Ischemic Preconditioning Blunts Loss of Knee Extensor Torque Complexity with Fatigue

Introduction: Neuromuscular fatigue reduces the temporal structure, or complexity, of muscle torque output, purportedly through an effect on motor unit behaviour. Ischaemic pre-conditioning (IPC), an emerging ergogenic aid, has been demonstrated to have a potent effect on muscular output and endurance. We therefore tested the hypothesis that IPC would attenuate the fatigueinduced loss of muscle torque complexity. Methods: Ten healthy participants (6 male/4 female) performed intermittent isometric knee extension contractions (6 s contraction, 4 s rest) to task failure at 40% maximal voluntary contraction (MVC). Contractions were preceded by either IPC (three bouts of 5 minutes proximal thigh occlusion at 225 mmHg, interspersed with 5 minutes rest) or SHAM (as IPC, but occlusion at only 20 mmHg) treatments. Torque and EMG signals were sampled continuously. Complexity and fractal scaling were quantified using approximate entropy (ApEn) and the detrended fluctuation analysis (DFA) α scaling exponent. Muscle oxygen consumption (mV̇O\(_2\)) was determined using near-infrared spectroscopy. Results: IPC increased time to task failure by 43 ± 13% (mean ± SEM, P = 0.047). Complexity decreased in both trials (decreased ApEn, increased DFA α; both P < 0.001), though the rate of decrease was significantly lower following IPC (ApEn, –0.2 ± 0.1 vs. –0.4 ± 0.1, P = 0.013; DFA α, 0.2 ± 0.1 vs. 0.3 ± 0.1, P = 0.037). Similarly, the rates of increase in EMG amplitude (P = 0.022) and mV̇O\(_2\) (P = 0.043) were significantly slower following IPC. Conclusion: These results suggest the ergogenic effect of IPC observed here is of neural origin and accounts for the slowing of the rates of change in torque complexity, EMG amplitude and muscle oxygen consumption as fatigue develops.