Electrically and hybrid-induced muscle activations: effects of muscle size and fiber type


Submitted: 15 July 2016
Accepted: 15 July 2016
Published: 13 September 2016
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Authors

  • Kelly Stratton Biomedical Engineering Department at the University of Connecticut, Storrs, CT, United States.
  • Pouran D. Faghri Allied Health Sciences and Biomedical Engineering Departments at the University of Connecticut, Storrs, CT, United States.
The effect of three electrical stimulation (ES) frequencies (10, 35, and 50 Hz) on two muscle groups with different proportions of fast and slow twitch fibers (abductor pollicis brevis (APB) and vastus lateralis (VL)) was explored. We evaluated the acute muscles’ responses individually and during hybrid activations (ES superimposed by voluntary activations). Surface electromyography (sEMG) and force measurements were evaluated as outcomes. Ten healthy adults (mean age: 24.4 ± 2.5 years) participated after signing an informed consent form approved by the university Institutional Review Board. Protocols were developed to: 1) compare EMG activities during each frequency for each muscle when generating 25% Maximum Voluntary Contraction (MVC) force, and 2) compare EMG activities during each frequency when additional voluntary activation was superimposed over ES-induced 25% MVC to reach 50% and 75% MVC. Empirical mode decomposition (EMD) was utilized to separate ES artifacts from voluntary muscle activation. For both muscles, higher stimulation frequency (35 and 50Hz) induced higher electrical output detected at 25% of MVC, suggesting more recruitment with higher frequencies. Hybrid activation generated proportionally less electrical activity than ES alone. ES and voluntary activations appear to generate two different modes of muscle recruitment. ES may provoke muscle strength by activating more fatiguing fast acting fibers, but voluntary activation elicits more muscle coordination. Therefore, during the hybrid activation, less electrical activity may be detected due to recruitment of more fatigue-resistant deeper muscle fibers, not reachable by surface EMG.

Stratton, K., & Faghri, P. D. (2016). Electrically and hybrid-induced muscle activations: effects of muscle size and fiber type. European Journal of Translational Myology, 26(3). https://doi.org/10.4081/ejtm.2016.6163

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