Julien D. Pe ́riard • Matthew N. Cramer • Phillip G. Chapman • Corinne Caillaud • Martin W. Thompson
Accepted: 8 December 2010 / Published online: 28 December 2010 Springer-Verlag 2010
Muscle weakness following constant load exercise under heat stress has been associated with hyperthermia-induced central fatigue. However, evidence of central fatigue influencing intense self-paced exercise in the heat is lacking. The purpose of this investigation was to evaluate force production capacity and central nervous system drive in skeletal muscle pre- and post-cycle ergometer exercise in hot and cool conditions. Nine trained male cyclists performed a 20-s maximal voluntary isometric contraction (MVC) prior to (control) and following a 40-km time trial in hot (35C) and cool (20C) conditions. MVC force production and voluntary activation of the knee extensors was evaluated via percutaneous tetanic stimulation. In the cool condition, rectal temperature increased to 39.0C and reached 39.8C in the heat (P\0.01). Following exercise in the hot and cool conditions, peak force declined by ~90 and ~99 N, respectively, compared with control (P\0.01). Mean force decreased by 15% (hot) and 14% (cool) (P\0.01 vs. control). Voluntary activation during the post-exercise MVC declined to 93.7% (hot) and 93.9% (cool) (P\0.05 vs. control). The post-exercise decline in voluntary activation represented *20% of the decrease in mean force production in both conditions. Therefore, the additional increase in rectal temperature did not exacerbate the loss of force production following self-paced exercise in the heat. The impairment in force production indicates that the fatigue exhibited by the quadriceps is mainly of peripheral origin and a consequence of the prolonged contractile activity associated with exercise.
The influence of heat stress on exercise performance is well documented, with several studies suggesting that exercise in the heat is impaired by the effects of hyperthermia on the central nervous system (CNS) (Nielsen et al. 1993; Nielsen and Nybo 2003; Nybo and Nielsen 2001). Nielsen et al. (1993) initially reported force production to be unaltered during a brief maximal voluntary isometric contraction (MVC) of the knee extensors after cycling to exhaustion in the heat. This observation was supported by Nybo and Nielsen (2001) who noted a similar ability to maintain force production and voluntary activation during brief (2–3 s) repeated MVCs following constant load cycling in thermoneutral and hyperthermic conditions. However, force production capacity of both exercised (quadriceps) and non-exercised (hand grip flexors) muscle groups was impaired during a sustained (2 min) MVC after exercise in the heat. Superimposed electrical stimulation during the sustained MVC revealed the impairment was associated with a progressive decline in voluntary activation. It was proposed that the attainment of a high body core temperature reduced CNS drive to exercising muscles (i.e. central fatigue) (Nybo and Nielsen 2001). This hypothesis of a ‘‘critical’’ core temperature limiting CNS drive to exercising muscles is indirectly supported by both animal (Fuller et al. 1998; Walters et al. 2000) and human studies.