Biomechanical Determinants of Metabolic Cost of Submaximal Cycling Power

Abstract

Previous authors have reported that metabolic cost increases linearly with increasing net mechanical cycling power output. This metabolic cost has also been reported to increase with increasing pedaling rate. Several investigators have speculated that the mechanism responsible for the effect of pedaling rate might be increased negative work. PURPOSE: We conducted this study to determine the relationship of metabolic cost of producing submaximal cycling power with summed net and positive powers at the ankle, knee, and hip, and to test the hypothesis that positive joint powers would account for previously reported differences in metabolic cost associated with different pedaling rates. METHODS: Five trained cyclists [31.8±8 yrs, 178±9 cm, 74.8±11 kg] cycled at 60, 90, and 120 rev/min, at intensities of 30, 60, and 90% of ventilatory threshold. Pedal forces, and pedal and crank position were sampled at 120 Hz for 20 sec. Hip position and geometrically calculated leg kinematics were determined from 20 sec of recorded motion capture data. Net joint moments and reaction forces were determined using inverse dynamic techniques. Ankle, knee, and hip joint powers were calculated as the product of net joint moment and joint angular velocity and power transferred across the hip joint was calculated as the dot product of joint reaction force and joint linear velocity. Pedal force and position data were recorded in the fifth minute of each trial. Expired gases were collected throughout the trial and values during the fifth minute were averaged and used to calculate metabolic cost via the Weir equation. Multiple linear regression analyses were used to determine the relationship between metabolic cost and pedaling rate with two measures of power: net joint power and net positive joint power. RESULTS: Both regression analyses produced significant relationships (R2 > 0.94, p <0.001) but the pedaling rate coefficient was positive when predicting net joint power but negative when predicting positive joint power. CONCLUSIONS: Our novel finding was that the metabolic cost of producing positive power decreased with increasing pedaling rate. We interpret these results to suggest that a pedaling rate of 120 rpm, which has been reported to be the optimal pedaling rate for producing maximum cycling power, allowed for more efficient positive joint power production. The commonly reported increase in metabolic cost associated with increasing pedaling rate appears to be due to the additional power required to overcome negative joint powers. Whether negative powers represent poor technique or are required to stabilize the 2 degree of freedom leg system is a topic of ongoing investigation in our lab.