Evidence for neuromuscular fatigue during high-intensity cycling in warm, humid conditions

The purpose of this study was to examine and describe the neuromuscular cha nges associated with fatigue using a self-paced cycling protocol of 60-min duration, under warm, humid conditions. Eleven subjects [mean (SE) age 21.8 (0.8) years; height 174.9 (3.0) cm; body mass 74.8 (2.7) kg; maximum oxyge n consumption 50.3 (1.8) ml . kg . min(-1)] performed one 60-min self-paced cycling time trial punctuated with six 1-min "all out" sprints at 10-min i ntervals, while 4 subjects repeated the trial for the purpose of determinin g reproducibility. Power output, integrated electromyographic signal (IEMG) , and mean percentile frequency shifts (MPFS) were recorded at the mid-poin t of each sprint. There were no differences between trials for EMG variable s, distance cycled, mean heart rate, and subjective rating of perceived exe rtion for the subjects who repeated the trial (n = 4). The results from the repeated trials suggest that neuromuscular responses to self-paced cycling are reproducible between trials. The mean heart rate for the 11 subjects w as 163.6 (0.71) beats . min(-1). Values for power output and IEMG expressed as a percentage of that recorded for the initial sprint decreased during s prints 2-5, with normalised values being 94%, 91%, 87% and 87%, respectivel y, and 71%, 71%, 73%, and 77%, respectively. However, during the final spri nt normalised power output and IEMG increased to 94% and 90% of initial val ues, respectively. MPFS displayed an increase with time; however, this was not significant (P = 0.06). The main finding of this investigation is the a bility of subjects to return power output to near initial values during the final of six maximal effort sprints that were included as part of a self-p aced cycling protocol. This appears to be due to a combination of changes i n neuromuscular recruitment, central or peripheral control systems, or the EMG signal itself. Further investigations in which changes in multiple phys iological systems are assessed systematically are required so that the unde rlying mechanisms related to the development of fatigue during normal dynam ic movements such as cycling can be more clearly delineated.