Task-dependent contributions to finger flexion fatigue: a comparison of single- and multi-digit contractions

Several physiological factors contribute to the task-dependent nature of fatigue, including the amount of muscle mass engaged. Activation of group III and IV muscle afferents may be a critical factor regulating fatigue responses between tasks engaging a smaller or larger musculature. Finger flexion tasks provide a unique means to regulate the amount of active muscle mass by altering the number of fingers engaged in a contraction (i.e., single vs. all); however, the central and peripheral fatigue responses between these tasks are unknown. In 14 young adults (n = 7 females), maximal voluntary contraction (MVC) force, 100 Hz doublet force, and voluntary activation were recorded before and after a sustained finger flexion task involving either the index in isolation or all digits in unison at 40% MVC until task failure. At task termination, voluntary activation was similarly impaired for all digits irrespective of which digits were engaged in the fatiguing task (∼10%-20% reduction, P = 0.209), however, the index finger displayed a greater reduction in 100 Hz force following single (∼46% reduction) compared with the all-finger fatiguing task (∼27% reduction, P = 0.019). During all-finger MVCs, the force produced by the index finger was more impaired following the single (∼68% reduction) compared with the all-finger (∼54% reduction, P = 0.009) fatiguing task. A task engaging a larger musculature resulted in less peripheral impairment compared with a smaller muscle mass task, presumably due to a sensory tolerance being reached due to a more diffuse ensemble activation of group III and IV afferents at task termination.NEW & NOTEWORTHY By controlling the number of fingers engaged in a fatiguing task, we show that engaging a smaller musculature alters the degree of peripheral fatigue at task termination, presumably due to a sensory limit achieved from group III and IV muscle afferents. Moreover, irrespective of which finger was fatigued, reductions in neural drive are present to all fingers at task termination, indicating the synergistic control of the fingers are maintained in the fatigued state.