AGE-RELATED-CHANGES IN THE TWITCH CONTRACTILE PROPERTIES OF HUMAN THENAR MOTOR UNITS

The purpose of this study was to examine the effects of aging on the c ontractile and electrophysiological properties of human thenar motor u nits (MUs). Percutaneous electrical stimulation of single motor axons within the median nerve was used to isolate and examine the twitch ten sions, contractile speeds, and surface-detected MU action potential (S -MUAP) sizes of 48 thenar MUs in 17 younger subjects (25-53 yr) and 44 thenar MUs in 9 older subjects (64-77 yr). A wide range of twitch ten sions, contractile speeds, and S-MUAP sizes was observed in both age g roups. However, older subjects had significantly larger MU twitch tens ions and slower MU twitch contraction and half-relaxation times. These changes were accompanied by increased S-MUAP sizes. These findings su ggest that the human thenar MU pool undergoes significant age-related increase in MU size and slowing of contractile speed. Such adaptation may help to overcome previously reported age-related losses of thenar MUs.