The aim of the present study was to unravel the integration among component processes that jointly lead to a fatigue-induced spontaneous termination point (FISTP), and to pinpoint possible mechanisms underlying this phenomenon. On 5 days during 2 weeks, six participants, who were familiar with the task, performed a quasi-isometric arm-curl exercise holding an Olympic bar with an initial elbow flexion of 90 degrees to the point of spontaneous termination of the exercise due to exhaustion. The repeated measurements ANOVA of windowed variance measures based on the time series of the elbow angles revealed a highly significant effect of the exertion time on the intra- and inter-trial enhancement of the elbow-angle variability when approaching a FISTP. Spectral analysis showed that the variability was generated predominantly in time spans of 1 to about 10s suggesting that slower and hence higher order control loops are destabilized close to termination points. There was a significant, positive correlation between termination angle and angle variability. The discontinuous change of the elbow angle at the moment of exercise termination was preceded by an enhancement of intra- and inter-trial fluctuations of the elbow angle. This may hint at a nonlinear coupling between the participating neuromuscular components or, more generally, a nonlinear dynamical process underlying the FISTP phenomenon. This dynamical characteristic may indeed explain why other accounts based on separable sites of local, physiological limitations fail to elucidate the occurrence of FISTP.
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