Motor unit number estimates in masters runners: use it or lose it?

INTRODUCTION A contributing factor to the loss of muscle mass and strength during aging is the reduction in the number of functioning motor units (MU). It has been shown that lifelong physically active older rats have greater numbers of MU compared with age-matched sedentary controls, suggesting that chronic exercise may preserve MU function with advancing age. This has not previously been examined in humans. PURPOSE Thus, the purpose of this study was to estimate the number of functioning MU in the tibialis anterior of masters runners (approximately 65 yr) and to compare the values with recreationally active young (approximately 25 yr) and healthy age-matched controls (approximately 65 yr). METHODS Decomposition-enhanced spike-triggered averaging was used to collect surface and intramuscular EMG signals during dorsiflexion at 25% of maximum voluntary isometric contraction. RESULTS The estimated number of MU did not differ between masters runners and young, but MU number estimates were lower in the old (91 +/- 22 MU) compared with the masters runners (140 +/- 53 MU) and young (150 +/- 43 MU). CONCLUSION These results demonstrate that lifelong high-intensity physical activity could potentially mitigate the loss of MU associated with aging well into the seventh decade of life.