Oxygen cost of internal work during cycling

Abstract

The energy cost of internal work and its relationships with lower limb mass and pedalling frequency were studied in four male subjects [age 22.2 (SD 1.5) years, body mass 81.0 (SD 5.1) kg, maximal O₂ uptake ($\dot VO_{2\max }$) above resting 3.06 (SD 0.4) l · min⁻¹]. The subjects cycled at 40, 60, 80 and 100 rpm and at five different exercise intensities for every pedalling frequency (unloaded condition, UL); the same exercises were repeated after having increased the lower limbs' masses by 40% (loaded condition, L). The exercise intensities were chosen so that the oxygen consumption ($\dot VO_2$) did not exceed 75% of$\dot VO_{2\max }$. For all the subjects and all the conditions, the rate of$\dot VO_2$ above resting increased linearly with the mechanical power ($\dot W$). They-intercepts of the linear regressions of$\dot VO_2$ on$\dot W$, normalised per kilogram of overall lower limbs' mass were the same in both UL and L and increased with the 4.165 power of pedalling frequency (f _p). These intercepts were taken to represent the metabolic counterpart of the internal power dissipation in cycling; they amounted to 0.78, 0.34, 3.29 and 10.30 W · kg⁻¹ for pedalling frequencies of 40, 60, 80 and 100 rpm respectively. The slope of the regression lines ($\Delta \dot W/\Delta \dot VO_2$) represents the Δ efficiency of cycle ergo this was also affected byf _p, ranging, on average; from 22.9% to 32.0%. These data allowed us to obtain a comprehensive description of the effects off _p (per minute), exercise intensity ($\dot W$, watts) and lower limbs' mass with or without added loads (m _L, kg), on$\dot VO_2$ (ml ·min⁻¹) during cycling:$\dot VO_2$ = [m _L · (4.3 · 10⁻⁸.f ^4.165_p /0.35)] + {1/[(3.594 · 10⁻⁵ ·f ²_p − 0.003 ·f _p + 0.326) · 0.35]} ·$\dot W$. The mean percentage error between the$\dot VO_2$ predicted from this equation and the actual value was 12.6%. This equation showed that the fraction of the overall$\dot VO_2$ due to internal work, for a normal 70-kg subject pedalling at 60 rpm and 100 W was of the order of 0.2.