Abstract
This study investigated the effect of isometrically induced fatigue on Hill-type muscle model parameters and related task-dependent effects. Parameter identification methods were used to extract fatigue-related parameter trends from isometric and ballistic dynamic maximum voluntary knee extensions. Nine subjects, who completed ten fatiguing sets, each consisting of nine 3 s isometric maximum voluntary contractions with 3 s rest plus two ballistic contractions with different loads, were analyzed. Only at the isometric task, the identified optimized model parameter values of muscle activation rate and maximum force generating capacity of the contractile element decreased from 20.8 pm 8.4 to 11.2 pm 4.1 Hz and from 18{,}137 pm 150 to 10{,}666 pm 2139 N, respectively. For all tasks, the maximum efficiency of the contractile element, mathematically related to the curvature of the force–velocity relation, increased from 0.35 pm 0.04 to 0.42 pm 0.05. The model parameter maximum contraction velocity decreased from 0.93 pm 0.1 to 0.9 pm 0.1 m/s and the stiffness of the serial elastic element from 1936 pm 227 to 1432 pm 245 N/mm. Thus, models of fatigue should consider fatigue dependencies in active as well as in passive elements, and muscle activation dynamics should account for the task dependency of fatigue.
Highlights
Human movement is possible through the interaction of several structures including nervous system, muscles, tendons, and bones
The identification of different values of nmax for isometric task (ISO) and BAL contractions is in accordance with previous model tests (Penasso and Thaller 2017)
In 12.1% of all datasets, the identification routine converged toward bound constraints and these data had to be replaced with estimations based on the EM algorithm
Summary
Human movement is possible through the interaction of several structures including nervous system, muscles, tendons, and bones. The magnitude and the effect of fatigue will depend on the modality of the movement, e.g., task dependency (Enoka and Stuart 1992; Taylor et al 2016), contraction type (Babault et al 2006; Griffin et al 2000; Liu et al 2003), contraction velocity (Harwood et al 2011; Morel et al 2015), and the level of achieved voluntary activation (Bigland-Ritchie et al 1978; Gandevia et al 1998; Sidhu et al 2013)
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