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

Maximum force (F-max) 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% F-max) to failure using both limbs assigned as Arm(control) or Arm(training). Experiment 2 involved five bouts of intermittent calf contractions (60% F-max) to failure using the same limb where data from the longest single trial (Calf(single)) or averaged across five bouts (Calf(averaged)) were analysed. F-max 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 Arm(control) (9/10), Arm(training) (10/10), Calf(single) (7/10) and Calf(averaged) (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. (C) 2014 Elsevier Ltd. All rights reserved.