Introduction
Constant power output may be attained by muscle contractions at different combinations of force and velocity, e.g. cycling at constant power output may be performed with a variety of pedal rates, PR. Gross efficiency, GE, depends on the combination of force and velocity during cyclic movements and GE has been shown to decrease with increasing PR above approx. 60 rpm. Surprisingly, cyclists may, according to perceived exertion ratings, prefer higher PR than those resulting in maximal GE when these variables are studied at a limited number of preset PR. However, with such protocol design the freely chosen pedaling rate, PRFC, is not determined directly and subjects have not a continuum of PR to select from, which may conceal subtle individual differences. Our intend was to examine the effect of PR on GE during cycling with preset PR and PRFC, respectively.
Methods
Twenty male subjects performed 5 min cycling sessions on a motorized treadmill with 61, 88, and 115 rpm, termed PR61, PR88 and PR115, as well as PRFC. Power output was adjusted to result in 40% and 70% VO2max by adjusting the weight attached to a wire running over a pulley and tied in behind the bicycle. The SRM crank system and Douglas bag methods were used to measure power output and oxygen uptake, respectively, GE being calculated as the ratio between the two.
Results
The PRFC showed a wide range between subjects: 56-88 rpm at 40% VO2max and 61-102 rpm at 70% VO2max. However, individual values of GE were relatively similar between subjects: 15-23% at 40% VO2max and 19-23% at 70% VO2max, and were unrelated to the PRFC at both exercise intensities. In contrast GE at PR61, PR88 and PR115 showed larger ranges: 12-23% at 40% VO2max and 15-26% at 70% VO2max. Also, significant decreases were found in GE with increasing PR when testing group mean values (ANOVA), and high correlation's with negative regression line slopes were found for both 40% VO2max (GE = 26.21-0.94PR, R=0.76, p<0.001) and 70% VO2max (GE = 26.15-0.65PR, R=0.65, p<0.001). Interestingly, GE at 70% VO2max with 61 rpm, was positively related to PRFC at 70% VO2max (GE = 16.17-0.07PRFC, R=0.46, p<0.05).
Conclusions
In spite of large inter individual differences in PRFC no relationship between this and GE was found. In contrast, a negative relationship was found between PR and GE on group mean values, corresponding with previous results (Gaesser & Brooks (1975)). The positive relationship between PRFC and GE at 70% VO2max with 61 rpm indicates that subjects choosing high PR may have a fundamentally higher GE. This could be caused by higher percentage of slow twitch muscle fibers, since a positive relationship between % ST fibers and GE has been reported (Coyle et al. (1992)).
References
1. Coyle, E. F., Sidossis, L. S., Horowitz, J. F. & Beltz, J. D. 1992. Med Sci Sports Exerc 24, 782-788.
2. Gaesser, G. A. & Brooks, G. A. 1975. J Appl Physiol 38, 1132-1139.