Empirical modelling of the dynamic response of fatigue during intermittent submaximal contractions of human forearm and calf muscles

Maximum force (Fmax) declines during intermittent submaximal contractions, but the linearity of this fatigue response and number of underlying phases is not clear. Healthy men were studied during two experiments (n = 10 each). Experiment 1 involved single bouts of intermittent forearm contractions (50% Fmax) to failure using both limbs assigned as Armcontrol or Armtraining. Experiment 2 involved five bouts of intermittent calf contractions (60% Fmax) to failure using the same limb where data from the longest single trial (Calfsingle) or averaged across five bouts (Calfaveraged) were analysed. Fmax was assessed at 25–30 s intervals during exercise and fitted to ten mono- and biphasic functions consisting of linear and/or nonlinear terms. For each fatigue response, the function which provided the best fit was determined on statistical grounds. Biphasic functions provided the majority of best fits during Armcontrol(9/10), Armtraining (10/10), Calfsingle (7/10) and Calfaveraged (9/10). For each condition, linear functions provided the best fit in 4–5 out of 10 responses. Two biphasic functions differentiated only by their first term (linear versus exponential) provided the best fit for 29/40 fatigue responses. These outcomes suggest that fatigue during intermittent contractions exhibits a biphasic response characterised by nonlinear and linear behaviour.