Muscle force redistributes segmental power for body progression during walking

Coordination of the non-paretic leg during hemiparetic gait: expected and novel compensatory patterns

BACKGROUND

Post-stroke hemiparesis is usually considered a unilateral motor control deficit of the paretic leg, while the non-paretic leg is assumed to compensate for paretic leg impairments and have minimal to no deficits. While the non-paretic leg electromyography (EMG) patterns are clearly altered, how the non-paretic leg acts to compensate remains to be established.

METHODS

Kinesiological data were recorded from sixty individuals with chronic hemiparesis (age: 60.9, SD=12.6 years, 21 females, 28 right hemiparetic, time since stroke: 4.5 years, SD 3.9 years), divided into three speed-based groups, and twenty similarly aged healthy individuals (age: 65.1, SD=10.4 years, 15 females). All walked on an instrumented split-belt treadmill at their self-selected speed and control subjects also walked at slower speeds matching those of the persons with hemiparesis. We determined the differences in magnitude and timing of non-paretic EMG activity relative to healthy control subjects in four pre-defined regions of stance phase of the gait cycle.

FINDINGS

Integrated EMG activity and EMG timing in the non-paretic leg were different in many muscles. Multiple compensatory patterns identified included: increased EMG output when the muscle was typically active in controls and novel compensatory EMG patterns that appeared to provide greater propulsion or support with little evidence of impaired motor performance.

INTERPRETATION

Most novel compensations were made possible by altered kinematics of the paretic and non-paretic leg (i.e., early stance plantarflexor activity provided propulsion due to the decreased advancement of the non-paretic foot) while others (late single limb stance knee extensor and late stance hamstring activity) appeared to be available mechanisms for increasing propulsion.

Kinematics of bipedal locomotion while carrying a load in the arms in bearded capuchin monkeys (Sapajus libidinosus)

Understanding the selective pressures that drove the evolution of bipedalism in the human lineage may help inform researchers about the locomotor mode(s) of pre-hominin ancestors. Several selective pressures have been hypothesized, including the need to carry food, tools, or infants. Bearded capuchin monkeys are an excellent primate in which to examine the hypothesis that carrying supported the evolution of bipedalism because they are morphologically generalized and in some ways similar to Miocene hominoids, from which the transitional biped evolved. Additionally, bearded capuchins regularly move bipedally while carrying tools that represent a significant portion of their body mass. Here, we examined the spatio-temporal and kinematic gait parameters in a wild setting of Sapajus libidinosus moving bipedally while carrying a stone tool, as well as unloaded bipedal tufted capuchins in the lab. Results indicate that compared with humans, the monkeys move with a more bent-hip, bent-knee posture during both types of bipedalism, as expected. Few differences exist in spatio-temporal or kinematic parameters within species across load-carrying and unloaded bipedalism. The capuchin ankle, however, during load-carrying goes through a greater range of motion in relatively less time than both humans and unloaded capuchins. Data from this study provide the first quantitative data on bipedalism during load-carrying by wild primates in a natural setting. As such, they are a useful comparative reference for understanding bipedalism, particularly during load-carrying.

Biomechanical and neurophysiological mechanisms related to postural control and efficiency of movement: a review

Understanding postural control requires considering various mechanisms underlying a person's ability to stand, to walk, and to interact with the environment safely and efficiently. The purpose of this paper is to summarize the functional relation between biomechanical and neurophysiological perspectives related to postural control in both standing and walking based on movement efficiency. Evidence related to the biomechanical and neurophysiological mechanisms is explored as well as the role of proprioceptive input on postural and movement control.

Coactivation of lower leg muscles during body weight-supported treadmill walking decreases with age in adolescents

The kinematics of children's walking are nearly adult-like by about age 3-4 years, but metabolic efficiency of walking does not reach adult values until late in adolescence or early adulthood, perhaps due to higher coactivation of agonist/antagonist muscle pairs in adolescents. Additionally, it is unknown how use of a body weight-supported treadmill device affects coactivation, but because unloading will alter the activity of anti-gravity muscles, it was hypothesized that muscle coactivation will be altered as well. Muscle coactivation during treadmill walking was evaluated for adolescents (ages 10 to 17 years, M = 13.2, SD = 2.2) and adults (ages 22 to 35 years, M = 25.2, SD = 4.3), for thigh muscles (vastus lateralis/biceps femoris) and lower leg muscles (tibialis anterior/gastrocnemius). Conditions included body weight unloadings from nearly 0% to 80% of body weight, while walking at a preferred speed (self-selected, overground speed) or a reduced speed. Unloading was accomplished using a lower body positive pressure support system. Coactivation was found to be higher in adolescents than in adults, but only for the lower leg muscles.

Gait strategy changes with acceleration to accommodate the biomechanical constraint on push-off propulsion

To maintain steady and level walking, push-off propulsion during the double support phase compensates for the energy loss through heel strike collisions in an energetically optimal manner. However, a large portion of daily gait activities also contains transient gait responses, such as acceleration or deceleration, during which the observed dominance of the push-off work or the energy optimality may not hold. In this study, we examined whether the push-off propulsion during the double support phase served as a major energy source for gait acceleration, and we also studied the energetic optimality of accelerated gait using a simple bipedal walking model. Seven healthy young subjects participated in the over-ground walking experiments. The subjects walked at four different constant gait speeds ranging from a self-selected speed to a maximum gait speed, and then they accelerated their gait from zero to the maximum gait speed using a self-selected acceleration ratio. We measured the ground reaction force (GRF) of three consecutive steps and the corresponding leg configuration using force platforms and an optical marker system, respectively, and we compared the mechanical work performed by the GRF during each single and double support phase. In contrast to the model prediction of an increase in the push-off propulsion that is proportional to the acceleration and minimizes the mechanical energy cost, the push-off propulsion was slightly increased, and a significant increase in the mechanical work during the single support phase was observed. The results suggest that gait acceleration occurs while accommodating a feasible push-off propulsion constraint.

Avaliação da atividade eletromiográfica com ou sem o uso de diversos tipos de calçado, em diferentes planos de locomoção

INTRODUÇÃO: São muito recorrentes as queixas de mulheres com desconforto na região lombar durante o uso de sapato de salto. OBJETIVO: O objetivo deste estudo foi avaliar a atividade eletromiográfica de músculos da perna e eretores da espinha associada a tipos de calçados e ao andar descalço, nos diferentes planos de locomoção, nas situações pré e pós-fadiga. MATERIAIS E MÉTODOS: Trata-se de um estudo observacional transversal, no qual foi utilizado uma amostra de conveniência com 15 mulheres jovens, com idades entre 18 e 35 anos, sedentárias. Os músculos analisados foram: tibial anterior, gastrocnêmio medial e lateral e eretores. A atividade muscular foi avaliada durante a marcha em superfície plana, subindo e descendo escada e rampa. Os calçados utilizados foram salto alto, salto baixo, tênis, chinelo, e também foi feita a avaliação sem o uso deles, ou seja, descalço. RESULTADOS: Os resultados da análise eletromiográfica mostrou que os músculos da perna tiveram maior atividade eletromiográfica pré-fadiga e os eretores, pós-fadiga dos membros inferiores, na maioria das condições e situações. CONCLUSÃO: Condições adaptativas associadas ao uso regular de salto alto foram amplamente questionadas em relação à incidência de lesões. Após fadiga de membros inferiores, houve declínio na atividade dos músculos da perna e aumento do recrutamento dos músculos eretores, justificado pela necessidade de maior estabilidade da coluna e da pelve durante a locomoção de mulheres jovens.

Assessing the effect of high-repetitive single limb exercises (HRSLE) on exercise capacity and quality of life in patients with chronic obstructive pulmonary disease (COPD): study protocol for randomized controlled trial

Background

Single-limb knee extension exercises have been found to be effective at improving lower extremity exercise capacity in patients with chronic obstructive pulmonary disease (COPD). Since the positive local physiological effects of exercise training only occur in the engaged muscle(s), should upper extremity muscles also be included to determine the effect of single limb exercises in COPD patients.

Methods/design

Trial design: a prospective, assessor-blind, block randomized controlled, parallel-group multicenter trial. Participants: stage II-IV COPD patients, > 40 years of age, ex-smokers, with stable medical treatment will be included starting May 2011. Recruitment at three locations in Sweden. Interventions: 1) high-repetitive single limb exercise (HRSLE) training with elastic bands, 60 minutes, three times/week for 8 weeks combined with four sessions of 60 minutes patient education, or 2) the same patient education alone. Outcomes: Primary: determine the effects of HRSLE on local muscle endurance capacity (measured as meters walked during 6-minute walk test and rings moved on 6-minute ring and pegboard test) and quality of life (measured as change on the Swedish version of the Chronic Respiratory Disease Questionnaire). Secondary: effects on maximal strength, muscular endurance, dyspnea, self-efficacy, anxiety and depression. The relationship between changes in health-related variables and changes in exercise capacity, sex-related differences in training effects, feasibility of the program, strategies to determine adequate starting resistance and provide accurate resistance for each involved movement and the relationship between muscle fatigue and dyspnea in the different exercise tests will also be analyzed. Randomization: performed by a person independent of the recruitment process and using a computer random number generator. Stratification by center and gender with a 1:1 allocation to the intervention or control using random block sizes. Blinding: all outcome assessors will be blinded to group assignment.

Discussion

The results of this project will contribute to increase the body of knowledge regarding COPD and HRSLE.

Trial registration

ClinicalTrials.gov Identifier: NCT01354067. Registration date: 2011-05-11. First participant randomized: 2011-09-02

Three-dimensional modular control of human walking

Recent studies have suggested that complex muscle activity during walking may be controlled using a reduced neural control strategy organized around the co-excitation of multiple muscles, or modules. Previous computer simulation studies have shown that five modules satisfy the sagittal-plane biomechanical sub-tasks of 2D walking. The present study shows that a sixth module, which contributes primarily to mediolateral balance control and contralateral leg swing, is needed to satisfy the additional non-sagittal plane demands of 3D walking. Body support was provided by Module 1 (hip and knee extensors, hip abductors) in early stance and Module 2 (plantarflexors) in late stance. In early stance, forward propulsion was provided by Module 4 (hamstrings), but net braking occurred due to Modules 1 and 2. Forward propulsion was provided by Module 2 in late stance. Module 1 accelerated the body medially throughout stance, dominating the lateral acceleration in early stance provided by Modules 4 and 6 (adductor magnus) and in late stance by Module 2, except near toe-off. Modules 3 (ankle dorsiflexors, rectus femoris) and 5 (hip flexors and adductors except adductor magnus) accelerated the ipsilateral leg forward in early swing whereas Module 4 decelerated the ipsilateral leg prior to heel-strike. Finally, Modules 1, 4 and 6 accelerated the contralateral leg forward prior to and during contralateral swing. Since the modules were based on experimentally measured muscle activity, these results provide further evidence that a simple neural control strategy involving muscle activation modules organized around task-specific biomechanical functions may be used to control complex human movements.

Potential of lower-limb muscles to accelerate the body during cerebral palsy gait

Two of the most common gait patterns in children with spastic diplegic cerebral palsy (CP) are termed 'crouch gait' and 'jump gait'. While outcomes of surgical interventions designed to improve functional mobility are generally positive, many children displaying these gait patterns show minimal or no improvement post-surgery. A poor response to treatment may be partially attributable to incorrect interpretations of muscle function. Computational techniques that assess muscle function may help address this issue, but before studying specific surgeries, the gait patterns themselves must be better understood. The aim of this study was to identify differences in lower-limb muscle function when comparing crouch, jump and able-bodied gait patterns by quantifying the potential of lower-limb muscles to accelerate the body's center of mass. A muscle's potential acceleration was defined as the acceleration induced by a unit of muscle force. Dynamic simulations of walking using musculoskeletal models were developed for eight children with crouch gait, ten with jump gait, and ten controls. There were significant differences (p<0.05) in muscle potential accelerations between crouch and able-bodied gait patterns, and between jump and able-bodied gait patterns, for most of the major muscles of the hip, knee, and ankle. One important outcome was the identification of the significantly reduced potential of gluteus medius to extend the hip in both crouch gait and jump gait. Potential acceleration analyses appear to be suitable for evaluating differences between common gait patterns and may also be applied to study the effects of surgical treatments. The results of such studies may lead to improved treatment outcomes for individuals with impaired mobility.

Hybrid models of the neuromusculoskeletal system improve subject-specificity

Muscle-actuated simulations of pathological gait have the capacity to identify muscle impairments and compensatory strategies, but the lack of subject-specific solutions prevents the prescription of personalized therapies. Conversely, electromyographic-driven models are limited to muscles for which data are available but can capture the true neural drive initiated by an individual subject. In order to improve subject-specificity and enforce physiological constraints on muscle activity, we propose a hybrid strategy for the optimization of subject-specific muscle patterns that involves forward dynamic simulation of whole body movement coupled with electromyographic-driven models of muscle subsets. In this paper we apply the hybrid approach to an example of post-stroke gait and demonstrate its unique ability to account for the unusual muscle activation patterns and muscle properties in patients with neuromuscular impairments.

3D intersegmental knee loading in below-knee amputees across steady-state walking speeds

BACKGROUND

Unilateral below-knee amputees often develop comorbidities that include knee joint disorders (e.g., intact leg knee osteoarthritis), with the mechanisms leading to these comorbidities being poorly understood. Mechanical knee loading of non-amputees has been associated with joint disorders and shown to be influenced by walking speed. However, the relationships between amputee knee loading and speed have not been identified. This study examined three-dimensional mechanical knee loading of amputees across a wide range of steady-state walking speeds.

METHODS

Fourteen amputees and 10 non-amputee control subjects were analyzed at four overground walking speeds. At each speed, intersegmental joint moment and force impulses (i.e., time-integrals over the stance phase) were compared between the control, intact and residual knees using repeated-measures ANOVAs.

FINDINGS

There were no differences in joint force impulses between the intact and control knees. The intact knee abduction moment impulse was lower than the non-amputees at 0.6 and 0.9 m/s. The intact knee flexion moment impulses at 0.6, 1.2 and 1.5m/s and knee external rotation moment impulses at all speeds were greater than the residual knee. The residual knee extension moment and posterior force impulses were insensitive to speed increases, while these quantities increased in intact and control knees.

INTERPRETATION

These results suggest the intact knees of asymptomatic and relatively new amputees are not overloaded during walking compared to non-amputees. Increased knee loads may develop in response to prolonged prosthesis usage or joint disorder onset. Further study is needed to determine if the identified bilateral loading asymmetries across speeds lead to diminished knee joint health.

Similar muscles contribute to horizontal and vertical acceleration of center of mass in forward and backward walking: implications for neural control

Leg kinematics during backward walking (BW) are very similar to the time-reversed kinematics during forward walking (FW). This suggests that the underlying muscle activation pattern could originate from a simple time reversal, as well. Experimental electromyography studies have confirmed that this is the case for some muscles. Furthermore, it has been hypothesized that muscles showing a time reversal should also exhibit a reversal in function [from accelerating the body center of mass (COM) to decelerating]. However, this has not yet been verified in simulation studies. In the present study, forward simulations were used to study the effects of muscles on the acceleration of COM in FW and BW. We found that a reversal in function was indeed present in the muscle control of the horizontal movement of COM (e.g., tibialis anterior and gastrocnemius). In contrast, muscles' antigravity contributions maintained their function for both directions of movement. An important outcome of the present study is therefore that similar muscles can be used to achieve opposite functional demands at the level of control of the COM when walking direction is reversed. However, some muscles showed direction-specific contributions (i.e., dorsiflexors). We concluded that the changes in muscle contributions imply that a simple time reversal would be insufficient to produce BW from FW. We therefore propose that BW utilizes extra elements, presumably supraspinal, in addition to a common spinal drive. These additions are needed for propulsion and require a partial reconfiguration of lower level common networks.

Analysis of ground reaction force and electromyographic activity of the gastrocnemius muscle during double support

Mechanisms associated with energy expenditure during gait have been extensively researched and studied. According to the double-inverted pendulum model energy expenditure is higher during double support, as lower limbs need to work to redirect the centre of mass velocity. This study looks into how the ground reaction force of one limb affects the muscle activity required by the medial gastrocnemius of the contralateral limb during step-to-step transition. Thirty-five subjects were monitored as to the medial gastrocnemius electromyographic activity of one limb and the ground reaction force of the contralateral limb during double support. After determination of the Pearson correlation coefficient (r), a moderate correlation was observed between the medial gastrocnemius electromyographic activity of the dominant leg and the vertical (Fz) and anteroposterior (Fy) components of ground reaction force of the non-dominant leg (r = 0.797, p < 0.0001; r = –0.807, p < 0.0001). A weak and moderate correlation was observed between the medial gastrocnemius electromyographic activity of the non-dominant leg and the Fz and Fy of the dominant leg, respectively (r = 0.442, p = 0.018; r = –0.684 p < 0.0001). The results obtained suggest that during double support, ground reaction force is associated with the electromyographic activity of the contralateral medial gastrocnemius and that there is an increased dependence between the ground reaction force of the non-dominant leg and the electromyographic activity of the dominant medial gastrocnemius.

Long-distance walking effects on trans-tibial amputees compensatory gait patterns and implications on prosthetic designs and training

Trans-tibial amputees are advised to walk as much as able people to achieve healthy and independent life. However, they usually have difficulties in doing so. Previous researches only included data from a few steps when studying the gait of amputees. Walking over a long distance was rarely examined. The objective of this study was to investigate the changes in spatial-temporal, kinetic and kinematic gait parameters of trans-tibial amputees after long-distance walking. Six male unilateral trans-tibial amputees performed two sessions of 30-min walking on a level treadmill at their self-selected comfortable speed. Gait analysis was undertaken over-ground: (1) before walking, (2) after the 1st walking session and (3) after the 2nd walking session. After the long-distance walking, changes in spatial-temporal gait parameters were small and insignificant. However, the sound side ankle rocker progression and push-off were significantly reduced. This was due to the fatigue of the sound side plantar flexors and was compensated by the greater effort in the prosthetic side. The prosthetic side knee joint showed significantly increased flexion and moment during loading response to facilitate the anterior rotation of the prosthetic shank. The prosthetic side hip extensors also provided more power at terminal stance to facilitate propulsion. Endurance training of the sound side plantar flexors, and improvements in the prosthetic design to assist anterior rotation of the prosthetic shank should improve long-distance walking in trans-tibial amputees.

Comparing gait performance of people with Charcot-Marie-Tooth disease who do and do not wear ankle foot orthoses

BACKGROUND AND PURPOSE

Ankle foot orthoses (AFOs) are commonly prescribed for people with Charcot-Marie-Tooth (CMT) disease. Scant evidence exists to guide the type and timing of orthotic prescription. This study explores the latter issue by investigating the differences in presentation and gait function of people with CMT disease who wore AFOs for daily mobility (n = 11) and a group who did not (n = 21). The aim was to see if there was a difference in the characteristics in people who regularly wear AFOs.

METHODS

Primary measures of gait function were a 10-m timed walk (comfortable and maximum speed) and a 6-minute walk test. Means of the variables were compared using independent t-tests. Secondary measures included disease severity, lower limb muscle strength, sensory impairment, walking effort, fatigue severity and perceived walking ability.

RESULTS

AFO wearers walked slower with higher effort. They also had greater disease severity, weaker leg muscles and perceived greater walking difficulty. Subjects not wearing AFOs showed significant relationships between gait variables and muscle strength, whereas AFO wearers showed significant relationships between gait variables and perceived walking ability, fatigue severity and effort.

CONCLUSIONS

People who regularly wore AFOs were more severely affected, had a slower maximum walking speed, higher energy cost of walking and worse perceived walking ability. Walking ability in this group was related to fatigue, perceived exertion during walking and perceived walking ability. Gait function of people not using AFOs was determined by lower limb muscle function. People prescribed AFOs, those who do not wear them and those not prescribed AFOs were similar in presentation, suggesting that people choose to wear orthoses when their condition becomes sufficiently severe.

Stabilisation of walking by intrinsic muscle properties revealed in a three-dimensional muscle-driven simulation

A fundamental question in movement science is how humans perform stable movements in the presence of disturbances such as contact with objects. It remains unclear how the nervous system, with delayed responses to disturbances, maintains the stability of complex movements. We hypothesised that intrinsic muscle properties (i.e. the force-length-velocity properties of muscle fibres and tendon elasticity) may help stabilise human walking by responding instantaneously to a disturbance and providing forces that help maintain the movement trajectory. To investigate this issue, we generated a 3D muscle-driven simulation of walking and analysed the changes in the simulation's motion when a disturbance was applied to models with and without intrinsic muscle properties. Removing the intrinsic properties reduced the stability; this was true when the disturbing force was applied at a variety of times and in different directions. Thus, intrinsic muscle properties play a unique role in stabilising walking, complementing the delayed response of the central nervous system.

The influence of energy storage and return foot stiffness on walking mechanics and muscle activity in below-knee amputees

BACKGROUND

Below-knee amputees commonly experience asymmetrical gait patterns and develop comorbidities in their intact and residual legs. Carbon fiber prosthetic feet have been developed to minimize these asymmetries by utilizing elastic energy storage and return to provide body support, forward propulsion and leg swing initiation. However, how prosthetic foot stiffness influences walking characteristics is not well-understood. The purpose of this study was to identify the influence of foot stiffness on kinematics, kinetics, muscle activity, prosthetic energy storage and return, and mechanical efficiency during amputee walking.

METHODS

A comprehensive biomechanical analysis was performed on 12 unilateral below-knee amputees. Subjects walked overground at 1.2 m/s with three prosthetic feet of varying keel and heel stiffness levels, which were created using additive manufacturing.

FINDINGS

As stiffness decreased, peak residual and intact leg ankle angles and residual leg knee flexion angle increased. The residual and intact leg braking ground reaction forces and knee extensor moments, residual leg vastus and gluteus medius activity, and intact leg vastus and rectus femoris activity also increased. The second vertical ground reaction force peak and hamstring activity in the residual leg and first vertical ground reaction force peak in the intact leg decreased. In addition, prosthetic energy storage and return increased and mechanical efficiency decreased as stiffness decreased.

INTERPRETATION

Decreasing foot stiffness can increase prosthesis range of motion, mid-stance energy storage and late-stance energy return, but the net contributions to forward propulsion and swing initiation may be limited as additional muscle activity to provide body support becomes necessary.

Is hip muscle strength the key to walking as a bilateral amputee, whatever the level of the amputations?

BACKGROUND

Little data have been reported on the factors that are important in bilateral amputee walking ability especially the role of hip strength.

STUDY DESIGN

Observational, case-control study where participants were evaluated at a single point in time.

OBJECTIVES

The aim of this study was to investigate the factors involved in bilateral amputee walking ability by assessment of walking speed, perceived exertion, exercise intensity, physiological cost index (PCI) and hip muscle strength.

METHODS

For a group of 10 bilateral amputees, with different levels of amputation, and a non-pathological reference group, walking ability was assessed using the two-minute walk test. Hip muscle strength was assessed using isokinetic strength tests.

RESULTS

Bilateral amputees were found to have slower walking speeds and increased PCI of walking which were correlated to higher levels of amputation. Peak hip torques were reduced in the amputees, which was only significant for concentric extension torque (p = 0.029), and approaching significance for concentric flexion (p = 0.061) and abduction (p = 0.057). Bilateral amputee peak hip strength suggested a positive trend with increasing walking ability.

CONCLUSIONS

Bilateral amputee walking ability was reduced and mainly related to level of amputation. The role of hip strength in bilateral amputee walking ability requires further investigation.

Evaluation of movements of lower limbs in non-professional ballet dancers: hip abduction and flexion

Background

The literature indicated that the majority of professional ballet dancers present static and active dynamic range of motion difference between left and right lower limbs, however, no previous study focused this difference in non-professional ballet dancers. In this study we aimed to evaluate active movements of the hip in non-professional classical dancers.

Methods

We evaluated 10 non professional ballet dancers (16-23 years old). We measured the active range of motion and flexibility through Well Banks. We compared active range of motion between left and right sides (hip flexion and abduction) and performed correlation between active movements and flexibility.

Results

There was a small difference between the right and left sides of the hip in relation to the movements of flexion and abduction, which suggest the dominant side of the subjects, however, there was no statistical significance. Bank of Wells test revealed statistical difference only between the 1^st and the 3^rd measurement. There was no correlation between the movements of the hip (abduction and flexion, right and left sides) with the three test measurements of the bank of Wells.

Conclusion

There is no imbalance between the sides of the hip with respect to active abduction and flexion movements in non-professional ballet dancers.

Comparative triceps surae morphology in primates: a review

Primate locomotor evolution, particularly the evolution of bipedalism, is often examined through morphological studies. Many of these studies have examined the uniqueness of the primate forelimb, and others have examined the primate hip and thigh. Few data exist, however, regarding the myology and function of the leg muscles, even though the ankle plantar flexors are highly important during human bipedalism. In this paper, we draw together data on the fiber type and muscle mass variation in the ankle plantar flexors of primates and make comparisons to other mammals. The data suggest that great apes, atelines, and lorisines exhibit similarity in the mass distribution of the triceps surae. We conclude that variation in triceps surae may be related to the shared locomotor mode exhibited by these groups and that triceps surae morphology, which approaches that of humans, may be related to frequent use of semiplantigrade locomotion and vertical climbing.

Bionic ankle-foot prosthesis normalizes walking gait for persons with leg amputation

Over time, leg prostheses have improved in design, but have been incapable of actively adapting to different walking velocities in a manner comparable to a biological limb. People with a leg amputation using such commercially available passive-elastic prostheses require significantly more metabolic energy to walk at the same velocities, prefer to walk slower and have abnormal biomechanics compared with non-amputees. A bionic prosthesis has been developed that emulates the function of a biological ankle during level-ground walking, specifically providing the net positive work required for a range of walking velocities. We compared metabolic energy costs, preferred velocities and biomechanical patterns of seven people with a unilateral transtibial amputation using the bionic prosthesis and using their own passive-elastic prosthesis to those of seven non-amputees during level-ground walking. Compared with using a passive-elastic prosthesis, using the bionic prosthesis decreased metabolic cost by 8 per cent, increased trailing prosthetic leg mechanical work by 57 per cent and decreased the leading biological leg mechanical work by 10 per cent, on average, across walking velocities of 0.75-1.75 m s(-1) and increased preferred walking velocity by 23 per cent. Using the bionic prosthesis resulted in metabolic energy costs, preferred walking velocities and biomechanical patterns that were not significantly different from people without an amputation.

Relationships between muscle contributions to walking subtasks and functional walking status in persons with post-stroke hemiparesis

BACKGROUND

Persons with post-stroke hemiparesis usually walk slowly and asymmetrically. Stroke severity and functional walking status are commonly predicted by post-stroke walking speed. The mechanisms that limit walking speed, and by extension functional walking status, need to be understood to improve post-stroke rehabilitation methods.

METHODS

Three-dimensional forward dynamics walking simulations of hemiparetic subjects (and speed-matched controls) with different levels of functional walking status were developed to investigate the relationships between muscle contributions to walking subtasks and functional walking status. Muscle contributions to forward propulsion, swing initiation and power generation were analyzed during the pre-swing phase of the gait cycle and compared between groups.

FINDINGS

Contributions from the paretic leg muscles (i.e., soleus, gastrocnemius and gluteus medius) to forward propulsion increased with improved functional walking status, with the non-paretic leg muscles (i.e., rectus femoris and vastii) compensating for reduced paretic leg propulsion in the limited community walker. Contributions to swing initiation from both paretic (i.e., gastrocnemius, iliacus and psoas) and non-paretic leg muscles (i.e., hamstrings) also increased as functional walking status improved. Power generation was also an important indicator of functional walking status, with reduced paretic leg power generation limiting the paretic leg contribution to forward propulsion and leg swing initiation.

INTERPRETATION

These results suggest that deficits in muscle contributions to the walking subtasks of forward propulsion, swing initiation and power generation are directly related to functional walking status and that improving output in these muscle groups may be an effective rehabilitation strategy for improving post-stroke hemiparetic walking.

The effect of subtalar inversion/eversion on the dynamic function of the tibialis anterior, soleus, and gastrocnemius during the stance phase of gait

The purpose of this study was to determine how gait deviation in one plane (i.e. excessive subtalar inversion/eversion) can affect the dynamic function of the tibialis anterior, gastrocnemius, and soleus to accelerate the subtalar, ankle, knee and hip joints, as well as the body center of mass. Induced acceleration analysis was performed based on a subject-specific three-dimensional linkage model configured by stance phase gait data and driven by one unit of muscle force. Eight healthy adult subjects were examined in gait analysis. The subtalar inversion/eversion was modeled by offsetting up to 20° from the normal subtalar angle while other configurations remained unaltered. This study showed that the gastrocnemius, soleus and tibialis anterior generally functioned as their anatomical definition in normal gait, but counterintuitive function was occasionally found in the bi-articular gastrocnemius. The plantarflexors play important roles in the body support and forward progression. Excessive subtalar eversion was found to enlarge the plantarflexors and tibialis anterior's function. Induced acceleration analysis demonstrated its ability to isolate the contributions of individual muscle to a given factor, and as a means of studying effect of pathological gait on the dynamic muscle functions.

Step length asymmetry is representative of compensatory mechanisms used in post-stroke hemiparetic walking

Post-stroke hemiparetic subjects walk with asymmetrical step lengths that are highly variable between subjects and may be indicative of the underlying impairments and compensatory mechanisms used. The goal of this study was to determine if post-stroke hemiparetic subjects grouped by step length asymmetry have similar abnormal walking biomechanics compared to non-impaired walkers. Kinematic and ground reaction force data were recorded from 55 hemiparetic subjects walking at their self-selected speed and 21 age and speed-matched non-impaired control subjects. Hemiparetic subjects were grouped by paretic step ratio, which was calculated as the paretic step-length divided by the sum of paretic and nonparetic step-lengths, into high (>0.535), symmetric (0.535-0.465) and low (<0.465) groups. Non-parametric Wilcoxin signed-rank tests were used to test for differences in joint kinetic measures between hemiparetic groups and speed-matched control subjects during late single-leg stance and pre-swing. The paretic leg ankle moment impulse was reduced in all hemiparetic subjects regardless of their paretic step ratio. The high group had increased nonparetic leg ankle plantarflexor and knee extensor moment impulses, the symmetric group had increased hip flexor moment impulses on both the paretic and nonparetic leg and the low group had no additional significant differences in joint moment impulses. These results suggest that the direction of asymmetry can be used to identify both the degree of paretic plantarflexor impairment and the compensatory mechanisms used by post-stroke hemiparetic subjects.

Braking and propulsive impulses increase with speed during accelerated and decelerated walking

The ability to accelerate and decelerate is important for daily activities and likely more demanding than maintaining a steady-state walking speed. Walking speed is modulated by anterior-posterior (AP) ground reaction force (GRF) impulses. The purpose of this study was to investigate AP impulses across a wide range of speeds during accelerated and decelerated walking. Kinematic and GRF data were collected from 10 healthy subjects walking on an instrumented treadmill. Subjects completed trials at steady-state speeds and at four rates of acceleration and deceleration across a speed range of 0-1.8 m/s. Mixed regression models were generated to predict AP impulses, step length and frequency from speed, and joint moment impulses from AP impulses during non-steady-state walking. Braking and propulsive impulses were positively related to speed. The braking impulse had a greater relationship with speed than the propulsive impulse, suggesting that subjects modulate the braking impulse more than the propulsive impulse to change speed. Hip and knee extensor, and ankle plantarflexor moment impulses were positively related to the braking impulse, and knee flexor and ankle plantarflexor moment impulses were positively related to the propulsive impulse. Step length and frequency increased with speed and were near the subjects' preferred combination at steady-state speeds, at which metabolic cost is minimized in nondisabled walking. Thus, these variables may be modulated to minimize metabolic cost while accelerating and decelerating. The outcomes of this work provide the foundation to investigate motor coordination in pathological subjects in response to the increased task demands of non-steady-state walking.

Immediate effects of different ankle pushoff instructions during walking exercise on hip kinematics and kinetics in individuals with total hip arthroplasty

Residual hip impairments, such as decreased hip muscle moment and power during walking, have been reported in patients with total hip arthroplasty (THA). Meanwhile, greater ankle power has also been reported in these patients. We investigated the interaction between hip and ankle joints during walking to determine the effects of different ankle pushoff instructions on hip biomechanics in patients with THA. Twenty-four women (age, 60.8±5.5 years) were randomly assigned to walking exercise groups with either decreased pushoff or increased pushoff. Patients in the decreased pushoff group and increased pushoff group were given the instructions "push less with your foot when you walk" and "push more with your foot when you walk," respectively. Exercises lasted approximately 10-15 min. A series of gait-related parameters were analyzed during pre-exercise, exercise, and post-exercise session. In the decreased ankle pushoff group, hip flexor power absorption and hip/ankle power ratio were higher during post-exercise than during pre-exercise. An increase in hip power from -9.8% to 32.1% was identified. The effect of increase in the hip power by the decreasing ankle pushoff was higher in the patients with greater ankle pushoff in their natural gaits. The patients in the increased ankle pushoff group showed decreased hip flexion angle and hip muscle moment and power after the walking exercise, although ankle pushoff was not increased. Walking exercise with decreased ankle pushoff may help improve the distribution of muscle power between hip flexors and ankle plantarflexors during walking in patients with THA.

The effect of prosthetic ankle energy storage and return properties on muscle activity in below-knee amputee walking

In an effort to improve amputee gait, energy storage and return (ESAR) prosthetic feet have been developed to provide enhanced function by storing and returning mechanical energy through elastic structures. However, the effect of ESAR feet on muscle activity in amputee walking is not well understood. Previous studies have analyzed commercial prosthetic feet with a wide range of material properties and geometries, making it difficult to associate specific ESAR properties with changes in muscle activity. In contrast, prosthetic ankles offer a systematic way to manipulate ESAR properties while keeping the prosthetic heel and keel geometry intact. In the present study, ESAR ankles were added to a Seattle Lightfoot2 to carefully control the energy storage and return by altering the ankle stiffness and orientation in order to identify its effect on lower extremity muscle activity during below-knee amputee walking. A total of five foot conditions were analyzed: solid ankle (SA), stiff forward-facing ankle (FA), compliant FA, stiff reverse-facing ankle (RA) and compliant RA. The ESAR ankles decreased the activity of muscles that contribute to body forward propulsion and increased the activity of muscles that provide body support. The compliant ankles generally caused a greater change in muscle activity than the stiff ankles, but without a corresponding increase in energy return. Ankle orientation also had an effect, with RA generally causing a lower change in muscle activity than FA. These results highlight the influence of ESAR stiffness on muscle activity and the importance of prescribing appropriate prosthetic foot stiffness to improve rehabilitation outcomes.

Leg extension is an important predictor of paretic leg propulsion in hemiparetic walking

Forward propulsion is a central task of walking that depends on the generation of appropriate anterior-posterior ground reaction forces (AP GRFs). The AP impulse (i.e., time integral of the AP GRF) generated by the paretic leg relative to the non-paretic leg is a quantitative measure of the paretic leg's contribution to forward propulsion and is variable across hemiparetic subjects. The purpose of this study was to investigate the underlying mechanisms of propulsion generation in hemiparetic walking by identifying the biomechanical predictors of AP impulses. Three-dimensional kinematics and GRFs were recorded from 51 hemiparetic and 21 age-matched control subjects walking at similar speeds on an instrumented treadmill. Hierarchical regression models were generated for each leg to predict the AP impulse from independent biomechanical variables. Leg extension was a significant predictor and positively related to the propulsive impulse in the paretic, non-paretic and control legs. Secondarily, the hip flexor moment impulse was negatively related to the propulsive impulse. Also, the relationship of paretic and non-paretic ankle moments with the propulsive impulse depended on the paretic step ratio, suggesting the plantar flexor contribution to the propulsive impulse depends on leg angle. These results suggest that increasing paretic leg extension will increase propulsion. Increasing paretic plantar flexor output and decreasing paretic hip flexor output could also increase paretic leg propulsion. While increased pre-swing hip flexor output has been suggested to compensate for decreased plantar flexor output, such output may further impair propulsion by the paretic leg if it occurs too soon in the gait cycle.

Pre-swing deficits in forward propulsion, swing initiation and power generation by individual muscles during hemiparetic walking

Clinical studies of hemiparetic walking have shown pre-swing abnormalities in the paretic leg suggesting that paretic muscle contributions to important biomechanical walking subtasks are different than those of non-disabled individuals. Three-dimensional forward dynamics simulations of two representative hemiparetic subjects with different levels of walking function classified by self-selected walking speed (i.e., limited community=0.4-0.8 m/s and community walkers = or > 0.8m/s) and a speed-matched control were generated to quantify individual muscle contributions to forward propulsion, swing initiation and power generation during the pre-swing phase (i.e., double support phase proceeding toe-off). Simulation analyses identified decreased paretic soleus and gastrocnemius contributions to forward propulsion and power generation as the primary impairment in the limited community walker compared to the control subject. The non-paretic leg did not compensate for decreased forward propulsion by paretic muscles during pre-swing in the limited community walker. Paretic muscles had the net effect to absorb energy from the paretic leg during pre-swing in the community walker suggesting that deficits in swing initiation are a primary impairment. Specifically, the paretic gastrocnemius and hip flexors (i.e., iliacus, psoas and sartorius) contributed less to swing initiation and the paretic soleus and gluteus medius absorbed more power from the paretic leg in the community walker compared to the control subject. Rehabilitation strategies aimed at diminishing these deficits have much potential to improve walking function in these hemiparetic subjects and those with similar deficits.

Age-related modifications of muscle synergies and spinal cord activity during locomotion

Recent findings have shown that neural circuits located in the spinal cord drive muscular activations during locomotion while intermediating between descending signals and peripheral sensory information. This relationship could be modified by the natural aging process. To address this issue, the activity of 12 ipsilateral leg muscles was analyzed in young and elderly people (7 subjects per group) while walking at six different cadences (40-140 steps/min). These signals were used to extract synergies underlying muscle activation and to map the motoneuronal activity of the pools belonging to the lumbosacral enlargement (L(2)-S(2)). The comparison between the two groups showed that neither temporal patterning of motor primitives nor muscles loading synergies seemed to be significantly affected by aging. Conversely, as the cadence increased, spinal maps differ significantly between the groups, showing higher and scattered activity during the whole gait cycle in elders and well-defined bursts in young subjects. The results suggested that motor primitives lead the synchronization of muscle activation mainly depending on the biomechanical demand of the locomotion; hence they are not significantly affected by aging. Nevertheless, at the spinal cord level, biomechanical requirements, peripheral afference, and descending inputs are differently integrated between the two groups, probably reflecting age-related changes of both nervous system and motor control strategies during locomotion.

Individual muscle contributions to the axial knee joint contact force during normal walking

Muscles are significant contributors to the high joint forces developed in the knee during human walking. Not only do muscles contribute to the knee joint forces by acting to compress the joint, but they also develop joint forces indirectly through their contributions to the ground reaction forces via dynamic coupling. Thus, muscles can have significant contributions to forces at joints they do not span. However, few studies have investigated how the major lower-limb muscles contribute to the knee joint contact forces during walking. The goal of this study was to use a muscle-actuated forward dynamics simulation of walking to identify how individual muscles contribute to the axial tibio-femoral joint force. The simulation results showed that the vastii muscles are the primary contributors to the axial joint force in early stance while the gastrocnemius is the primary contributor in late stance. The tibio-femoral joint force generated by these muscles was at times greater than the muscle forces themselves. Muscles that do not cross the knee joint (e.g., the gluteus maximus and soleus) also have significant contributions to the tibio-femoral joint force through their contributions to the ground reaction forces. Further, small changes in walking kinematics (e.g., knee flexion angle) can have a significant effect on the magnitude of the knee joint forces. Thus, altering walking mechanics and muscle coordination patterns to utilize muscle groups that perform the same biomechanical function, yet contribute less to the knee joint forces may be an effective way to reduce knee joint loading during walking.

A dynamic model of quadriceps and hamstrings function

The mechanical effect of hamstrings and quadriceps contractions on hip and knee joint motion was investigated using a dynamic model of the musculoskeletal system. The model consisted of 13 anatomically linked segments. The geometry of bones, joints, and muscle attachments was derived from magnetic resonance imaging of a healthy adult. The knee joint was represented by a crossing bars linkage to simulate cruciate ligament function, and muscles were represented by spring actuators. The effects of hamstring and quadriceps contractions, in various combinations, were tested on different configurations of hip and knee joint position in the absence of gravity. In the standing posture, with the foot free to move and the pelvis fixed in space, the effect of semimembranosus (SM) contraction was hip and knee flexion. If the foot was fixed to the ground, SM contraction produced hip extension and knee flexion. The addition of quadriceps contraction reduced or abolished the knee flexion and enhanced hip extension. In all other simulations, SM alone produced knee flexion and hip extension and the combination of SM with vastus (VA) and rectus femoris (RF) contractions resulted in knee extension and enhanced hip extension. Our findings suggest that co-contraction of quadriceps and hamstrings may be a strategy to increase the hip extension function of the hamstrings.

Merging of healthy motor modules predicts reduced locomotor performance and muscle coordination complexity post-stroke

Evidence suggests that the nervous system controls motor tasks using a low-dimensional modular organization of muscle activation. However, it is not clear if such an organization applies to coordination of human walking, nor how nervous system injury may alter the organization of motor modules and their biomechanical outputs. We first tested the hypothesis that muscle activation patterns during walking are produced through the variable activation of a small set of motor modules. In 20 healthy control subjects, EMG signals from eight leg muscles were measured across a range of walking speeds. Four motor modules identified through nonnegative matrix factorization were sufficient to account for variability of muscle activation from step to step and across speeds. Next, consistent with the clinical notion of abnormal limb flexion-extension synergies post-stroke, we tested the hypothesis that subjects with post-stroke hemiparesis would have altered motor modules, leading to impaired walking performance. In post-stroke subjects (n = 55), a less complex coordination pattern was shown. Fewer modules were needed to account for muscle activation during walking at preferred speed compared with controls. Fewer modules resulted from merging of the modules observed in healthy controls, suggesting reduced independence of neural control signals. The number of modules was correlated to preferred walking speed, speed modulation, step length asymmetry, and propulsive asymmetry. Our results suggest a common modular organization of muscle coordination underlying walking in both healthy and post-stroke subjects. Identification of motor modules may lead to new insight into impaired locomotor coordination and the underlying neural systems.

Simultaneous prediction of muscle and contact forces in the knee during gait

Musculoskeletal models are currently the primary means for estimating in vivo muscle and contact forces in the knee during gait. These models typically couple a dynamic skeletal model with individual muscle models but rarely include articular contact models due to their high computational cost. This study evaluates a novel method for predicting muscle and contact forces simultaneously in the knee during gait. The method utilizes a 12 degree-of-freedom knee model (femur, tibia, and patella) combining muscle, articular contact, and dynamic skeletal models. Eight static optimization problems were formulated using two cost functions (one based on muscle activations and one based on contact forces) and four constraints sets (each composed of different combinations of inverse dynamic loads). The estimated muscle and contact forces were evaluated using in vivo tibial contact force data collected from a patient with a force-measuring knee implant. When the eight optimization problems were solved with added constraints to match the in vivo contact force measurements, root-mean-square errors in predicted contact forces were less than 10 N. Furthermore, muscle and patellar contact forces predicted by the two cost functions became more similar as more inverse dynamic loads were used as constraints. When the contact force constraints were removed, estimated medial contact forces were similar and lateral contact forces lower in magnitude compared to measured contact forces, with estimated muscle forces being sensitive and estimated patellar contact forces relatively insensitive to the choice of cost function and constraint set. These results suggest that optimization problem formulation coupled with knee model complexity can significantly affect predicted muscle and contact forces in the knee during gait. Further research using a complete lower limb model is needed to assess the importance of this finding to the muscle and contact force estimation process.

Modular control of human walking: Adaptations to altered mechanical demands

Studies have suggested that the nervous system may adopt a control scheme in which synergistic muscle groups are controlled by common excitation patters, or modules, to simplify the coordination of movement tasks such as walking. A recent computer modeling and simulation study of human walking using experimentally derived modules as the control inputs provided evidence that individual modules are associated with specific biomechanical subtasks, such as generating body support and forward propulsion. The present study tests whether the modules identified during normal walking could produce simulations of walking when the mechanical demands were substantially altered. Walking simulations were generated that emulated human subjects who had their body weight and/or body mass increased and decreased by 25%. By scaling the magnitude of five module patterns, the simulations could emulate the subjects' response to each condition by simply scaling the mechanical output from modules associated with specific biomechanical subtasks. Specifically, the modules associated with providing body support increased (decreased) their contribution to the vertical ground reaction force when body weight was increased (decreased) and the module associated with providing forward propulsion increased its contribution to the positive anterior-posterior ground reaction force and positive trunk power when the body mass was increased. The modules that contribute to controlling leg swing were unaffected by the perturbations. These results support the idea that the nervous system may use a modular control strategy and that flexible modulation of module recruitment intensity may be sufficient to meet large changes in mechanical demand.