Purpose: Although it is anecdotally believed that fluctuating running speed during competition has a negative impact on running performance, its influence on physiological strain remains unclear. Therefore, the purpose of the present study was to determine the effect of fluctuating running speed during 10-km running on physiological strain compared with constant running speed. Methods: Ten male endurance runners completed 2 trials on different days, consisting of either fluctuating (FLU) or constant (CON) running speed during 10-km running on a treadmill. In the CON trial, running speed was constant at 80% of maximal oxygen uptake (?VO2max), while running speed fluctuated between 75% and 85% of VO2max in the FLU trial. Blood lactate levels were evaluated before and after running. Moreover, oxygen uptake (VO2) and expired minute ventilation (VE) were measured with running. Results: The time to complete 10-km running was the same between the 2 trials (FLU 38.6 [5.1] min, CON 38.6 [5.1] min). The FLU trial showed greater exercise-induced blood lactate elevation compared with the CON trial (FLU 4.5 [1.5] mmol/L, CON 2.9 [1.4] mmol/L). The mean values of VO2 (FLU 48.6 [3.5] mL·kg-1·min-1, CON 47.1 [3.6] mL·kg-1·min-1) and VE (FLU 95 [14] L/min, CON 87 [11] L/min) during running were significantly higher in the FLU trial than in the CON trial. Conclusion: Fluctuating running speed elevated physiological strain compared with constant running speed during 10-km running, even if the time to complete 10-km running was the same.
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