A motor unit-based model of muscle fatigue

Muscle fatigue is a temporary decline in the force and power capacity of skeletal muscle resulting from muscle activity. Because control of muscle is realized at the level of the motor unit (MU), it seems important to consider the physiological properties of motor units when attempting to understand and predict muscle fatigue. Therefore, we developed a phenomenological model of motor unit fatigue as a tractable means to predict muscle fatigue for a variety of tasks and to illustrate the individual contractile responses of MUs whose collective action determines the trajectory of changes in muscle force capacity during prolonged activity. An existing MU population model was used to simulate MU firing rates and isometric muscle forces and, to that model, we added fatigue-related changes in MU force, contraction time, and firing rate associated with sustained voluntary contractions. The model accurately estimated endurance times for sustained isometric contractions across a wide range of target levels. In addition, simulations were run for situations that have little experimental precedent to demonstrate the potential utility of the model to predict motor unit fatigue for more complicated, real-worldapplications. Moreover the model provided insight, into the complex orchestration of MU force contributions during fatigue, that would be unattainable with current experimental approachesAuto21 Network of Centres of Excellence [A506-AWH]; National Institutes of Health [R01NS079147]Open access journal.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Wrist rotations about one or two axes affect maximum wrist strength

© 2015 Elsevier Ltd and The Ergonomics Society. Most wrist strength studies evaluate strength about one axis, and postural deviations about that same axis. The purpose of this study was to determine if wrist posture deviations about one axis (e.g. flexion/extension), or two axes (e.g. flexion/extension and pronation/supination), affect the strength about another axis (e.g. ulnar deviation). A custom-built instrumented handle was used to measure maximum static isometric torque exertions at 18 wrist postures (combinations of flexion/extension, radial/ulnar deviation, and pronation/supination). Ulnar deviation torques were highest when the wrist was in neutral. This pattern was not maintained for the other torque directions; the generated torque tended to be highest when the wrist posture was not neutral. The effects were similar for male and female subjects, although male subjects exerted significantly larger torques in all directions. This study illustrates that there is a complex relationship between wrist posture and maximal wrist torques

The decline in force capacity with a 100% MVC load, from Bigland-Ritchie et al [36], Bigland-Ritchie [37], Kent-Braun et al [38], Jones et al [39], and Kennedy et al. [40], are compared to the fatigue model output with and without excitation adaptation.

<p>The decline in force capacity with a 100% MVC load, from Bigland-Ritchie et al [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005581#pcbi.1005581.ref036" target="_blank">36</a>], Bigland-Ritchie [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005581#pcbi.1005581.ref037" target="_blank">37</a>], Kent-Braun et al [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005581#pcbi.1005581.ref038" target="_blank">38</a>], Jones et al [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005581#pcbi.1005581.ref039" target="_blank">39</a>], and Kennedy et al. [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005581#pcbi.1005581.ref040" target="_blank">40</a>], are compared to the fatigue model output with and without excitation adaptation.</p

The total muscle and motor unit capacities for initial target forces of 15, 50 and 85% until total muscle capacity decreased to 15% MVC (ie. 85% muscle fatigue for each trial).

<p>(A) Fatigue model outputs for total muscle capacity. Arrows indicate when force fell below 15% MVC. (B) Final force capacity of each MU, normalized to its rested capacity, when total muscle capacity reached 15% MVC for each initial force condition (shown with a vertical arrow in 9A).</p

Fatigue model outputs for a sustained 100% MVC load for 200 s.

<p>(A) Increased excitation in response to fatigue. Force capacity is shown with and without firing rate adaptation and the modeled force remains at the target load until the endurance time. (B) Firing rate of each MU, over the course of the trial. Lines begin when the MU was recruited. Each 20th MU is highlighted and labelled, but all 120 MUs are shown as lighter lines. (C) Force contribution of each MU. (D) Relative force capacity of each MU (normalized to its rested capacity). Note the higher y-axis scale (57) than with the 20% MVC (22), 50% MVC (30), and 80% MVC (50) force.</p

Fatigue model outputs for a sustained 50% MVC load with an endurance time of 95.5 s.

<p>(A) Increased excitation in response to fatigue. Force capacity is shown with and without firing rate adaptation and the modeled force remains at the target load until the endurance time. (B) Firing rate of each MU, over the course of the trial. Lines begin when the MU was recruited. Each 20th MU is highlighted and labelled, but all 120 MUs are shown as lighter lines. (C) Force contribution of each MU. (D) Relative force capacity of each MU (normalized to its rested capacity). Note the higher y-axis scale (30) than with the 20% MVC force (22).</p