go into a highly kyphotic posture, subjects may be taking advantage of stretch-shortening behavior in extensor musculature and associated tendons to reduce the energy required to raise the torso. Such a kyphotic posture during extension, however, may put excessive loading on the elastic structures of the spine and torso musculature increasing the risk of injury.

Cerebellar activation during discrete and not continuous timed movements: An fMRI study

Individuals with cerebellar lesions are impaired in the timing of repetitive movements that involve the concatenation of discrete events such as tapping a finger. In contrast, these individuals perform comparably to controls when producing continuous repetitive movements. Based on this, we have proposed that the cerebellum plays a key role in event timing—the representation of the temporal relationship between salient events related to the movement (e.g., flexion onset or contact with a response surface). In the current study, we used fMRI to examine cerebellar activity during discrete and continuous rhythmic movements. Participants produced rhythmic movements with the index finger either making smooth, continuous transitions between flexion and extension or with a pause inserted before each flexion phase making the movement discrete. Lateral regions in lobule VI, ipsilateral to the moving hand were activated in a similar manner for both conditions. However, activation in the superior vermis was significantly greater when the movements were discrete compared to when the movements were continuous. This pattern was not evident in cortical regions within the field of view, including M1 and SMA. The results are consistent with the hypothesis that subregions of the cerebellum are selectively engaged during tasks involving event timing.

Height of Chair Seat and Movement Characteristics in Sit-to-Stand by Young and Elderly Adults

The height of a chair seat affects the burden on the lower limbs during the sit-to-stand (STS). To develop an objective test to evaluate muscle function of the lower limbs using floor reaction force during a STS, the relationship between chair-seat height and the burden on the lower legs must be assessed. To examine the influence of the chair-seat height, floor reaction forces during a STS performed with 5 chair heights adjusted to each subject's lower leg length were compared. The force production and quickness of movement tended to decrease in the phases of trunk flexion and knee and hip joint extension when performing a STS from a lower chair, when the chair height differed by 20% (6.2 cm) from the lower leg length, and was marked when the difference between chair height and lower leg length became larger. In 52 elderly and 50 young adults, floor reaction forces during a STS performed from a chair of the same height as subjects' lower leg lengths were compared. Elderly persons were inferior in force production (strength) and quickness of movement, which decreased as elderly stood up from a chair of a lower height.

Validity of Muscle Force Estimation Utilizing Musculoskeletal Model

Few methods are available for measurement of muscle strength during physical activities. This study addresses the validity of estimating the force of the quadriceps femoris during the forward lunge and squat carried out by 18 healthy men. Data were collected by the utilization of a musculoskeletal model through computer simulation. From the electromyographic (EMG) recordings of the vasti medialis (VM) and lateralis (VL) and rectus femoris (RF) muscles a correlation coefficient (r) was calculated between the estimated value for the muscle force and the root mean square (RMS) value. The median r for VM and VL was found to exceed 0.65 during the flexion and extension phases of forward lunge and below 0.44 for VM and VL during squat for the same phases. For RF, the median r for both tasks was below 0.39 except during the flexion phase of squat. The coefficient of the similarity in the EMG patterns exceeded 0.93 between the estimated muscle force and EMG RMS for both the forward lunge and squat. The value of the estimated muscle force for RF in these 2 tasks was found to be very small. The results show that the musculoskeletal model can be considered a valid measure for estimating muscle force.

Hip Adduction Does not Affect VMO EMG Amplitude or VMO:VL Ratios during a Dynamic Squat Exercise

Context:Clinicians commonly attempt to facilitate vastus medialis oblique (VMO) activity by instructing patients to squeeze a ball between their knees during squatting exercises.Objective:To determine whether VMO activation amplitude and the VMO to vastus lateralis (VL) activation ratio (VMO:VL) were altered when performing active hip adduction during a dynamic squat exercise.Design:Single test session.Participants:Fifteen healthy subjects, with no history of knee pain, volunteered for this study.Intervention:Surface EMG of the VMO, VL, and hip adductor (ADD) muscles were recorded while subjects performed 10 consecutive squats against their body weight through a range of 0° to 90° of knee flexion. Subjects performed the squat exercises during two different conditions: (1) active hip adduction and (2) no hip adduction.Main Outcome Measures:Average VMO EMG amplitude and VMO:VL ratio were determined during the knee flexion (0° to 90°) and knee extension (90° to 0°) phases of the squat exercise.Results:Active hip adduction did not significantly change VMO amplitude or VMO:VL ratio during the knee flexion or knee extension phases of the dynamic squat exercise.Conclusions:Based on these results, we conclude that VMO amplitude and the VMO: VL ratio are not influenced by performing active hip adduction during a dynamic squat exercise in healthy subjects.

Muscle Fatigue Alters Neuromuscular Performance during Resisted Weight Bearing Exercise

Neuromuscular control is essential to the maintenance of lower extremity stability during functional activities. Numerous studies suggest that muscle fatigue contributes to deterioration of neuromuscular performance and may increase the risk of injury. However, limited information is currently available regarding muscle synergistic patterns and triggered reflex responses during dynamic weight bearing activities in the presence of muscle fatigue. PURPOSE: To examine the effect of muscle fatigue on 1) voluntary and reflexive patterns of muscle activation and 2) performance accuracy while tracking a visual target during the weight bearing lateral step down exercise. METHODS: A specially designed apparatus was used to provide controlled resistance to knee motion during the lateral step down exercise. Motion of the knee was measured as subjects were asked to track a sinusoidal target waveform as accurately as possible during the exercise. Absolute, constant, variable, and endpoint errors were calculated to evaluate tracking performance. Random perturbations were delivered in 20% of the trials by unexpectedly releasing the resistance during the flexion phase of the exercise. Quadriceps and hamstring muscle activity was recorded during both perturbed and unperturbed trials. Tests were conducted in a group of 12 healthy females before and after completing a repetitive submaximal eccentric quadriceps fatigue protocol. A second group of 12 females served as controls. RESULTS: Muscle fatigue increased the level of quadriceps muscle activity throughout most of the exercise. In the flexion phase, average rectus femoris (RF) and vastus lateralis (VL) EMG increased by 2.5% (p=.008) and 3.7% (p=.002) MVIC, respectively. In the extension phase, RF activity increased by 5.5% MVIC (p < .001), while VL activity increased by 8.5% MVIC (p < .001). Muscle fatigue also resulted in a reversal in the direction of the constant errors during the extension phase of the exercise, causing subjects to lag behind the target. Unexpected perturbations elicited long latency responses characterized by facilitation of the quadriceps and inhibition of the hamstrings. Muscle fatigue increased the amplitude of this response in VL by 4.3% MVIC (p=.004). Despite the increased amplitude of the response, errors at the endpoint of the flexion phase increased by .67 cm (p=.034) after fatigue. CONCLUSION: These results indicate that muscle fatigue alters patterns of coordinated muscle activity, which may render subjects less able to cope with unexpected perturbations during activities that require a high degree of accuracy.

Interaction of Gender and Type of Landing on Landing Mechanics

Gender differences during landings have been suggested to provide a rationale for the greater number of knee injuries sustained by female athletes. Several studies have reported gender differences in landing mechanics, but no standardized protocol to examine the jump landing has been established. While all jump landings may exhibit some similar characteristics it may be that observed gender differences in lower extremity mechanics vary depending on the type of landing protocol used. PURPOSe: To evaluate gender differences in landing mechanics under two different types of landings. METHODS: Thirteen male (M) and female (F) varsity intercollegiate soccer players were selected to participate in the study. Kinematic and kinetic data were collected using a Flock of Birds motion analysis system with a Bertec force platform during a jumping task. For the task, individuals jumped off a 20cm platform that was placed half of the subj ect's height from the center of the force platform. Upon landing on the force platform (T1) the subject performed a counter movement jump and was instructed to jump as high as possible and then land on the force platform (T2). The data were processed using Motion Monitor software and custom Lab View programs were developed to identify the variables of interest. Sagittal plane motion (peak (PK) and excursion (EXC)) at the hip (H) and knee (K) were analyzed during the eccentric phase of knee flexion during both periods on the force platform. As well, peak vertical ground reaction force (PVGRF) and time to PVGRF were examined. Gender differences between the two types of landings were analyzed using a mixed factor repeated measures ANOVA. RESULTS: F demonstrated increased HEXC (43.9 deg vs. 55.8 deg) from T1 to T2 compared to M (40.7 deg vs. 41.7 deg, p = 0.04). However, F demonstrated a smaller decrease in KPK (F= 5.2 deg, M= 12.3 deg, p=0.07) and HPK (F= +0.6 deg, M= 8.4 deg, p=0.09) than M from T1 to T2. Main effects were observed for type of landing for KEXC and PVGRF. T2 was associated with decreased KECX (82.9 v 71.5) but greater PVGRF (2.6 BW v 2.8 BW) than T1. No significant differences were observed for time to PVGRF. CONCLUSIONS: Males and females utilize different strategies at the hip and knee during different types of landings. These results may shed light on discrepancies in gender-based differences in lower extremity sagittal plane mechanics in recent literature.

Scaling and non-scaling of muscle activity, kinematics, and dynamics in sit-ups with different degrees of difficulty

The purpose of this study was to investigate how the CNS adjusts motor patterns for variants of a complex axial movement—the sit-up. Adjustments were induced by changing the support surface contact and mass distribution of the body. Healthy adults performed straight-legged sit-ups, 3 s in duration, with support added to or removed from the lumbar trunk, or with mass added to the head or to the legs. Each of these interventions either increased or decreased the difficulty of the task. The study addressed the extent to which changes in sit-up difficulty are compensated by scaling of muscle activity, kinematics, and dynamics versus the extent to which they are compensated by changing discretely the motor pattern. The analysis of muscle activity, kinematics, and dynamics focused on the first 30–40% of the sit-up—the trunk flexion phase—since this is the most critical part of the movement. Our results demonstrate that, in some respects, sit-up kinematics and dynamics scaled with difficulty, but in other respects, they did not. Muscle activity also scaled, in many respects, but in more difficult sit-ups, abdominal flexor activity decreased instead of increased. Non-scaling changes in these parameters suggest that complex movements, such as the sit-up, may require discrete changes in motor pattern in order to deal with large loads, which challenge the available leverage.

B14 筋骨格モデルを用いた推定筋張力の妥当性(筋骨格系のバイオメカニクス1)

The purpose of this study was to examine the validity of estimated muscular tension using the musculoskeletal model during forward lunge and squat. Eighteen healthy men were enrolled in this study. A correlation coefficient was calculated between the estimated value for the muscular tension and the root mean square (RMS) value for the electromyographic recordings. Electromyographic data was obtained from vasti medialis (VM) and lateralis (VL) and rectus femoris (RF). The mean values of coefficient correlation for VM and VL exceeded 0.61 during the flexion and extension phase of forward lunge. The coefficient correlation for squat fell below 0.44 for during the flexion and extension phase, and RF, it fell below 0.30 during forward lunge and squat. Similarities of the waveform patterns exceeded 0.93 between estimated muscular tension and electromyographic RMS for both forward lunge and squat. The value of the estimated muscular tension for RF for both exercises was found to be very small. In conclusion, the output pattern for the estimated muscular tension was validated, although further study is required to examine the value of muscular tension, in particularly, of a two-joint muscle such as RF.

MODULATION OF THE SOLEUS H-REFLEX DURING STATIC AND DYNAMIC IMPOSED HIP ANGLE CHANGES

The aim of the study was to investigate the modulation of the soleus H-reflex during static and dynamic imposed hip angle changes. Five healthy subjects participated. H-reflexes were measured during hip joint passively flexed and extended in the sagittal plane. In flexion phase, the soleus H-reflex during dynamic conditions was lower than the stationary controls. By contrast, it was conversely higher in extension phase. The findings suggest that the modulation of the soleus H-reflex from hip proprioceptors is a major factor in passive hip movement. Additionally, the central pattern generator might modulate the soleus H-reflex.

Contributions of the Radiocarpal and Midcarpal Jointin Flexion-extension Motion of Wrist

Purpose : Flexion and extension of the wrist occurs at the radiocarpal and midcarpal joints. This study examined the angular contribution of the radiocarpal and midcarpal joints to the total arc of motion. Materials and Methods : Five healthy adults were selected and ten wrists were tested. Lateral X-rays were taken with the wrist in the neutral position, 30˚ flexion, 60˚ flexion, 90˚ flexion, 30˚ extension, 60˚ extension, and 90˚ extension. The radiocarpal and midcarpal angle were measured. The angular contribution of the radiocarpal and midcarpal angle was calculated at each phase of motion; neutral to 30˚, 30˚ to 60˚, 60˚ to 90˚. Results : During wrist flexion, the angular contribution of the midcarpal joint was 65%, 72%, 71% at each phase of motion, respectively. During wrist extension, the angular contribution of midcarpal joint was 37%, 53%, 78% at each phase of motion, respectively. Therefore, during wrist extension, the main contributor of motion is changed from the radiocarpal joint in the early phase to the midcarpal joint in the late phase. Conclusion : It is suggested that the proximal carpal low is not moved passively by just the link system but the motion is guided by the peculiar shape of midcarpal articulation and ligaments complex at each phase. The link system is believed to act in the early phase of flexion and in late phase of extension.

INFLUENCE OF AGE ON NEUROMUSCULAR CONTROL OF THE KNEE.

Purpose/Hypothesis: An increased incidence of injury has made neuro-muscular control of the knee joint a focus of attention in motor control research. The central nervous system (CNS) controls the knee against unpredictable perturbations that occur during movement. Age associated changes in the musculoskeletal, neuromuscular and sensory systems compromise the ability of the elderly to respond effectively to a perturbation, thereby placing the older population at a higher risk of injury. Weight bearing exercises have been primarily used to re-educate neuromuscular control of the knee following pathology. Limited information is available on the strategies used by the elderly to control the knee during dynamic activities. This study examined the effect of age on the neuromuscular control of the knee by examining accuracy of performance, muscle synergies and long latency reflexes (LLR) during a functional weight bearing exercise. We hypothesized that the elderly would show greater impairment in neuro-muscular control than the young. Number of Subjects: Ten young and ten elderly subjects performed a single leg squat exercise (SLS) while tracking a sinusoidal target with knee motion at different levels of resistances (4%, 8% and 12% body weight). Materials/Methods: Subjects first performed a set of learning trials to gain proficiency with the task. Later, they performed the SLSs with random perturbations (drop in resistance level to 0% BW) during the knee flexion phase. A split plot repeated measures ANOVA was used to assess between and within group differences in movement accuracy, synergistic activation patterns (EMG), and long latency reflexes. Results: The elderly group 1) had a 50% greater absolute error in tracking the target angle when compared to the younger group (p < 0.05); 2) showed a 60% greater activation of the quadriceps and hamstrings at all levels of resistances as compared to the younger group (p < 0.05); and showed 15% greater LLR for the quadriceps and biceps femoris when compared to the younger group (p < 0.05). Conclusions: These results indicate that the elderly recruit higher percentages of MVC and have greater error when performing the SLS weight bearing exercise. Clinical Relevance: Age associated changes of the sensorimotor system necessitates greater muscle activity to help stabilize the knee in the elderly. These findings provide important clinical implications when developing rehabilitation programs for the young and elderly.