Regulation of pacing strategies during successive 4-km time trials.

PURPOSE Athletes adopt a pacing strategy to delay fatigue and optimize athletic performance. However, many current theories of the regulation of muscle function during exercise do not adequately explain all observed features of such pacing strategies. We studied power output, oxygen consumption, and muscle recruitment strategies during successive 4-km cycling time trials to determine whether alterations in muscle recruitment by the central nervous system could explain the observed pacing strategies. METHODS Seven highly trained cyclists performed three consecutive 4-km time trial intervals, each separated by 17 min. Subjects were instructed to perform each trial in the fastest time possible but were given no feedback other than distance covered. Integrated electromyographic (iEMG) readings were measured at peak power output anti from 90 s before the end of each trial. RESULTS Subjects attained VO2 values similar to their VO2(peak) in each interval. Time taken to complete the first and third intervals was similar. Peak power output was highest in the first interval, but average power output, oxygen consumption, heart rate, and postexercise plasma lactate concentrations were not different between intervals. Power output and iEMG activity rose similarly during the final 60 s in all intervals but was not different between trials. CONCLUSION The increase in power output and the parallel upward trend in iEMG at the end of each interval indicate that the iEMG changes "tracked" the power output changes dynamically and that therefore the observed pacing strategies were not regulated by peripheral mechanisms. Rather, these findings are compatible with the action of a centrally regulated mechanism that alters the number of motor units that are recruited and de-recruited during exercise based upon peripheral feedback or anticipatory feed-forward.