Effects of continuous cycling training on motor unit firing rates, input excitation, and myosin heavy chain of the vastus lateralis in sedentary females

• Published in: Experimental brain research Vol. 240; no. 3; pp. 825 - 839
• Main Authors: Trevino, Michael A., Dimmick, Hannah L., Parra, Mandy E., Sterczala, Adam J., Miller, Jonathan D., Deckert, Jake A., Gallagher, Philip M., Fry, Andrew C., Weir, Joseph P., Herda, Trent J.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-03-2022; Springer; Springer Nature B.V
• Subjects: Action Potentials - physiology; Aerobics; Analysis; Biomedical and Life Sciences; Biomedicine; Biopsy; Biopsy, Needle; Cycling; Decomposition; Electromyography; Female; Females; Firing rate; Health aspects; Humans; Isoforms; Isometric Contraction - physiology; Methods; Muscle, Skeletal - physiology; Myosin; Myosin Heavy Chains; Neurology; Neurosciences; Quadriceps Muscle - diagnostic imaging; Quadriceps Muscle - physiology; Recruitment, Neurophysiological - physiology; Research Article; Sedentary behavior; Strengthening exercises; Torque; United States; Myosin heavy chain; Endurance training; Vastus lateralis; Motor unit
• ISSN: 0014-4819; 1432-1106
• DOI: 10.1007/s00221-021-06278-3

This study examined the effects of continuous endurance training on motor unit (MU) mean firing rates (MFR), percent myosin heavy chain (%MHC) isoforms, and muscle cross-sectional area (mCSA) of the vastus lateralis (VL). Twelve females completed 5-weeks of continuous cycling-training (CYC), while 8 females were controls (CON). Participants performed maximal voluntary contractions (MVCs) and 40% MVCs of the knee extensors before (PRE) and after the 5-week treatment period at the same absolute pre-treatment submaximal torque (POST
ABS
) and relative to post-treatment MVCs (POST
REL
). Surface electromyographic (EMG) signals were decomposed with the Precision Decomposition III algorithm. MU firing times and waveforms were validated with reconstruct-and-test and spike trigger average procedures. MFRs at steady torque, recruitment thresholds (RT), and normalized EMG amplitude (N-EMG
RMS
) were analyzed.
Y
-intercepts and slopes were calculated for the MFR vs. RT relationships. MHC isoforms and mCSA were determined with muscle biopsies and ultrasonography. CYC decreased MVCs and type IIX %MHC isoform without changes in mCSA. The slopes for the MFR vs. RT relationships decreased for CYC during POST
REL
and POST
ABS
while N-EMG
RMS
increased for POST
ABS
with no differences between PRE and POST
REL
. Type I %MHC isoform was correlated with the slope for the MFR vs. RT relationship during POST
ABS
and POST
REL
for CYC. This study provides evidence that decreases in the MFRs of higher threshold MUs post-CYC is likely a function of changes in input excitation (POST
ABS
) and the firing frequency–excitation relationships (POST
REL
). Evidence is provided that MHC isoforms influence the firing rate scheme of the muscle following short-term training.