title: Action potential amplitude as a noninvasive indicator of motor unit-specific hypertrophy.
abstract: 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 multichannel electrode arrays, the firing properties of individual motor units (MUs) can now be detected noninvasively. 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 noninvasively assessed the relationship between MU APSIZE and RT before and immediately after an 8-wk 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 noninvasive, electrophysiological assessment of longitudinal changes to MU APSIZE appears to reflect hypertrophy specific to MUs across the RT continuum.
authors.affiliation: Applied Musculoskeletal and Human Physiology Laboratory, Oklahoma State University, Stillwater, Oklahoma.
authors.forename: Zachary K
authors.initials: ZK
authors.lastname: Pope
authors.affiliation: Applied Musculoskeletal and Human Physiology Laboratory, Oklahoma State University, Stillwater, Oklahoma.
authors.forename: Garrett M
authors.initials: GM
authors.lastname: Hester
authors.affiliation: Applied Musculoskeletal and Human Physiology Laboratory, Oklahoma State University, Stillwater, Oklahoma.
authors.forename: Franklin M
authors.initials: FM
authors.lastname: Benik
authors.affiliation: Applied Musculoskeletal and Human Physiology Laboratory, Oklahoma State University, Stillwater, Oklahoma Jason.defreitas@okstate.edu.
authors.forename: Jason M
authors.identifier: 0000-0002-5108-8079
authors.initials: JM
authors.lastname: DeFreitas
mesh_terms: D000200:Action Potentials; D000328:Adult; D019054:Evoked Potentials, Motor; D006801:Humans; D008297:Male; D009119:Muscle Contraction; D053580:Muscle Strength; D052097:Quadriceps Muscle / Q000000981:diagnostic imaging / Q000294:innervation / Q000502:physiology*; D011999:Recruitment, Neurophysiological; D055070:Resistance Training; D014463:Ultrasonography
keywords: size principle; skeletal muscle fiber type; strength training; surface EMG decomposition