Fatigue During Maximal Cycling is Determined by the Number of Cycles

Abstract

Fatigue during maximal cycling has been reported to increase with increased pedaling rate. Increasing pedaling rate imposes two changes on the neuromuscular system: decreased time for muscle excitation and relaxation, and increased muscle shortening velocity. Using two crank lengths allows the effects of pedal speed and pedaling rateto be dissected. PURPOSE: Our purposes for conducting this investigation were to determine whether pedaling rate or pedal speed was mainly responsible for the increased fatigue previously observed at increased pedaling rates and to explore other possible fatigue mechanisms. METHODS: Ten trained cyclists performed 30-second maximal isokinetic cycling trials using two crank lengths; 120 and 220 mm. Pedaling rate was optimized for maximal power production for each crank length: 135 rpm for the 120 mm cranks (1.7 m/s pedal speed) and 109 rpm for the 220 mm cranks (2.5 m/s pedal speed). Power was recorded with a SRM power meter. RESULTS: Peak power did not differ: 901 ± 309 W for 120 mm vs. 898 ± 311 W for 220 mm. Fatigue index was greater (57.5 ± 8.4% vs. 51.1 ± 11.3%; p< 0.01) and total work (18.0 ± 5.5 kJ vs. 19.2 ± 5.9 kJ; p< 0.01) was less with the 120 mm cranks. Regression analysis indicated that cumulative crank revolutions accounted for 99.4% of the variability in power throughout the trials for the combined data for both crank lengths. CONCLUSION: These results support previous findings of increased fatigue with increased pedaling rate and eliminate the confounding effects of pedal speed and unequal initial peak power. The novel result from our data was that power decreased by a specific amount with each crank cycle, independent of time or work. The mechanism responsible for our results is likely membrane depolarization but additional research must be performed to confirm this mechanism.