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 to 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 to the all-finger (~54% reduction, p=0.009) fatiguing task. A task engaging a larger musculature resulted in less peripheral impairment compared to 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.