Quadriceps muscle strength, contractile properties, and motor unit firing rates in young and old men

Changes with age in the voluntary static and dynamic strength of the quadri ceps muscle group have been well characterized. and the importance of the m uscle group for locomotion and independent living have been highlighted in both normal human aging and in clinical studies. Surprisingly few studies o f this muscle group have described age-related changes in voluntary activat ion ability using twitch interpolation and changes in stimulated contractil e properties, and none have assessed the influence of old age on motor unit firing rates. We compared in 13 young (mean age 26 years) and 12 old (mean age 80 years) men the voluntary isometric strength, stimulated contractile properties, and average steady state motor unit firing rates in the quadri ceps muscle. Maximum voluntary contraction (MVC) force and twitch tension w ere similar to 50% lower in the old men, but contractile speed was only sim ilar to 10% slower than in the young men. There was no difference in the ab ility of either group to activate the quadriceps to a high degree (94-96%), At all isometric force levels tested (10%, 25%, 50%, 75%, and 100% MVC), t here were no differences in mean motor unit firing rates. In both groups, t he range of firing rates was similar and not large (similar to 8 Hz at 10% MVC and 26 Hz at MVC), Thus, the substantial age-related weakness in this m uscle does not seem to be related to changes in neural drive. (C) 1999 John Wiley & Sons, Inc.