Properties of sprouted rat motor units: effects of period of enlargement and activity level.

The effects of short and prolonged partial denervation of lateral gastrocnemius muscles in sedentary and active rats (running) were examined. In PD muscles of sedentary animals the mean motor unit (MU) tetanic force after 30 days was not different than that measured after 90 days. Increased locomotor activity over the same period (voluntary running, approximately equal to 6 km/day) resulted in an increase in mean MU tetanic force of enlarged MUs (28%). The absence of a significant increase in mean muscle fiber area suggested an activity-related enhancement of motoneuron sprouting. However, the small magnitude of this increase, relative to the potential for further sprouting, indicates the activity effect is not strong and may be partly due to fiber area changes not evident with whole muscle analysis. Nonetheless, these data demonstrate that daily locomotor activity can enhance the tension-generating capacity of chronically enlarged MUs.