Muscular efficiency at low power outputs, comparable to daily living activities, is relevant to understanding obesity. Calculating muscular efficiency has remained controversial due to the difficulty of isolating the energy expenditure associated with the skeletal muscle mass performing mechanical work. While subtracting baselines (e.g. metabolic rate at rest or during zero load cycling) is common, these subtractions are problematic. The purpose of this study was to investigate the effects of different baseline subtractions on the calculation of efficiency during cycling, particularly at low power outputs.

15 active subjects (7 females, 8 males;​ age 20-35) completed a one-visit cycle ergometer protocol. They performed 14, 5-minute trials including:​ seated rest – with cranks vertical, seated rest – with cranks horizontal, passive (motordriven) cycling, no-chain cycling, no-load cycling, cycling at very low (10-30 Watts), low (40-60 Watts), and moderate (80-120 Watts) power outputs. All cycling trials were performed at 90 revolutions per minute (rpm) on a panloaded, mechanically braked cycle ergometer. During each trial, indirect calorimetry was performed continuously.

The average rate of oxygen consumption, (V̇O₂), during rest was 0.29, ±0.06 L/min (mean, ±S.D.), and nearly doubled during passive cycling (0.57, ±0.14 L/min). V̇O₂ values for no-chain (0.76, ±0.12 L/min) and no-load (0.79, ±0.11 L/min) trials were not statistically different from each other. Thus, volitional muscle contraction constituted only about half of the increased cost of no-load cycling vs. rest. The regression equation for V̇O₂ vs. power output for 10-120 Watt trials was 0.007x + 0.752, (R² = 0.994). Mean delta efficiency for low power outputs was 68.4%, compared to 36.3% and 36.9% for middle, and high power outputs. Means for gross (3.6%) and net (5.6%) efficiencies were rather low at low power outputs. Furthermore, at low power outputs, delta and work efficiencies demonstrated values exceeding theoretical values for muscular efficiency.

In conclusion, no method of baseline subtractions gave satisfactory values for muscular efficiency at low power outputs.