Skeletal muscle fibers hypertrophy in response to strength training, with type II fibers generally demonstrating the greatest plasticity in regards to cross-sectional area (CSA). However, assessing fiber type specific CSA in humans requires invasive muscle biopsies. With advancements in the decomposition of surface electromyographic (sEMG) signals recorded using multi-channel electrode arrays, the firing properties of individual motor units (MU) can now be detected non-invasively. Since action potential amplitude (APSIZE) has a documented relationship with muscle fiber size, as well as with its' parent MU's recruitment threshold (RT) force, our purpose was to examine if MU APSIZE, as a function of its' RT (i.e. the size principle), could potentially be used as a longitudinal indicator of MU specific hypertrophy. By decomposing the sEMG signals from the vastus lateralis muscle of 10 subjects during maximal voluntary knee extensions, we non-invasively assessed the relationship between MU APSIZE and RT before and immediately after an 8 week strength training intervention. In addition to significant increases in muscle size and strength (p < 0.02), our data show that training elicited an increase in MU APSIZE of high-threshold MUs. Additionally, a large portion of the variance (83.6%) in the change in each individual's relationship between MU APSIZE and RT was explained by training-induced changes in whole muscle CSA (obtained via ultrasonography). Our findings suggest that the non-invasive, electrophysiological assessment of longitudinal changes to MU APSIZE appears to reflect hypertrophy specific to MUs across the RT continuum.
- size principle
- surface EMG decomposition
- strength training
- skeletal muscle fiber type
- Copyright © 2016, Journal of Neurophysiology