ADAPTATIONS OF RAT LATERAL GASTROCNEMIUS MOTOR UNITS IN RESPONSE TO VOLUNTARY RUNNING

This study investigated the effects of 12 wk of voluntary wheel runnin g on motor units from rat lateral gastrocnemius. Motor units were isol ated via ventral root splitting (L(5)) from active or sedentary rats a nd were classified into slow, fast-fatigue-resistant, and fast-fatigab le (FF) units. An overall increase in mean motor unit tetanic tension (35%) was accompanied by a decrease in mean motor unit fatigue resista nce (-10%). These adaptations were localized in the fast unit populati on but varied among fast motor unit subtypes. The overall increase in tetanic force was due primarily to increases in fast-fatigue-resistant units (300%), whereas changes in fatigue resistance (-43%) were confi ned to FF units. However, the changes seen with activity may have been partly obscured by classifying fast motor units based on fatigability , since a significant increase in tetanic force accompanied by a decre ased twitch one-half relaxation time was apparent in units falling in the midrange of the tetanic force continuum and included a number of F F units. These data provide direct demonstration of nonuniform motor u nit adaptations subsequent to increases in normal functional activity.