The effect of distal femoral extension osteotomy on muscle lengths after surgery.

PURPOSE: The distal femoral extension osteotomy (DFEO) is often used in the treatment of crouch gait to help compensate for knee flexion contractures. The effects of DFEO on skeletal and muscle lengths are incompletely understood, but are important to consider in planning concomitant surgeries such as patellar tendon advancement (PTA). Therefore, the purpose of this study was to quantify the changes in femur, quadriceps, and hamstring lengths with DFEO, and to determine the sensitivity to surgical factors such as wedge location and magnitude. METHODS: A musculoskeletal model with six degrees of freedom tibiofemoral and patellofemoral joints was used for analysis. A wedge was removed from the distal femur and the remaining bone segments were plated together to simulate the DFEO. After simulating the knee's post-operative equilibrium, the surgically-induced changes in muscle and bone lengths were analysed. RESULTS: Relative to the pre-operative state, DFEO stretches the hamstrings while shortening the femur and quadriceps. A more posterior wedge apex location (i.e. creation of a cuneiform wedge) diminished the stretch of the hamstrings, but induced greater shortening of the femur and quadriceps. More proximal wedge locations necessitated greater translation of the distal fragment to maintain the knee joint axis. CONCLUSION: Reduced quadriceps length after DFEO shown in this study is consistent with the need for simultaneous PTA. The induced hamstring stretch also may represent a potential mechanism for post-operative nerve palsies. Overall, the numerical results provide a firmer basis for planning the specifics of DFEO such that desired muscle lengths and joint alignment are achieved.

Length and activation dependent variations in muscle shear wave speed.

Muscle stiffness is known to vary as a result of a variety of disease states, yet current clinical methods for quantifying muscle stiffness have limitations including cost and availability. We investigated the capability of shear wave elastography (SWE) to measure variations in gastrocnemius shear wave speed induced via active contraction and passive stretch. Ten healthy young adults were tested. Shear wave speeds were measured using a SWE transducer positioned over the medial gastrocnemius at ankle angles ranging from maximum dorsiflexion to maximum plantarflexion. Shear wave speeds were also measured during voluntary plantarflexor contractions at a fixed ankle angle. Average shear wave speed increased significantly from 2.6 to 5.6 m s(-1) with passive dorsiflexion and the knee in an extended posture, but did not vary with dorsiflexion when the gastrocnemius was shortened in a flexed knee posture. During active contractions, shear wave speed monotonically varied with the net ankle moment generated, reaching 8.3 m s(-1) in the maximally contracted condition. There was a linear correlation between shear wave speed and net ankle moment in both the active and passive conditions; however, the slope of this linear relationship was significantly steeper for the data collected during passive loading conditions. The results show that SWE is a promising approach for quantitatively assessing changes in mechanical muscle loading. However, the differential effect of active and passive loading on shear wave speed makes it important to carefully consider the relevant loading conditions in which to use SWE to characterize in vivo muscle properties.

Electrical stimulation of the rectus femoris during pre-swing diminishes hip and knee flexion during the swing phase of normal gait.

Individuals who have suffered cerebral insults often exhibit stiff-knee gait, a condition characterized by reduced knee flexion during swing. We investigated the effect that an increment in normal rectus femoris (RF) activity can have on hip and knee joint angles during swing, as a first step to determining this muscle's involvement in stiff-knee gait. For this, we developed a protocol that electrically stimulated the RF during pre-swing or after toe-off in randomly selected strides of treadmill walking, consistent with the timing of RF activity during normal gait. Seven healthy young adults participated in the study. Pre-swing stimulation induced a significant (p < 0.05) reduction in peak knee flexion (avg 7.5°) in all subjects, with an accompanying decrease in hip flexion in four of the subjects. RF stimulation after toe-off diminished peak knee flexion in three subjects and reduced hip flexion in four subjects. When compared to muscle-actuated gait simulations that were similarly perturbed, the induced motion measures were generally consistent in direction but exhibited greater variability across strides and subjects. We conclude that excess RF activity during pre-swing has the potential to contribute to stiff-knee gait, and that clinical treatment should consider the "counter-intuitive" function that the RF has in extending the hip.

Age and gender differences in peak lower extremity joint torques and ranges of motion used during single-step balance recovery from a forward fall.

Previous studies have found substantial age and gender group differences in the ability of healthy adults to regain balance with a single step after a forward fall. It was hypothesized that differences in lower extremity joint strengths and ranges of motion (ROM) may have contributed to these observed differences. Kinematic and forceplate data were therefore used with a rigid-link biomechanical model simulating stepped leg dynamics to examine the joint torques and ROM used by subjects during successful single-step balance recoveries after release from a forward lean. The peak ROM and torques used by subjects in the study were compared to published estimates or measured values of the available maxima. No significant age or gender group differences were found in the mean ROM used by the subjects for any given initial lean angle. As initial lean angle increased, larger knee ROM and significantly larger hip ROM were used in the successful recoveries. There were substantial gender differences and some age group differences in peak lower extremity joint torques used in successful recoveries. Both young and older females often used nearly maximal joint torques to recover balance. Subjects' maximum joint strengths in plantarflexion and hip flexion were not good predictors of single-step balance recovery ability, particularly among the female subjects.

Self-reported walking ability predicts functional mobility performance in frail older adults.

OBJECTIVE: To determine how self-reported physical function relates to performance in each of three mobility domains: walking, stance maintenance, and rising from chairs. DESIGN: Cross-sectional analysis of older adults. SETTING: University-based laboratory and community-based congregate housing facilities. PARTICIPANTS: Two hundred twenty-one older adults (mean age, 79.9 years; range, 60-102 years) without clinical evidence of dementia (mean Folstein Mini-Mental State score, 28; range, 24-30). INTERVENTION AND MAIN OUTCOME MEASURES: We compared the responses of these older adults on a questionnaire battery used by the Established Populations for the Epidemiologic Study of the Elderly (EPESE) project, to performance on mobility tasks of graded difficulty. Responses to the EPESE battery included: (1) whether assistance was required to perform seven Katz activities of daily living (ADL) items, specifically with walking and transferring; (2) three Rosow-Breslau items, including the ability to walk up stairs and walk a half mile; and (3) five Nagi items, including difficulty stooping, reaching, and lifting objects. The performance measures included the ability to perform, and time taken to perform, tasks in three summary score domains: (1) walking ("Walking," seven tasks, including walking with an assistive device, turning, stair climbing, tandem walking); (2) stance maintenance ("Stance," six tasks, including unipedal, bipedal, tandem, and maximum lean); and (3) chair rise ("Chair Rise," six tasks, including rising from a variety of seat heights with and without the use of hands for assistance). A total score combines scores in each Walking, Stance, and Chair Rise domain. We also analyzed how cognitive/ behavioral factors such as depression and self-efficacy related to the residuals from the self-report and performance-based ANOVA models. RESULTS: Rosow-Breslau items have the strongest relationship with the three performance domains, Walking, Stance, and Chair Rise (eta-squared ranging from 0.21 to 0.44). These three performance domains are as strongly related to one Katz ADL item, walking (eta-squared ranging from 0.15 to 0.33) as all of the Katz ADL items combined (eta-squared ranging from 0.21 to 0.35). Tests of problem solving and psychomotor speed, the Trails A and Trails B tests, are significantly correlated with the residuals from the self-report and performance-based ANOVA models. CONCLUSIONS: Compared with the rest of the EPESE self-report items, self-report items related to walking (such as Katz walking and Rosow-Breslau items) are better predictors of functional mobility performance on tasks involving walking, stance maintenance, and rising from chairs. Compared with other self-report items, self-reported walking ability may be the best predictor of overall functional mobility.

Muscle activities used by young and old adults when stepping to regain balance during a forward fall.

The current study was undertaken to determine if age-related differences in muscle activities might relate to older adults being significantly less able than young adults to recover balance during a forward fall. Fourteen young and twelve older healthy males were released from forward leans of various magnitudes and asked to regain standing balance by taking a single forward step. Myoelectric signals were recorded from 12 lower extremity muscles and processed to compare the muscle activation patterns of young and older adults. Young adults successfully recovered from significantly larger leans than older adults using a single step (32.2 degrees vs. 23.5 degrees ). Muscular latency times, the time between release and activity onset, ranged from 73 to 114 ms with no significant age-related differences in the shortest muscular latency times. The overall response muscular activation patterns were similar for young and older adults. However older adults were slower to deactivate three stance leg muscles and also demonstrated delays in activating the step leg hip flexors and knee extensors prior to and during the swing phase. In the forward fall paradigm studied, age-differences in balance recovery performance do not seem due to slowness in response onset but may relate to differences in muscle activation timing during the stepping movement.

Age and gender differences in single-step recovery from a forward fall.

BACKGROUND: Previous work has found that healthy older men were significantly less able than young male adults to recover balance by taking a single rapid step upon sudden release from forward leans. In light of the higher rates of falls and fall-related injuries among older women compared to older men, we hypothesized that healthy older women would perform more poorly than either female young adults or older men in this test of abilities to recover balance rapidly. METHODS: Ten young (mean age 25.0 years) and 10 older (73.7 years) healthy women were released from forward leans and instructed to regain standing balance by taking a single step forward. The lean angle was incrementally increased from its smallest value, approximately 14 degrees, until the subject failed to regain balance as instructed. Lower extremity kinematics were measured, and findings were compared with those of the earlier study of healthy young and old men. RESULTS: Five of the 10 older women could not recover balance with a single step after release from the smallest of the imposed forward leans. For the 5 older women who succeeded in recovering as instructed from at least one lean, the mean maximum lean angle was significantly smaller than that for young women (16.2 degrees vs 30.7 degrees, p < .001) or older men (16.2 degrees vs 23.9 degrees, p = .014). In contrast, there was no significant difference in mean maximum lean angle between female and male young adults. CONCLUSIONS: Healthy older women, compared to either young women or older men, were significantly less able to recover balance by taking a single rapid step during a forward fall. The decreased abilities of older women appeared to result from limitations in the maximum speeds at which they moved their swing foot during recovery.

A retina with parallel input and pulsed output, extracting high-resolution information.

Animal eyes resolve images 10-100 times better than either the acceptance angle of a single photoreceptor or the center-to-center distance between neighboring photoreceptors. A new model of the fly's visual system emulates this improved performance, offering a different approach to subpixel resolution. That an animal without a cortex is capable of this performance suggests that high level computation is not involved. The model takes advantage of a photoreceptor cell's internal structure for capturing and transducing light. This organelle is a waveguide. Neurocircuitry exploits the waveguide's optical nonlinearities, namely the shoulder region of its gaussian, angular-sensitivity profile, to extract high resolution information from the visual scene. The receptive fields of optically disparate inputs overlap in space. Photoreceptor input is continuous rather than discretely sampled. The output of the integrating module is a signal proportional to the position of the target within the detector array. Input imbalance at the level of the photodiode modules is detected by circuitry connecting neighboring visual elements. A pulsed network of these connections forms a parallel array that segments edges of an object and continuously reports its position to the underlying layer of feature extractors, offering a new approach to real time processing with high resolution and reduced computational load.

Thresholds for sensing foot dorsi- and plantarflexion during upright stance: effects of age and velocity.

BACKGROUND: The objective of this study was to determine in healthy young and old adult females the influence of age, rotation direction, angle, and speed on the threshold for sensing foot dorsi- and plantarflexion when standing and bearing weight on the limb. METHODS: Twelve young (YF, mean age 22 years) and 12 old (OF, 70 years) healthy adult females stood with their dominant foot on a servo-controlled platform and the other foot on a fixed platform. The platform induced either dorsi- or plantarflexion rotations at angular velocities of 0.1, 0.5, or 2.5 degrees/s to angles of 0.05, 0.1, 0.2, 0.4 or 0.8 degree. Subjects performed five trials at each velocity-angle combination and 30 dummy trials in which no platform rotation occurred, for a total of 180 trials. Success rates were determined for detecting both rotation (SRR) and rotation direction (SRD) for each test condition. The angular thresholds required to achieve an SRD of 75% were estimated using logistic regression. RESULTS: Age, rotation angle, and rotation speed significantly affected SRD (repeated measures ANOVA: p < .001). For the YF, DF thresholds were 0.04, 0.09, and 0.41 degree at the fast, moderate, and slow velocities, respectively. Threshold angles were three to four times larger in the OF than in the YF. A 10-fold reduction in the angular threshold was observed upon increasing the speed of rotation from 0.1 to 2.5 degrees/s. CONCLUSIONS: Both age and speed significantly affected the thresholds for sensing foot dorsiflexion and plantarflexion in women.

Age differences in using a rapid step to regain balance during a forward fall.

BACKGROUND: Earlier studies showed that healthy old adults have substantially reduced abilities to develop joint torques rapidly. We hypothesized that this age decline would reduce abilities to regain balance once a forward fall is underway. The present study examined whether aging in fact reduces ability to regain balance by taking a single, rapid step upon release from a forward lean. METHODS: Ten young (mean age 24.3 yr) and ten old (72.8 yr) healthy males were released from a forward-leaning position and instructed to regain standing balance by taking a single step forward. Lean angle was successively increased until a subject failed to regain balance as instructed. Lower extremity motions and foot-floor reactions were measured during the responses. Total response time was divided into reaction, weight transfer, and step times. RESULTS: At small lean angles, responses of old subjects were similar to those of the young. However, the mean maximum lean angle from which old could regain balance as instructed was significantly smaller than that for young (23.9 vs 32.5 deg, p < .0005). Within each age group, maximum lean angle correlated strongly with weight transfer time and step velocity. CONCLUSIONS: Substantial age-related declines in the ability to regain balance by taking a rapid step exist among healthy adults when the time available for recovery is short. The source of the decline seems largely to lie in the decrease with age of maximum response execution speed rather than in the sensory or motor programming processes involved in response initiation.

Effects of age on rapid ankle torque development.

BACKGROUND: When balance is disturbed, often only fractions of a second are available in which to make the initial responses needed for its restoration. Abilities to develop joint torques rapidly may be critical to such responses. We undertook this study to quantify age effects among healthy adults in abilities to develop ankle joint torques rapidly. METHODS: Ankle dorsiflexion (DF) and plantarflexion (PF) torque development during rapid isometric and during isokinetic (30, 60, 120, 180, and 240 deg/sec) exertions was assessed in 24 healthy young (mean age 23 years) and 24 healthy old adults (mean age 72 years). The effects of age, gender, and torque direction on the times needed to reach given torque magnitudes, maximum rates of isometric torque development (MRTD), and maximum isokinetic torques were examined. RESULTS: The old adults required substantially more time to reach given torque magnitudes than the young adults. For example, the young and old females needed approximately 236 and 337 msec to develop 15 Nm of DF torque, of which 141 and 164 msec were reaction times. Isometric MRTD were 25 to 36% lower in the old than in the young adults. The age declines in isometric torque development time were associated with losses in maximum isometric strength. Maximum isokinetic torques developed by the old were 20 to 40% lower than those of young adults. The percent losses in isokinetic torques with age were independent of joint angular velocity for PF, but increased with velocity for DF. CONCLUSIONS: We found substantial age declines in abilities of healthy old adults to rapidly develop ankle joint torques. The capacities of even healthy old adults to recover balance or to carry out other time-critical actions that require moderate-to-substantial strengths may be considerably degraded by these declines.

Do neural factors underlie age differences in rapid ankle torque development?

OBJECTIVES: Rapid torque development is substantially slower in healthy old adults compared with young adults, but the underlying cause of this age-related loss remains unclear. Measurements of myoelectric signals in ankle dorsi- and plantarflexor muscles during rapid exertions were used to explore the extent to which the loss might be attributed to neural factors. METHODS: Myoelectric signals were measured in a laboratory setting in 24 healthy young and 24 healthy old adult volunteers during rapid isometric and isokinetic torque development. Premotor times, muscle activation rates, and myoelectric activity levels of agonistic and antagonistic muscles were quantified. RESULTS: There were few marked age differences in the premotor times or in the onset rates or magnitudes of agonistic muscles activities during maximum isometric and during isokinetic exertions. Premotor times were statistically associated with age but, in the mean, were only approximately 10 to 25 ms longer in the old. Age effects on agonist muscle activity magnitudes were significant only in the lateral gastrocnemius. Small decreases in antagonistic muscle activity levels with age were found. CONCLUSIONS: Given the outcomes of this study, the differences observed previously in rapid torque development abilities in healthy older adults, compared with healthy younger adults, seem attributable largely to differences in muscle contraction mechanisms rather than to differences in speeds of stimulus sensing or central processing of motor commands, or to differences in muscle recruitment strategies.

Lumbar muscle activities in rapid three-dimensional pulling tasks.

STUDY DESIGN: An experimental and theoretical study in healthy young adults. OBJECTIVE: To utilize myoelectric signals to investigate muscle recruitment and estimate spine loads during rapid, three-dimensional pulling tasks. SUMMARY OF BACKGROUND DATA: Most previous research concerning lumbar trunk loads has focused on quasi-static exertions and sagittally symmetric dynamic tasks. METHODS: Nine young males performed dynamic pulls in five prescribed directions, requiring the active development of combined sagittal, frontal, and/or transverse moments by the lumbar muscles. Myoelectric activities were recorded from 14 lumbar muscles. Individual muscle activities were compared with their biomechanical capability to equilibrate the external moments at the L3-4 lumbar cross-section. Myoelectric signal-to-force models were used to estimate the peak loads on the lumbar spine. RESULTS: The largest muscle activities always occurred in those muscles having the greatest spatial effectiveness to develop the task moments. However, abdominal oblique and latissimus dorsi muscles were at times active during pulling tasks involving substantial lateral and/or axial moments despite poor spatial effectiveness to equilibrate the task moments. Of ergonomic significance is the finding that the estimated spine compression was substantially greater when asymmetric pulls imposed twisting loads about the spine compared to equivalent symmetric pulls, reflecting the additional muscle activities required to equilibrate the twisting moments. CONCLUSIONS: Asymmetric pulls resulted in some activities in obliquely oriented muscles not primarily associated with equilibrating the task moments at a single level of the lumbar trunk. Therefore, other factors, such as equilibrium requirements at other lumbar levels and trunk stiffness, may be important determinants of lumbar muscle activities during three-dimensional loadings of the trunk.

Co-contraction of lumbar muscles during the development of time-varying triaxial moments.

The recruitment and co-contraction of lumbar muscles were investigated during the voluntary development of slowly and rapidly varying trunk flexion and extension, lateral bending, and axial twisting moments. Myoelectric signals were recorded from 14 lumbar muscles in nine young men during maximum voluntary exertions and cyclic isometric exertions. System identification techniques were used to calibrate dynamic models of the relationship between myoelectric signals and force. To assess co-contraction, the predicted muscle forces were subdivided into a task-moment set of muscle forces that minimally satisfied moment equilibrium and a co-contraction set of muscle forces that produced zero net moment. The sum of co-contraction muscle forces was used to quantify the degree of co-contraction present. Co-contraction was largely dependent on the direction of exertion and relatively less dependent on the subject or the rate of exertion. Co-contractions were estimated to contribute approximately 16-19% to the sum of muscle forces at a lumbar cross section during attempted extension of the trunk. Estimated co-contractions during attempted lateral bending and axial twisting were two to three times greater, which demonstrates that co-contraction is a major determinant of spinal loading in these tasks. This analysis suggests that substantial contractions of lumbar muscles, especially during asymmetric exertions, are used for reasons other than equilibrating moments at the L3-L4 level.

Identification of dynamic myoelectric signal-to-force models during isometric lumbar muscle contractions.

A 14-muscle myoelectric signal (MES)-driven muscle force prediction model of the L3-L4 cross section is developed which includes a dynamic MES-force relationship and allows for cocontraction. Model parameters are estimated from MES and moments data recorded during rapid exertions in trunk flexion, extension, lateral bending and axial twist. Nine young healthy males participated in the experimental testing. The model used in the parameter estimation is of the output error type. Consistent and physically feasible parameter estimates were obtained by normalizing the RMS MES to maximum exertion levels and using nonlinear constrained optimization to minimize a cost function consisting of the trace of the output error covariance matrix. Model performance was evaluated by comparing measured and MES-predicted moments over a series of slow and rapid exertions. Moment prediction errors were on the order of 25, 30 and 40% during attempted trunk flexion-extensions, lateral bends and axial twists, respectively. The model and parameter estimation methods developed provide a means to estimate lumbar muscle and spine loads, as well as to empirically investigate the use and effects of cocontraction during physical task performances.

Quantitative interpretation of lumbar muscle myoelectric signals during rapid cyclic attempted trunk flexions and extensions.

The quantitative relationship between lumbar myoelectric signals (MES) and rapidly varying isometric trunk muscle forces was investigated. Ten young adult males were asked to cycle harmonically between attempted trunk flexion and attempted trunk extension in an upright position at rates of 0.33, 0.67 and 1.0 Hz to peak efforts of 20, 40 and 60% of maximum voluntary exertion levels. The forces voluntarily exerted against a load cell were measured and used along with acquired kinematic data to calculate the time course of the net sagittal moment at the level of the third lumbar vertebra during task performances. A 22 muscle double linear programming biomechanical model was used to predict the lumbar trunk muscle contraction forces from the calculated moments. Rectified and bidirectionally low-pass filtered myoelectric activities were acquired at the L3 level from four abdominal muscles and four back muscles. The processed MES were found to be well correlated (r > 0.90) with predicted muscle forces when the MES were time-shifted to account for electromechanical delay as well as the dynamic phase shift between muscle electrical activity and contraction force. Mean time shifts that maximized the linear MES-force relationship ranged from 111 to 218 ms, were greater for the trunk extensors than the trunk flexors and generally exhibited lateral symmetry. The corresponding approximate phase angles averaged 20 degrees at the slowest rate and 50 degrees at the fastest rate. MES-force phase angles decreased as effort level was increased indicating that the dynamic MES-force relationship is nonlinear. These results illustrate the importance of accounting for the phase lag between muscle electrical activity and force when using MES to quantify muscle loads during rapidly varying exertions.