Exercise performance during repetitive sprints is carefully regulated by the central motor drive to limit the development of peripheral fatigue upon a critical threshold
Résumé
Introduction Recently, Amann et al. (2009) demonstrated that somatosensory feedback from working muscle plays a metabo-meter role for the central nervous system so that the central motor drive is adapted to limit the peripheral fatigue development upon a critical threshold. Because during 5 km time trials voluntary muscle contractions are submaximal it is argued that the central motor command and power output are much more dependent upon a conscious self-paced cycling strategy rather than feedback from the contracting muscles (Marcora, 2010). We asked whether inputs from the fatiguing locomotor muscles reduce the maximal voluntary drive of central motor command, in order to tightly regulate the total degree of peripheral fatigue development during all-out sprints. Methods Twelve healthy subjects randomly performed, at least 72 h apart, two sets of 10 all-out 10 sec sprints intersped with 30 sec of passive recovery. One test was performed in the fresh state (control condition), the other with pre-induced quadriceps fatigue using neuromuscular electrical stimulation (NMES). NMES - and exercise - induced quadriceps fatigue was quantified via pre vs. post changes in potentiated quadriceps twitch force in response to supra-maximal femoral nerve stimulation (Qtw-pot) and in maximal isometric voluntary contraction (MVC). Central motor drive was estimated using the integral of each burst of the quadriceps EMG (iEMG). Mean power output was assessed for every sprint as an index of performance. Results NMES pre-induced quadriceps fatigue (ΔQs-pot = −28.9% ± 3,9 ; ΔMVC = −15.0% ± 3,1) resulted in a significant reduction in central motor drive (−8.6% ± 2,5) and performance (−3.2% ± 1,9) compared to control, which was more pronounced during the first five sprints of the test. Despite these significant differences, the magnitude of locomotor muscle fatigue following sprints was not different between control and pre-fatigue conditions (ΔQs-pot = −50.6% ± 2.8 vs. −50.6% ± 3.4; ΔMVC = −18.3% ± 3.4 vs. −20.0% ± 4.2). Discussion Our findings demonstrate that power output is tightly regulated during repetitive all-out sprints to avoid the development of peripheral muscle fatigue upon an individual critical level. We thus suggest that feedback from the fatiguing locomotor muscles exerts an inhibitory effect on the central motor drive and plays a key role in exercise performance. References Amann M, Proctor LT, Sebranek JJ, Pegelow DF, Dempsey JA. (2009). J Physiol, 587, 271-283. Marcora S. (2010). J Appl Physiol, 108, 454-456.