Altered Achilles Tendon Morphology in Individuals With Chronic Post-Stroke Hemiparesis: A Case Report

Background: Individuals post-stroke walk slowly and with more effort, which puts them at higher risks for falls. The slow walking speed results from insufficient propulsive forces generated by the paretic leg. Current rehabilitative efforts to improve walking function target increasing propulsive forces, but overlook the muscle-tendon unit. Case presentations: Two individuals with chronic post-stroke hemiparesis are presented. In both individuals post-stroke, paretic ankle plantarflexors presented with increased muscle tone. Gait kinetics revealed asymmetric propulsive forces, specifically, insufficient propulsive forces by the paretic legs, consistent with previous literature. Sonography revealed increased thickness of paretic Achilles tendon at the calcaneal insertion, in both stroke cases, in contrast to comparable Achilles tendon thickness between limbs in the non-neurologically impaired controls. Conclusion: Tendon unit integrity should be considered in individuals post-stroke who demonstrate abnormal muscle tone and insufficient propulsion during gait

Dynamics of Human Walking

The problem of biped locomotion at steady speeds is discussed through a Lagrangian formulation developed for velocity-dependent, body driving forces. Human walking on a level surface is analyzed in terms of the data on the resultant ground-reaction force and the external work. It is shown that the trajectory of the center of mass is due to a superposition of its rectilinear motion with a given speed and a backward rotation along a shortened hypocycloid. A stiff-to-compliant crossover between walking gaits is described and the maximum speed for human walking, given by an instability of the trajectory, is predicted. Key words: locomotion, integrative biology, muscles, bipedalism, human walking, biomechanics.Comment: 9 pages, 4 figure

Muscle Synergies Facilitate Computational Prediction of Subject-Specific Walking Motions.

Researchers have explored a variety of neurorehabilitation approaches to restore normal walking function following a stroke. However, there is currently no objective means for prescribing and implementing treatments that are likely to maximize recovery of walking function for any particular patient. As a first step toward optimizing neurorehabilitation effectiveness, this study develops and evaluates a patient-specific synergy-controlled neuromusculoskeletal simulation framework that can predict walking motions for an individual post-stroke. The main question we addressed was whether driving a subject-specific neuromusculoskeletal model with muscle synergy controls (5 per leg) facilitates generation of accurate walking predictions compared to a model driven by muscle activation controls (35 per leg) or joint torque controls (5 per leg). To explore this question, we developed a subject-specific neuromusculoskeletal model of a single high-functioning hemiparetic subject using instrumented treadmill walking data collected at the subject's self-selected speed of 0.5 m/s. The model included subject-specific representations of lower-body kinematic structure, foot-ground contact behavior, electromyography-driven muscle force generation, and neural control limitations and remaining capabilities. Using direct collocation optimal control and the subject-specific model, we evaluated the ability of the three control approaches to predict the subject's walking kinematics and kinetics at two speeds (0.5 and 0.8 m/s) for which experimental data were available from the subject. We also evaluated whether synergy controls could predict a physically realistic gait period at one speed (1.1 m/s) for which no experimental data were available. All three control approaches predicted the subject's walking kinematics and kinetics (including ground reaction forces) well for the model calibration speed of 0.5 m/s. However, only activation and synergy controls could predict the subject's walking kinematics and kinetics well for the faster non-calibration speed of 0.8 m/s, with synergy controls predicting the new gait period the most accurately. When used to predict how the subject would walk at 1.1 m/s, synergy controls predicted a gait period close to that estimated from the linear relationship between gait speed and stride length. These findings suggest that our neuromusculoskeletal simulation framework may be able to bridge the gap between patient-specific muscle synergy information and resulting functional capabilities and limitations

Biomechanics and Energetics of Bipedal Locomotion on Uneven Terrain.

Humans navigate uneven terrain in their everyday lives. From trails, grass, and uneven sidewalks, we constantly adapt to various surfaces in our environment. Past research has shown that walking on natural terrain, compared to walking on smooth flat surfaces, results in increased energy expenditure during locomotion. However, the biomechanical adaptations responsible for this energetic increase are unclear, since locomotion research is often conducted either on short walkways or in an outdoor setting, thus limiting data collections. To further our understanding of human locomotion on uneven terrain, I focused on quantifying the biomechanical and energetic changes due to increased terrain variability during walking and running. First, this thesis presents modifications to a regular exercise treadmill to allow for attachment of a separate uneven surface. Using this treadmill, I collected kinetic, kinematic, electromyographic, and energy expenditure data during continuous human walking and running. I showed that humans walking at 1.0m/s on an uneven surface, with a 2.5cm height variability, increased energy expenditure by 0.73W/kg (approx. 28%) compared to walking on smooth terrain. Greater energy expenditure was primarily caused by increased positive work at the hip and knee, with minor contributions from increased muscle activity and step parameter adaptations. I then showed that running at 2.3m/s on the same surface resulted in an energetic increase of 0.48W/kg (approx. 5%) compared to running on even terrain. In contrast to walking, humans compensated for uneven terrain during running by reducing positive work produced by the ankle and adapting a more crouched leg posture. The similar absolute increases in energetic cost between walking and running implied that much of this increase is likely due to surface height variability and changes in mechanical work. Finally, this work presents analytical and simulated analyses for the rimless wheel and simplest walker models. These analyses explored the relationship between gait dynamics, energy input strategies, surface unevenness and the energetic cost of walking. Together, these studies advance our understanding of the relationship between mechanics and energetics of human walking on uneven surfaces and could potentially lead to more robust and energetically efficient legged robots, prostheses and more effective clinical rehabilitation interventions.PhDKinesiology and Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111616/1/voloshis_1.pd

The influence of push-off timing in a robotic ankle-foot prosthesis on the energetics and mechanics of walking

Background: Robotic ankle-foot prostheses that provide net positive push-off work can reduce the metabolic rate of walking for individuals with amputation, but benefits might be sensitive to push-off timing. Simple walking models suggest that preemptive push-off reduces center-of-mass work, possibly reducing metabolic rate. Studies with bilateral exoskeletons have found that push-off beginning before leading leg contact minimizes metabolic rate, but timing was not varied independently from push-off work, and the effects of push-off timing on biomechanics were not measured. Most lower-limb amputations are unilateral, which could also affect optimal timing. The goal of this study was to vary the timing of positive prosthesis push-off work in isolation and measure the effects on energetics, mechanics and muscle activity. Methods: We tested 10 able-bodied participants walking on a treadmill at 1.25 m.s(-1). Participants wore a tethered ankle-foot prosthesis emulator on one leg using a rigid boot adapter. We programmed the prosthesis to apply torque bursts that began between 46% and 56% of stride in different conditions. We iteratively adjusted torque magnitude to maintain constant net positive push-off work. Results: When push-off began at or after leading leg contact, metabolic rate was about 10% lower than in a condition with Spring-like prosthesis behavior. When push-off began before leading leg contact, metabolic rate was not different from the Spring-like condition. Early push-off led to increased prosthesis-side vastus medialis and biceps femoris activity during push-off and increased variability in step length and prosthesis loading during push-off. Prosthesis push-off timing had no influence on intact-side leg center-of-mass collision work. Conclusions: Prosthesis push-off timing, isolated from push-off work, strongly affected metabolic rate, with optimal timing at or after intact-side heel contact. Increased thigh muscle activation and increased human variability appear to have caused the lack of reduction in metabolic rate when push-off was provided too early. Optimal timing with respect to opposite heel contact was not different from normal walking, but the trends in metabolic rate and center-of-mass mechanics were not consistent with simple model predictions. Optimal push-off timing should also be characterized for individuals with amputation, since meaningful benefits might be realized with improved timing

Musculoskeletal stiffness and Achilles tendon mechanical property changes following exercise-induced muscle damage

This thesis investigated the affect of exercise induced muscle damage (EIMD) on musculoskeletal stiffness (MSS), Achilles tendon (AT) stiffness and AT strain. Furthermore, this thesis determined the reliability of a protocol used to measure MSS with the aim to apply this protocol in the investigation of the EIMD associated changes in MSS, AT stiffness and AT strain. Three studies were conducted as part of this thesis

On the effects of variable leg-spring properties during hopping

Das Federmassemodell, auch spring-loaded inverted pendulum (SLIP), beschreibt die Schwerpunktsbewegung biologischer Laufsysteme. Dieses Modell bildet die Beine als lineare Federn mit konstanten Parametern ab. In biologischen Systemen können sich federartige Eigenschaften der Gliedmaßen jedoch zeitlich ändern. Daher wurde in der vorliegenden Arbeit untersucht, inwieweit Variationen der Federparameter während des Bodenkontaktes die Dynamik des Federmassemodells beeinflussen