Muscular strength and power are correlated with motor unit action potential amplitudes, but not myosin heavy chain isoforms in sedentary males and females

• Published in: Journal of biomechanics Vol. 86; pp. 251 - 255
• Main Authors: Herda, Trent J., Trevino, Michael A., Sterczala, Adam J., Miller, Jonathan D., Wray, Mandy E., Dimmick, Hannah L., Gallagher, Philip M., Fry, Andrew C.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 27-03-2019; Elsevier Limited
• Subjects: Action potential; Action Potentials - physiology; Adult; Algorithms; Amplitudes; Chains; Conflicts of interest; Correlation; Decomposition; Electromyography; EMG decomposition; Female; Females; Humans; Isoforms; Isokinetic; Male; Males; Motor unit action potential amplitude; Motor units; Motors; Muscle contraction; Muscle Contraction - physiology; Muscle cross-sectional area; Muscle Fibers, Skeletal - metabolism; Muscle strength; Muscle Strength - physiology; Muscle, Skeletal - diagnostic imaging; Muscle, Skeletal - physiology; Myosin; Myosin heavy chain; Myosin Heavy Chains - metabolism; Protein Isoforms; Quadriceps Muscle - physiology; Sedentary Behavior; Sex; Sex differences; Sex Factors; Slopes; Torque; Ultrasonic imaging; Ultrasonography; Ultrasound; Muscle cross-sectional area; EMG decomposition; Motor unit action potential amplitude; Isokinetic; Myosin heavy chain; Ultrasound
• ISSN: 0021-9290; 1873-2380
• DOI: 10.1016/j.jbiomech.2019.01.050

It remains unclear if the sizes of higher-threshold motor units (MU) are associated with muscular strength and power. Therefore, the purpose of this study was to examine sex-related differences in muscle cross-sectional area (mCSA), percent myosin heavy chain (%MHC) isoform expression, and the MU action potential amplitudes (MUAPAMPS)-recruitment threshold (RT) relationships of the vastus lateralis and isometric peak torque, isokinetic peak torque and mean power at 1.05 rad·s−1 of the leg extensors. Surface electromyographic decomposition techniques were used to quantify MUAPAMPS recorded during isometric muscle actions at 70% of maximal voluntary contractions and regressed against RTs with the slopes calculated. Ultrasound images were used to measure mCSA. Males had greater slopes from the MUAPAMP-RT relationship than the females (P < 0.05). The greater slopes likely reflected larger higher-threshold MUs for the males. The mCSAs and slopes from the relationships were strongly correlated with isometric and isokinetic peak torque and isokinetic mean power (r = 0.78–0.82), however, type I %MHC isoform was only moderately correlated with isometric peak torque (r = −0.54). The results indicated that sex-related differences in muscular strength and power were associated more so with the sizes of the higher-threshold MUs (slopes) and mCSA than MHC isoforms. The amount of cross-bridge activity within muscle fibers that comprise higher-threshold MUs may be the primary contributor to muscular strength and power rather than the contractile properties of the muscle.