Motor unit loss is accompanied by decreased peak muscle power in the lower limb of older adults

This study investigated the relationship between motor unit (MU) properties and the isometric strength and power of two lower limb muscles in healthy young and older adults. Twelve older adults (6 men, mean age, 77 +/- 5 years) and twelve young adults (6 men, mean age, 24 +/- 3 years) were studied. MU properties of the tibialis anterior (TA) and vastus medialis (VM) muscles were determined electrophysiologically using decomposition-enhanced spike-triggered averaging (DE-STA). Motor unit number estimates (MUNEs) of the TA were significantly reduced (p < 0.05) in older adults (102 +/- 76) compared to young adults (234 +/- 109), primarily as a result of significantly larger surface-detected motor unit potentials (S-MUPs) in older adults (63 +/- 29 mu V) compared to young adults (27 +/- 14 mu V). Although VM S-MUP values were larger in older adults (60 +/- 31 mu V) compared to young (48 +/- 42 mu V), the difference was not significant. Maximal isometric strength was significantly larger in both the TA and knee extensors of young adults (TA: 0.56 Nm/kg, KE: 22 Nm/kg) compared to old (TA: 0.4 Nm/kg, KE: 1.3 Nm/kg). Similar reductions in peak muscle power were observed between young (TA: 33W, KE: 357W) and old adults (TA: 26W, KE: 224W). The greatest deficit between young and old subjects in peak power output occurred at 20% MVC for the TA and 40% MVC for the knee extensors. Results from this study indicate that there are changes in MU properties with age, and that this effect may be greater in the more distal TA muscle. Further, this study demonstrates that muscle power may be a sensitive marker of changes in neuromuscular function with aging. (C) 2015 Elsevier Inc. All rights reserved.