Fatigue and recovery in children and adults during sustained contractions at 2 different submaximal intensities

The purpose of this study was to examine the effects of submaximal sustained contractions on fatigue and recovery properties in untrained prepubescent boys (n = 14) and men (n = 14). All participants performed, in random order, an isometric plantar flexion at 20% and 60% of their maximal voluntary contraction (MVC) until exhaustion (20%(fatigue) and 60%(fatigue), respectively). During both fatigue protocols, surface electromyogram (sEMG) of the soleus and tibialis anterior muscles was recorded. Before and after the fatigue protocol, MVCs and blood lactate concentration were obtained. During 20%(fatigue) and 60%(fatigue), agonist and antagonist sEMG increased gradually to a similar extent in both groups. Immediately after fatigue, MVC torque decreased in both groups, compared with prefatigue values, and boys recovered faster than men in both fatigue protocols. The reduction in agonist sEMG during MVC after fatigue was not significantly different between men and boys (p < 0.05), independent of the fatigue protocol. sEMG of boys recovered to baseline values 3 min after the 20%(fatigue) and 60%(fatigue) protocols, whereas men did not recover. Lactate concentration increased immediately after the end of the fatigue protocols, compared with the prefatigue values (p < 0.05). This increase was higher for the 60%(fatigue) than for 20%(fatigue) protocol, and did not differ between boys and men. It is concluded that low-and middle-intensity submaximal sustained isometric fatigue protocols induce similar fatigue effects in boys and men. Furthermore, it was shown that boys can recover faster than men, independent of the contraction intensity that induces fatigue.