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William H. Clark and Jason R. Franz

The triceps surae muscle–tendon units are important in governing walking performance, acting to regulate mechanical behavior of the ankle through interaction between active muscle and passive elastic structures. Ankle joint quasi-stiffness (the slope of the relation between ankle moment and ankle rotation, kA) is a useful aggregate measure of this mechanical behavior. However, the role of muscle activation and length–tension behavior in augmenting kA remains unclear. In this study, 10 subjects completed eccentric isokinetic contractions at rest and at 2 soleus activation levels (25% and 75% isometric voluntary contraction) prescribed using electromyographic biofeedback. Ultrasound imaging quantified activation-dependent modulation of soleus muscle length–tension behavior and its role in augmenting kA. The authors found that soleus muscle stiffness (kM) and kA exhibit nonlinear relations with muscle activation and both were more sensitive to the onset of activation than to subsequent increases in activation. Our findings also suggest that kA can be modulated via activation through changes in soleus muscle length–tension behavior. However, this modulation is more complex than previously appreciated—reflecting interaction between active muscle and passive elastic tissues. Our findings may have implications for understanding normal and pathological ankle joint function and the design of impedance-based prostheses.

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Katie A. Conway and Jason R. Franz

The authors elucidated functional limitations in older adult gait by increasing horizontal impeding forces and walking speed to their maximums compared with dynamometry and with data from their young counterparts. Specifically, the authors investigated which determinants of push-off intensity represent genuine functionally limiting impairments in older adult gait versus biomechanical changes that do not directly limit walking performance. They found that older adults walked at their preferred speed with hallmark deficits in push-off intensity. These subjects were fully capable of overcoming deficits in propulsive ground reaction force, trailing limb positive work, trailing leg and hip extension, and ankle power generation when the propulsive demands of walking were increased to maximum. Of the outcomes tested, age-related deficits in ankle moment emerged as the lone genuine functionally limiting impairment in older adults. Distinguishing genuine functional limitations from age-related differences masquerading as limitations represents a critical step toward the development and prescription of effective interventions.

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Katie A. Conway, Randall G. Bissette and Jason R. Franz

Aging and many gait pathologies are characterized by reduced propulsive forces and ankle moment and power generation during trailing leg push-off in walking. Despite those changes, we posit that many individuals retain an underutilized reserve for enhancing push-off intensity during walking that may be missed using conventional dynamometry. By using a maximum ramped impeding force protocol and maximum speed walking, we gained mechanistic insight into the factors that govern push-off intensity and the available capacity thereof during walking in young subjects. We discovered in part that young subjects walking at their preferred speed retain a reserve capacity for exerting larger propulsive forces of 49%, peak ankle power of 43%, and peak ankle moment of 22% during push-off—the latter overlooked by maximum isometric dynamometry. We also provide evidence that these reserve capacities are governed at least in part by the neuromechanical behavior of the plantarflexor muscles, at least with regard to ankle moment generation. We envision that a similar paradigm used to quantify propulsive reserves in older adults or people with gait pathology would empower the more discriminate and personalized prescription of gait interventions seeking to improve push-off intensity and thus walking performance.