Neuromuscular fatigue and recovery in maximal compared to explosive strength loading.

The purpose of this study was to examine acute neuromuscular fatigue and its recovery in men (n = 8) and women (n = 8). Two strenuous, but clearly different exercises were compared: maximal (MSL) versus explosive strength loading (ESL). The MSL included five sets of ten repetition maximum bilateral leg extensions. The same task and the same number of sets was performed in ESL but with 40% from the load used in MSL and performed as explosively as possible. Isometric force-time curves were measured during maximal voluntary bilateral isometric action of the leg extensors before the fatigue loading and after each set. The measurements were repeated after resting for 1 h, 2 h, 1 day and two days. Surface elecrtomyogram (EMG) activity was recorded from the vastus lateralis, vastus medialis and rectus femoris muscles. Blood samples from the fingertips were also taken throughout the experiment to determine blood lactate concentration. Marked changes took place in both sexes in force production during both MSL and ESL but the overall decreases were greater and the recovery was slower after MSL. Pronounced decreases occurred also in maximal integrated EMG (iEMG). In the men, the decrease in iEMG for the early contraction phase (0-100 ms) during ESL was greater (P < 0.05) than that of MSL, whereas the decrease in iEMG in the peak force phase (500-1500 ms) was similar. As expected, the increase in blood lactate concentration was greater during MSL in both sexes. It was concluded that heavy resistance loading may result in considerable acute fatigue of central and peripheral origin. The reduced electrical activity in the muscles accompanied by an accumulation of blood lactate led to marked decreases in strength. Explosive type loading, especially in men, appeared to lead primarily to central fatigue with less involvement of peripheral fatigue than MSL. The women seemed unable to fatigue themselves as much as the men, particularly in ESL.