Changes in muscle activation can prolong the endurance time of a submaximal isometric contraction in humans

Fourteen young subjects (7 men and 7 women) performed a fatiguing isometric contraction with the elbow flexor muscles at 20% of maximal voluntary contraction (MVC) force on three occasions. Endurance time for session 3 [1,718+/-1,189 (SD) s] was longer than for session 1 (1,225+/-683 s) and session 2 (1,410+/-977 s). Five men and four women increased endurance time between session 1 and 3 by 60+/-28% (responders), whereas two men and three women did not (-3+/-11%; nonresponders). The MVC force was similar for the responders and nonresponders, both before and after the fatiguing contraction. Fatiguing contractions were characterized by an increase in the electromyogram. (EMG) amplitude and number of bursts during the fatiguing contractions. The responders achieved a similar level of EMG at exhaustion but a reduced rate of increase in the EMG across sessions. The rate of increase in EMG across sessions declined for the nonresponders, but it remained greater than that of the responders. The increase in burst rate during the contractions declined across sessions with a negative relation between burst rate and endurance time (r=-0.42). Normalized force fluctuations increased during the fatiguing contractions, and there was a positive relation (r=0.60) between the force fluctuations and burst rate. Changes in mean arterial pressure and heart rate during the fatiguing contraction were similar for the responders and nonresponders across the three sessions. The results indicate that those subjects who increased the endurance time of a submaximal contraction across three sessions did so by altering the level and pattern of muscle activation.