Time Required for the Restitution of Extra Initial Force in Fast Motor Units of the Rat Medial Gastrocnemius Muscle

During tetanic contractions of fast motor units (MUs), an early increase in force (boost) is followed by a slight decline to a plateau (sag). This boost is present at the onset of activity but disappears during rhythmically repeated contractions at short intervals; however, it may be restituted after a period of rest. Nevertheless, the background of the boost, especially the minimum time required to restore this effect, is unknown, and the present study aimed to address this gap.

The functional isolation of a single MU was achieved by splitting the L5 or L4 ventral roots into thin filaments, which were electrically stimulated with rectangular electrical pulses. We recorded a series of three 504-ms tetanic contractions (triplets) evoked at 35 Hz repeated once per second, with the boost visible in the first tetanus. The triplets were evoked at progressively shorter time intervals, ranging from 90 to 2 s.

The boost in successive triplets reduced when the intervals became critically short. The reduction of boost was estimated by the decrease in sag amplitude following the peak force associated with the boost. In fast fatigue-resistant MUs, the sag amplitude decreased by 25% at an average interval of 29.80 s and by 50% at an interval of 16.84 s. For fast fatigable MUs, the interval required to restitute the studied effect was longer: a 25% reduction in sag corresponded to an interval of 82.29 s, and a 50% reduction corresponded to 54.07 s.

The results suggest that the physiological basis for the restitution of initial force (boost) is the kinetics of muscle energy status recovery following initial exercise.