Differential changes in muscle oxygenation between voluntary and stimulated isometric fatigue of human dorsiflexors

The purpose of this study was to compare fatigue and recovery of maximal voluntary torque [ maximal voluntary contraction (MVC)] and muscle oxygenation after voluntary (Vol) and electrically stimulated (ES) protocols of equal torque production. On 1 day, 10 male subjects [25 yr (SD 4)] completed a Vol fatigue protocol and, on a separate day, an ES fatigue protocol of the right dorsiflexors. Each task involved 2 min of intermittent (2-s on, 1-s off) isometric contractions at 50% of MVC. For the ES protocol, stimulation was delivered percutaneously to the common peroneal nerve at a frequency of 25 Hz. Compared with the Vol protocol, the ES protocol caused a greater impairment in MVC (75 vs. 83% prefatigue value; Pre) and greater increase in 50-Hz half relaxation time (165 vs. 117% Pre) postexercise. After acute (1 min) recovery, MVC impairment was similar for both protocols, whereas 50-Hz half relaxation time was still greater in the ES than Vol protocol. Total hemoglobin decreased to a similar extent in both protocols during exercise, but it was elevated above the resting value to a significantly greater extent for the ES protocol in recovery (18 vs. 11 mu M). Oxygen saturation was significantly lower in the ES than Vol protocol during exercise (46 vs. 57% Pre), but it was significantly greater during recovery ( 120 vs. 105% Pre). These findings suggest that despite, equal torque production, ES contractions impose a greater metabolic demand on the muscle that leads to a transient greater impairment in MVC. The enforced synchronization and fixed frequency of excitation inherent to ES are the most likely causes for the exacerbated changes in the ES compared with the Vol protocol.