Effects of fatiguing, submaximal high‐ versus low‐torque isometric exercise on motor unit recruitment and firing behavior

The purpose of this investigation was to evaluate the effects of repeated, high‐ (HT: 70% MVIC) versus low‐torque (LT: 30% MVIC) isometric exercise performed to failure on motor unit (MU) recruitment and firing behavior of the vastus lateralis. Eighteen resistance‐trained males (23.1 ± 3.8 years) completed familiarization, followed by separate experimental sessions in which they completed either HT or LT exercise to failure in random order. LT exercise resulted in a greater time to task failure and a more dramatic decline in the muscle's force capacity, but the total work completed was similar for HT and LT exercise. An examination of the firing trains from 4670 MUs recorded during exercise revealed that firing rates generally increased during HT and LT exercise, but were higher during HT than LT exercise. Furthermore, recruitment thresholds (RT) did not significantly change during HT exercise, whereas the RT of the smallest MUs increased and the RT for the moderate to large MUs decreased during LT exercise. Both HT and LT exercise resulted in the recruitment of additional higher threshold MUs in order to maintain torque production. However, throughout exercise, HT required the recruitment of larger MUs than did LT exercise. In a few cases, however, MUs were recruited by individuals during LT exercise that were similar in size and original (pre) RT to those detected during HT exercise. Thus, the ability to achieve full MU recruitment during LT exercise may be dependent on the subject. Consequently, our data emphasize the task and subject dependency of muscle fatigue. This study expands on recent work examining the motor unit control scheme during muscle fatigue by examining the effect of varying torque levels on motor unit behavior. Our data suggest that as excitation to the motor neuron pool increases during fatigue, motor unit firing rates increase during both high‐ and low‐torque exercise. However, changes in motor unit recruitment thresholds were dependent on torque level. Our data also suggest that the net output of the motor neuron pool during fatigue at low‐torques may be constrained in comparison to high‐torques, although the degree to which this was true was dependent on the subject.