Age-related changes in the twitch contractile properties of human thenar motor units

Doherty, Timothy J., and William F. Brown. Age-relatedchanges in the twitch contractile properties of human thenar motorunits. J. Appl. Physiol. 82(1):93-101, 1997.The purpose of this study was to examine theeffects of aging on the contractile and electrophysiological propertiesof human thenar motor units (MUs). Percutaneous electrical stimulationof single motor axons within the median nerve was used to isolate andexamine the twitch tensions, contractile speeds, and surface-detectedMU action potential (S-MUAP) sizes of 48 thenar MUs in 17 youngersubjects (25-53 yr) and 44 thenar MUs in 9 older subjects(64-77 yr). A wide range of twitch tensions, contractile speeds,and S-MUAP sizes was observed in both age groups. However, oldersubjects had significantly larger MU twitch tensions and slower MUtwitch contraction and half-relaxation times. These changes wereaccompanied by increased S-MUAP sizes. These findings suggest that thehuman thenar MU pool undergoes significant age-related increase in MUsize and slowing of contractile speed. Such adaptation may help toovercome previously reported age-related losses of thenar MUs.