Muscle Strength and Size are Associated with Motor Unit Connectivity in Aged Mice

Title: Muscle Strength and Size are Associated with Motor Unit Connectivity in Aged Mice In older adults, loss of muscle strength (dynapenia) and loss of muscle mass (sarcopenia) are important contributors to loss of physical function. We sought to investigate dynapenia, sarcopenia, and loss of motor unit function in aging mice. C57BL/6J mice were analyzed with cross sectional (males: 3 vs. 27 months; males and females: 8 vs. 12 vs. 20 months) and longitudinal studies (males: 10-25 months) using in vivo electrophysiological measures of motor unit connectivity (triceps surae compound muscle action potential and motor unit number estimation), in vivo measures of plantar flexion torque, MRI hindlimb muscle volume, and grip strength. Compound muscle action potential amplitude, motor unit number estimation, and plantar flexion torque were decreased at 20 months. In contrast, grip strength was reduced at 24 months. Motor unit number estimates correlated with muscle torque and hindlimb muscle volume. Our results demonstrate that loss of motor unit connectivity is an early finding in aging male and female mice and that muscle size and contractility are both associated with motor unit number.