Energy cost and mechanical efficiency of riding a four-wheeled, human-powered, recumbent vehicle

Oxygen consumption at steady state (VO2, l(.)min(-1)) and mechanical power (W, W) were measured in five subjects riding a human-powered vehicle (HPV, the Karbyk, a four-wheeled recumbent cycle) on a flat concrete road at constant sub-maximal speeds. The external mechanical work spent per unit of distance (W, J(.)m(-1)), as calculated from the ratio of (W) over dot to the speed (v, m(.)s(-1)), was found to increase with the square of v: (W) over dot = 8.12 + (0.262(.)v(2)) (r = 0.986, n = 31), where the first term represents the mechanical energy wasted, over a unit of distance, against frictional forces (rolling resistance, R-r), and the second term (k(.)v(2)) is the work performed, per unit distance, to overcome the air drag. The rolling coefficient (C-r, obtained dividing R-r by m(.)g, where m is the overall mass and g is the acceleration of gravity) amounted to [mean (SD)] 0.0084 (0.0008), that is about 60% higher than that of a racing bicycle. The drag coefficient was calculated from the measured values of k, air density (rho) and frontal area (A) [C-x = k(.)(0.5(.)A(.)rho)(-1)], and amounted to 1.067 (0.029), that is about 20% higher than that of a racing bicycle. The energy cost of riding the HPV (C-k, J(.)m(-1)) was measured from the ratio of metabolic power above rest (net (V) over dot O-2, expressed in J(.)s(-1)) to the speed (v, m(.)s(-1)); the value of this parameter increased with the square of v, as described by: C-k = 61.45 + (0.675(.)v(2)) (r = 0.711, n = 23). The net mechanical efficiency (eta) was calculated from the ratio of W to Ck: over the investigated speed range this turned out to be 0.22 (0.021). Best performance times (BPTs) of a "typical" elite athlete riding the Karbyk were calculated over the distances of 1, 5 and 10 km: these were about 8% longer than the BPTs calculated, on the same subjects, when riding a conventional racing bicycle.