Efficiency of cycling exercise: Quantification, mechanisms, and misunderstandings

Abstract

The energetics of cycling represents a well‐studied area of exercise science, yet there are still many questions that remain. Efficiency, broadly defined as the ratio of energy output to energy input, is one key metric that, despite its importance from both a scientific as well as performance perspective, is commonly misunderstood. There are many factors that may affect cycling efficiency, both intrinsic (e.g., muscle fiber type composition) and extrinsic (e.g., cycling cadence, prior exercise, and training), creating a complex interplay of many components. Due to its relative simplicity, the measurement of oxygen uptake continues to be the most common means of measuring the energy cost of exercise (and thus efficiency); however, it is limited to only a small proportion of the range of outputs humans are capable of, further limiting our understanding of the energetics of high‐intensity exercise and any mechanistic bases therein. This review presents evidence that delta efficiency does not represent muscular efficiency and challenges the notion that the slow component of oxygen uptake represents decreasing efficiency. It is noted that gross efficiency increases as intensity of exercise increases in spite of the fact that fast‐twitch fibers are recruited to achieve this high power output. Understanding the energetics of high‐intensity exercise will require critical evaluation of the available data.

Effect of Gear Ratio and Cadence on Gross Efficiency and Pedal Force Effectiveness during Multistage Graded Cycling Test Using a Road Racing Bicycle

“…Based on physics theory, the energy demand of the primary work muscle relative to the same absolute pedaling load per revolution is steady when the cadence is increased. In contrast, an increasing work rate (exercise intensity) might be accompanied by increasing external energy costs that do not involve effective pedal force, such as stabilization in riding posture (recruiting previously inactive muscles) and changing muscle activation [ 10 ]. Moreover, it is assumed that various factors (e.g., air resistance, rolling resistance, alteration in the recruitment pattern of the muscle fiber type, and increased inertia force) induced by increasing the cadence could affect GE [ 10 , 19 ].…”

“…Theoretically, the energy demand of the primary working muscle relative to the same absolute pedaling load per revolution is steady whenever the cadence is increased. However, the energy demand in various tissues other than the working muscle may also increase (e.g., postural stabilization) when demanding a higher work rate, suggesting that external work relative to the total energy cost, which does not account for effective pedal cranking, would be increased [ 10 ]. Collectively, it is currently unclear whether increasing cadence impairs GE.…”

Complexities of defining the bioenergetic efficiency and VO2 slow component of skeletal muscle contraction

“…To the Editor, We would like to applaud MacDougall, Falconer, and MacIntosh 1 for their recent critical commentary on the measurements and assumptions involved in the computations of the bioenergetic efficiency of cycling exercise. Nevertheless, we would like to expand on the critical commentary, especially concerning the VO 2 slow component.…”

Response to “complexities of defining the bioenergetic efficiency and VO2 slow component of skeletal muscle contraction”

“…In our recent review article on cycling efficiency, 3 we presented the possibility that the VȮ 2 slow component may be a consequence of a relatively slow transition in the metabolism of the working muscles from substrate level phosphorylation to oxidative phosphorylation, while the total muscular ATP cost for a given work rate (i.e., efficiency) remains constant.…”

“…We agree that delta efficiency is not a reasonable estimate of muscular efficiency, and this point is substantiated in our review. 3 This does not mean that gross efficiency cannot be calculated. O'Malley and Robergs 1 also argue that the nonaerobic contribution to the energy cost of exercise cannot be estimated.…”