The importance of swing-phase initial conditions in stiff-knee gait

The diminished knee flexion associated with stiff-knee gait, a movement abnormality commonly observed in persons with cerebral palsy, is thought to be caused by an over-active rectus femoris muscle producing an excessive knee extension moment during the swing phase of gait. As a result, treatment for stiff-knee gait is aimed at altering swing-phase muscle function. Unfortunately, this treatment strategy does not consistently result in improved knee flexion. We believe this is because multiple factors contribute to stiff-knee gait. Specifically, we hypothesize that many individuals with stiff-knee gait exhibit diminished knee flexion not because they have an excessive knee extension moment during swing, but because they walk with insufficient knee flexion velocity at toe-off. We measured the knee flexion velocity at toe-off and computed the average knee extension moment from toe-off to peak flexion in 17 subjects (18 limbs) with stiff-knee gait and 15 subjects (15 limbs) without movement abnormalities. We used forward dynamic simulation to determine how adjusting each stiff-knee subject's knee flexion velocity at toe-off to normal levels would affect knee flexion during swing. We found that only one of the 18 stiff-knee limbs exhibited an average knee extension moment from toe-off to peak flexion that was larger than normal. However, 15 of the 18 limbs exhibited a knee flexion velocity at toe-off that was below normal. Simulating an increase in the knee flexion velocity at toe-off to normal levels resulted in a normal or greater than normal range of knee flexion for each of these limbs. These results suggest that the diminished knee flexion of many persons with stiff-knee gait may be caused by abnormally low knee flexion velocity at toe-off as opposed to excessive knee extension moments during swing.     

Muscles that influence knee flexion velocity in double support: implications for stiff-knee gait

Adequate knee flexion velocity at toe-off is important for achieving normal swing-phase knee flexion during gait. Consequently, insufficient knee flexion velocity at toe-off can contribute to stiff-knee gait, a movement abnormality in which swing-phase knee flexion is diminished. This work aims to identify the muscles that contribute to knee flexion velocity during double support in normal gait and the muscles that have the most potential to alter this velocity. This objective was achieved by perturbing the forces generated by individual muscles during double support in a forward dynamic simulation of normal gait and observing the effects of the perturbations on peak knee flexion velocity. Iliopsoas and gastrocnemius were identified as the muscles that contribute most to increasing knee flexion velocity during double support. Increased forces in vasti, rectus femoris, and soleus were found to decrease knee flexion velocity. Vasti, rectus femoris, gastrocnemius, and iliopsoas were all found to have large potentials to influence peak knee flexion velocity during double support. The results of this work indicate which muscles likely contribute to the diminished knee flexion velocity at toe-off observed in stiff-knee gait, and identify the treatment strategies that have the most potential to increase this velocity in persons with stiff-knee gait.     

Contributions of muscle forces and toe-off kinematics to peak knee flexion during the swing phase of normal gait: an induced position analysis

A three-dimensional dynamic simulation of walking was used together with induced position analysis to determine how kinematic conditions at toe-off and muscle forces following toe-off affect peak knee flexion during the swing phase of normal gait. The flexion velocity of the swing-limb knee at toe-off contributed 30 degrees to the peak knee flexion angle; this was larger than any contribution from an individual muscle or joint moment. Swing-limb muscles individually made large contributions to knee angle (i.e., as large as 22 degrees), but their actions tended to balance one another, so that the combined contribution from all swing-limb muscles was small (i.e., less than 3 degrees of flexion). The uniarticular muscles of the swing limb made contributions to knee flexion that were an order of magnitude larger than the biarticular muscles of the swing limb. The results of the induced position analysis make clear the importance of knee flexion velocity at toe-off relative to the effects of muscle forces exerted after toe-off in generating peak knee flexion angle. In addition to improving our understanding of normal gait, this study provides a basis for analyzing stiff-knee gait, a movement abnormality in which knee flexion in swing is diminished.     

Importance of preswing rectus femoris activity in stiff-knee gait

Stiff-knee gait is characterized by diminished and delayed knee flexion during swing. Rectus femoris transfer surgery, a common treatment for stiff-knee gait, is often recommended when a patient exhibits prolonged activity of the rectus femoris muscle during swing. Treatment outcomes are inconsistent, in part, due to limited understanding of the biomechanical factors contributing to stiff-knee gait. This study used a combination of gait analysis and dynamic simulation to examine how activity of the rectus femoris during swing, and prior to swing, contribute to knee flexion. A group of muscle-actuated dynamic simulations was created that accurately reproduced the gait dynamics of ten subjects with stiff-knee gait. These simulations were used to examine the effects of rectus femoris activity on knee motion by eliminating rectus femoris activity during preswing and separately during early swing. The increase in peak knee flexion by eliminating rectus femoris activity during preswing (7.5+/-3.1 degrees ) was significantly greater on average (paired t-test, p=0.035) than during early swing (4.7+/-3.6 degrees ). These results suggest that preswing rectus femoris activity is at least as influential as early swing activity in limiting the knee flexion of persons with stiff-knee gait. In evaluating rectus femoris activity for treatment of stiff-knee gait, preswing as well as early swing activity should be examined.     

Contributions of muscles and passive dynamics to swing initiation over a range of walking speeds

Stiff-knee gait is a common walking problem in cerebral palsy characterized by insufficient knee flexion during swing. To identify factors that may limit knee flexion in swing, it is necessary to understand how unimpaired subjects successfully coordinate muscles and passive dynamics (gravity and velocity-related forces) to accelerate the knee into flexion during double support, a critical phase just prior to swing that establishes the conditions for achieving sufficient knee flexion during swing. It is also necessary to understand how contributions to swing initiation change with walking speed, since patients with stiff-knee gait often walk slowly. We analyzed muscle-driven dynamic simulations of eight unimpaired subjects walking at four speeds to quantify the contributions of muscles, gravity, and velocity-related forces (i.e. Coriolis and centrifugal forces) to preswing knee flexion acceleration during double support at each speed. Analysis of the simulations revealed contributions from muscles and passive dynamics varied systematically with walking speed. Preswing knee flexion acceleration was achieved primarily by hip flexor muscles on the preswing leg with assistance from biceps femoris short head. Hip flexors on the preswing leg were primarily responsible for the increase in preswing knee flexion acceleration during double support with faster walking speed. The hip extensors and abductors on the contralateral leg and velocity-related forces opposed preswing knee flexion acceleration during double support.     

Do the hamstrings operate at increased muscle–tendon lengths and velocities after surgical lengthening?

Children with crouch gait frequently walk with improved knee extension during the terminal swing and stance phases following hamstrings lengthening surgery; however, the mechanisms responsible for these improvements are unclear. This study tested the hypothesis that surgical lengthening enables the hamstrings of persons with cerebral palsy to operate at longer muscle-tendon lengths or lengthen at faster muscle-tendon velocities during walking. Sixty-nine subjects who had improved knee extension after surgery were retrospectively examined. The muscle-tendon lengths and velocities of the subjects' semimembranosus muscles were estimated by combining kinematic data from gait analysis with a three-dimensional computer model of the lower extremity. Log-linear analyses confirmed that the subjects who walked with abnormally short muscle-tendon lengths and/or slow muscle-tendon velocities preoperatively tended to walk with longer lengths (21 of 29 subjects, p<0.01) or faster velocities (30 of 40 subjects, p<0.01) postoperatively. In these cases, surgical lengthening may have slackened the subjects' tight hamstrings and/or diminished the hamstrings' spastic response to stretch. Other subjects walked with muscle-tendon lengths and velocities that were neither shorter nor slower than normal preoperatively (22 of 69 subjects), and the semimembranosus muscles of most of these subjects did not operate at increased lengths or velocities after surgery; in these cases, the subjects' postsurgical improvements in knee extension may have been unrelated to the hamstrings surgery. Analyses of muscle-tendon lengths and velocities may help to distinguish individuals who have "short" or "spastic" hamstrings from those who do not, and thus may augment conventional methods used to describe patients' neuromusculoskeletal impairments and gait abnormalities.     

Manipulating post-stroke gait: Exploiting aberrant kinematics

Post-stroke individuals often exhibit abnormal kinematics, including increased pelvic obliquity and hip abduction coupled with reduced knee flexion. Prior examinations suggest these behaviors are expressions of abnormal cross-planar coupling of muscle activity. However, few studies have detailed the impact of gait-retraining paradigms on three-dimensional joint kinematics. In this study, a cross-tilt walking surface was examined as a novel gait-retraining construct. We hypothesized that relative to baseline walking kinematics, exposure to cross-tilt would generate significant changes in subsequent flat-walking joint kinematics during affected limb swing. Twelve post-stroke participants walked on a motorized treadmill platform during a flat-walking condition and during a 10-degree cross-tilt with affected limb up-slope, increasing toe clearance demand. Individuals completed 15 min of cross-tilt walking with intermittent flat-walking catch trials and a final washout period (5 min). For flat-walking conditions, we examined changes in pelvic obliquity, hip abduction/adduction and knee flexion kinematics at the spatiotemporal events of swing initiation and toe-off, and the kinematic event of maximum angle during swing. Pelvic obliquity significantly reduced at swing initiation and maximum obliquity in the final catch trial and late washout. Knee flexion significantly increased at swing initiation, toe-off, and maximum flexion across catch trials and late washout. Hip abduction/adduction was not significantly influenced following cross-tilt walking. Significant decrease in the rectus femoris and medial hamstrings muscle activity across catch trials and late washout was observed. Exploiting the abnormal features of post-stroke gait during retraining yielded desirable changes in muscular and kinematic patterns post-training.     

Changes in electromyographic activity after botulinum toxin injection of the rectus femoris in patients with hemiparesis walking with a stiff-knee gait

PurposeThis study was designed to evaluate the effects of botulinum toxin type-A (BoNTA) injection of the rectus femoris (RF) muscle on the electromyographic activity of the knee flexor and extensor and on knee and hip kinematics during gait in patients with hemiparesis exhibiting a stiff-knee gait.MethodTwo gait analyses were performed on fourteen patients: before and four weeks after BoNTA injection. Spatiotemporal, kinematic and electromyographic parameters were quantified for the paretic limb.ResultsBoNTA treatment improved gait velocity, stride length and cadence with an increase of knee angular velocity at toe-off and maximal knee flexion in the swing phase. Amplitude and activation time of the RF and co-activation duration between the RF and biceps femoris were significantly decreased. The instantaneous mean frequency of RF was predominantly lower in the pre-swing phase.ConclusionsThe results clearly show that BoNTA modified the EMG amplitude and frequency of the injected muscle (RF) but not of the synergist and antagonist muscles. The reduction in RF activation frequency could be related to increased activity of slow fibers. The frequency analysis of EMG signals during gait appears to be a relevant method for the evaluation of the effects of BoNTA in the injected muscle.     

Gait modification when decreasing double support percentage

Much is still unknown about walking stability, including which aspects of gait contribute to higher stability. Walking stability appears to be related to walking speed, although the exact relationship is unclear. As walking speed decreases, the double support (DS) period of gait increases both in time and as a percentage of the gait cycle. Because humans have more control over their center of mass movement during DS, increasing DS duration may alter stability. This study examined how human gait is affected by changing DS percentage independent of walking speed. Sixteen young, healthy adults walked on a treadmill at a single speed for six one-minute trials. These trials included normal gait as well as longer- and shorter-than-normal DS percentage gaits. Subjects were consistently able to decrease DS percentage but had difficulty increasing DS percentage. In some cases, subjects altered their cadence when changing DS percentage, particularly when attempting to increase DS percentage. The changes to gait when decreasing DS percentage were similar to changes when increasing walking speed but occurred mainly during the swing period. These changes include increased hip and knee flexion during the swing period, increased swing foot height, and larger magnitude peaks in ground reaction forces. The changes in gait when attempting to increase DS percentage trended toward changes when decreasing walking speed. Altering DS percentage induced gait changes that were similar to, yet clearly distinct from, gait changes due to walking speed. Further, the difficulty of increasing DS percentage when walking at a constant speed suggests that people walk more slowly when they want to increase time spent in DS.     

Effects of Quadriceps Muscle Fatigue on Stiff-Knee Gait in Patients with Hemiparesis

The relationship between neuromuscular fatigue and locomotion has never been investigated in hemiparetic patients despite the fact that, in the clinical context, patients report to be more spastic or stiffer after walking a long distance or after a rehabilitation session. The aim of this study was to evaluate the effects of quadriceps muscle fatigue on the biomechanical gait parameters of patients with a stiff-knee gait (SKG). Thirteen patients and eleven healthy controls performed one gait analysis before a protocol of isokinetic quadriceps fatigue and two after (immediately after and after 10 minutes of rest). Spatiotemporal parameters, sagittal knee and hip kinematics, rectus femoris (RF) and vastus lateralis (VL) kinematics and electromyographic (EMG) activity were analyzed. The results showed that quadriceps muscle weakness, produced by repetitive concentric contractions of the knee extensors, induced an improvement of spatiotemporal parameters for patients and healthy subjects. For the patient group, the increase in gait velocity and step length was associated with i) an increase of sagittal hip and knee flexion during the swing phase, ii) an increase of the maximal normalized length of the RF and VL and of the maximal VL lengthening velocity during the pre-swing and swing phases, and iii) a decrease in EMG activity of the RF muscle during the initial pre-swing phase and during the latter 2/3 of the initial swing phase. These results suggest that quadriceps fatigue did not alter the gait of patients with hemiparesis walking with a SKG and that neuromuscular fatigue may play the same functional role as an anti-spastic treatment such as botulinum toxin-A injection. Strength training of knee extensors, although commonly performed in rehabilitation, does not seem to be a priority to improve gait of these patients.     

Kinematic and kinetic changes in obese gait in bariatric surgery-induced weight loss

This study examines the effects of a radical bariatric surgery-induced weight loss on the gait of obese subjects. We performed a three-dimensional motion analysis of lower limbs, and collected force platform data in the gait laboratory to calculate knee and hip joint moments. Subjects (n=13) performed walking trials in the laboratory before and 8.8 months (SD 4.2) after the surgical procedure at two gait speeds (1.2m/s and 1.5m/s). The average weight loss was 26.7kg (SD 9.2kg), corresponding to 21.5% (SD 6.8%) of the initial weight. We observed a decrease in step width at both gait speeds, but no changes in relative double support or swing time or stride length. A significant decrease was noted in the absolute values of peak knee abductor, peak knee flexor and peak hip extensor moments. However, the moment values normalized by the body weight and height remained unchanged in most cases. Thus, we conclude that weight loss reduces hip and knee joint moments in proportion to the amount of weight lost.     

Mechanisms of improved knee flexion after rectus femoris transfer surgery

Rectus femoris transfer is frequently performed to treat stiff-knee gait in subjects with cerebral palsy. In this surgery, the distal tendon is released from the patella and re-attached to one of several sites, such as the sartorius or the iliotibial band. Surgical outcomes vary, and the mechanisms by which the surgery improves knee motion are unclear. The purpose of this study was to clarify the mechanism by which the transferred muscle improves knee flexion by examining three types of transfers. Muscle-actuated dynamic simulations were created of ten children diagnosed with cerebral palsy and stiff-knee gait. These simulations were altered to represent surgical transfers of the rectus femoris to the sartorius and the iliotibial band. Rectus femoris transfers in which the muscle remained attached to the underlying vasti through scar tissue were also simulated by reducing but not eliminating the muscle's knee extension moment. Simulated transfer to the sartorius, which converted the rectus femoris’ knee extension moment to a flexion moment, produced 32±8° improvement in peak knee flexion on average. Simulated transfer to the iliotibial band, which completely eliminated the muscle's knee extension moment, predicted only slightly less improvement in peak knee flexion (28±8°). Scarred transfer simulations, which reduced the muscle's knee extension moment, predicted significantly less (p<0.001) improvement in peak knee flexion (14±5°). Simulations revealed that improved knee flexion following rectus femoris transfer is achieved primarily by reduction of the muscle's knee extension moment. Reduction of scarring of the rectus femoris to underlying muscles has the potential to enhance knee flexion.     

Joint moment work during the stance-to-swing transition in hemiparetic subjects

Following stroke many individuals are left with neurological and functional deficits, including hemiparesis, which impair their ability to walk. Our previous work reported that propulsion of the paretic leg during pre-swing is impaired and may limit gait speed and knee flexion during swing. To elucidate the mechanism of this impairment, we assessed the mechanical work produced by the hip, knee, and ankle moments during pre-swing of the paretic limb in a group of stroke subjects and compared it with the work produced by non-disabled controls walking at similar speeds. Kinematic and kinetic gait data were collected from 23 hemiparetic and 10 control subjects. The hemiparetic subjects walked at their self-selected speeds. The controls walked at their self-selected and two or three slower speeds. Even when compared to controls walking at slow speeds, ankle plantarflexor work during pre-swing was greatly reduced (-0.136+/-0.062J/kg) in the hemiparetic subjects. Differences in hip (+0.006+/-0.020J/kg) and knee (+0.040+/-0.026J/kg) moment work partially offset the reduction in ankle work, but net joint moment work was still significantly reduced (-0.088+/-0.056J/kg). The reduction in work accounts for the low energy of the paretic limb at the stance-to-swing transition previously reported. Future investigation is needed to determine if targeted training of the plantarflexors in the paretic limb improves swing-phase function and locomotor performance in hemiparetic individuals.     

Walking with increased ankle pushoff decreases hip muscle moments

In a simple bipedal walking model, an impulsive push along the trailing limb (similar to ankle plantar flexion) or a torque at the hip can power level walking. This suggests a tradeoff between ankle and hip muscle requirements during human gait. People with anterior hip pain may benefit from walking with increased ankle pushoff if it reduces hip muscle forces. The purpose of our study was to determine if simple instructions to alter ankle pushoff can modify gait dynamics and if resulting changes in ankle pushoff have an effect on hip muscle requirements during gait. We hypothesized that changes in ankle kinetics would be inversely related to hip muscle kinetics. Ten healthy subjects walked on a custom split-belt force-measuring treadmill at 1.25m/s. We recorded ground reaction forces and lower extremity kinematic data to calculate joint angles and internal muscle moments, powers and angular impulses. Subjects walked under three conditions: natural pushoff, decreased pushoff and increased pushoff. For the decreased pushoff condition, subjects were instructed to push less with their feet as they walked. Conversely, for the increased pushoff condition, subjects were instructed to push more with their feet. As predicted, walking with increased ankle pushoff resulted in lower peak hip flexion moment, power and angular impulse as well as lower peak hip extension moment and angular impulse (p<0.05). Our results emphasize the interchange between hip and ankle kinetics in human walking and suggest that increased ankle pushoff during gait may help to compensate for hip muscle weakness or injury and reduce hip joint forces.     

Muscular coordination of knee motion during the terminal-swing phase of normal gait

Children with cerebral palsy often walk with diminished knee extension during the terminal-swing phase, resulting in a troublesome "crouched" posture at initial contact and a shortened stride. Treatment of this gait abnormality is challenging because the factors that extend the knee during normal walking are not well understood, and because the potential of individual muscles to limit terminal-swing knee extension is unknown. This study analyzed a series of three-dimensional, muscle-driven dynamic simulations to quantify the angular accelerations of the knee induced by muscles and other factors during swing. Simulations were generated that reproduced the measured gait dynamics and muscle excitation patterns of six typically developing children walking at self-selected speeds. The knee was accelerated toward extension in the simulations by velocity-related forces (i.e., Coriolis and centrifugal forces) and by a number of muscles, notably the vasti in mid-swing (passive), the hip extensors in terminal swing, and the stance-limb hip abductors, which accelerated the pelvis upward. Knee extension was slowed in terminal swing by the stance-limb hip flexors, which accelerated the pelvis backward. The hamstrings decelerated the forward motion of the swing-limb shank, but did not contribute substantially to angular motions of the knee. Based on these data, we hypothesize that the diminished knee extension in terminal swing exhibited by children with cerebral palsy may, in part, be caused by weak hip extensors or by impaired hip muscles on the stance limb that result in abnormal accelerations of the pelvis.     

Biomechanics of overground vs. treadmill walking in healthy individuals.

The goal of this study was to compare treadmill walking with overground walking in healthy subjects with no known gait disorders. Nineteen subjects were tested, where each subject walked on a split-belt instrumented treadmill as well as over a smooth, flat surface. Comparisons between walking conditions were made for temporal gait parameters such as step length and cadence, leg kinematics, joint moments and powers, and muscle activity. Overall, very few differences were found in temporal gait parameters or leg kinematics between treadmill and overground walking. Conversely, sagittal plane joint moments were found to be quite different, where during treadmill walking trials, subjects demonstrated less dorsiflexor moments, less knee extensor moments, and greater hip extensor moments. Joint powers in the sagittal plane were found to be similar at the ankle but quite different at the knee and hip joints. Differences in muscle activity were observed between the two walking modalities, particularly in the tibialis anterior throughout stance, and in the hamstrings, vastus medialis and adductor longus during swing. While differences were observed in muscle activation patterns, joint moments and joint powers between the two walking modalities, the overall patterns in these behaviors were quite similar. From a therapeutic perspective, this suggests that training individuals with neurological injuries on a treadmill appears to be justified.     

How early reactions in the support limb contribute to balance recovery after tripping

Tripping causes a forward angular momentum that has to be arrested to prevent a fall. The support limb, contralateral to the obstructed swing limb, can contribute to an adequate recovery by providing time and clearance for proper positioning of the recovery limb, and by restraining the angular momentum of the body during push-off. The present study investigated how such a contribution is achieved by the support limb in terms of response times and muscle moment generation, in order to provide more insight in the requirements for successful recovery after tripping. Twelve young adults repeatedly walked over a platform in which 21 obstacles were hidden. Each subject was tripped over one of these obstacles during mid-swing in at least five trials. Kinematics, dynamics and muscle activity were measured. Very rapid responses were seen in the muscles of the support limb (approximately 65 ms), causing fast increases in muscle moments in the joints during the primary phase of recovery. Especially a large ankle plantar flexion moment (204 Nm), a knee flexion moment (-54 Nm) and a hip extension moment (52 Nm), generated by triceps surae and hamstring muscle activity, brought about the necessary push-off reaction and simultaneously caused a restraining of the forward angular momentum of the body. These required joint moments could be a problem for the elderly, who might not be able to generate such powerful moments. Strength training in these muscle groups may be indicated in elderly subjects to reduce the risk of falling after a trip.     

Joint moment contributions to swing knee extension acceleration during gait in children with spastic hemiplegic cerebral palsy

Inadequate peak knee extension during the swing phase of gait is a major deficit in individuals with spastic cerebral palsy (CP). The biomechanical mechanisms responsible for knee extension have not been thoroughly examined in CP. The purpose of this study was to assess the contributions of joint moments and gravity to knee extension acceleration during swing in children with spastic hemiplegic CP. Six children with spastic hemiplegic CP were recruited (age=13.4±4.8 years). Gait data were collected using an eight-camera system. Induced acceleration analysis was performed for each limb during swing. Average joint moment and gravity contributions to swing knee extension acceleration were calculated. Total swing and stance joint moment contributions were compared between the hemiplegic and non-hemiplegic limbs using paired t-tests (p<0.05). Swing limb joint moment contributions from the hemiplegic limb decelerated swing knee extension significantly more than those of the non-hemiplegic limb and resulted in significantly reduced knee extension acceleration. Total stance limb joint moment contributions were not statistically different. Swing limb joint moment contributions that decelerated knee extension appeared to be the primary cause of inadequate knee extension acceleration during swing. Stance limb muscle strength did not appear to be the limiting factor in achieving adequate knee extension in children with CP. Recent research has shown that the ability to extend the knee during swing is dependent on the selective voluntary motor control of the limb. Data from individual participants support this concept.     

Contributions of muscles to terminal-swing knee motions vary with walking speed

Many children with cerebral palsy walk with diminished knee extension during terminal swing, at speeds much slower than unimpaired children. Treatment of these gait abnormalities is challenging because the factors that extend the knee during normal walking, over a range of speeds, are not well understood. This study analyzed a series of three-dimensional, muscle-driven dynamic simulations to determine whether the relative contributions of individual muscles and other factors to angular motions of the swing-limb knee vary with walking speed. Simulations were developed that reproduced the measured gait dynamics of seven unimpaired children walking at self-selected, fast, slow, and very slow speeds (7 subjects×4 speeds=28 simulations). In mid-swing, muscles on the stance limb made the largest net contribution to extension of the swing-limb knee at all speeds examined. The stance-limb hip abductors, in particular, accelerated the pelvis upward, inducing reaction forces at the swing-limb hip that powerfully extended the knee. Velocity-related forces (i.e., Coriolis and centrifugal forces) also contributed to knee extension in mid-swing, though these contributions were diminished at slower speeds. In terminal swing, the hip flexors and other muscles on the swing-limb decelerated knee extension at the subjects’ self-selected, slow, and very slow speeds, but had only a minimal net effect on knee motions at the fastest speeds. Muscles on the stance limb helped brake knee extension at the subjects’ fastest speeds, but induced a net knee extension acceleration at the slowest speeds. These data—which show that the contributions of muscular and velocity-related forces to terminal-swing knee motions vary systematically with walking speed—emphasize the need for speed-matched control subjects when attempting to determine the causes of a patient's abnormal gait.     

Muscular contributions to hip and knee extension during the single limb stance phase of normal gait: a framework for investigating the causes of crouch gait

Crouch gait, a troublesome movement abnormality among persons with cerebral palsy, is characterized by excessive flexion of the hips and knees during stance. Treatment of crouch gait is challenging, at present, because the factors that contribute to hip and knee extension during normal gait are not well understood, and because the potential of individual muscles to produce flexion or extension of the joints during stance is unknown. This study analyzed a three-dimensional, muscle-actuated dynamic simulation of walking to quantify the angular accelerations of the hip and knee induced by muscles during normal gait, and to rank the potential of the muscles to alter motions of these joints. Examination of the muscle actions during single limb stance showed that the gluteus maximus, vasti, and soleus make substantial contributions to hip and knee extension during normal gait. Per unit force, the gluteus maximus had greater potential than the vasti to accelerate the knee toward extension. These data suggest that weak hip extensors, knee extensors, or ankle plantar flexors may contribute to crouch gait, and strengthening these muscles--particularly gluteus maximus--may improve hip and knee extension. Abnormal forces generated by the iliopsoas or adductors may also contribute to crouch gait, as our analysis showed that these muscles have the potential to accelerate the hip and knee toward flexion. This work emphasizes the need to consider how muscular forces contribute to multijoint movements when attempting to identify the causes of abnormal gait.