SPECIAL COMMUNICATIONS: Letters to the Editor-in-Chief
Metabolic efficiency is the ratio of mechanical work done by the muscles relative to the energy expended by the body (5), and the latter is calculated from the oxygen consumption (O2) and substrate utilized (RER). Over the last 30 years, gross efficiency (GE) during cycling has been reported to range 18–22%.
Lucia et al. (10) recently presented GE from “world-class” road cyclists and reported an average GE of 24.5% with a peak of 28.1%. These data are unique and are indicative of either extreme physiological adaptations or methodological error.
Moseley et al. (11) reported average GE of 18.9% in world-class cyclists (O2max of 75.5 mL·kg−1·min−1) of similar caliber to those used by Lucia et al. In Moseley et al.’s study, the GE of professional cyclists varied from 17.7 to 22.3%, whereas others have reported GE between 18.4 and 22.5% (3,4,8). Measurements of GE in professional road cyclists, performed at the Australian Institute of Sport (AIS) over 15 yr, have generally ranged 20–22%. In a recent AIS study (9), the O2-W relationship of world-class cyclists (73.0–78.3 mL·kg−1·min−1) was similar to regression equations published by other labs (7,13) and the ACSM (1). Regression equations for the seven professional male road cyclists indicate that O2 at 385 W ranged 4.84–5.11 L·min−1; values noticeably higher than those suggested by Lucia and colleagues (10). O2max data also demonstrate a large deviation from these regressions. Lucia and colleagues report exceptionally low and variable O2max at an admirable peak power output (4.8–5.7 L·min−1 at ∼500 W).
Efficiency reported by Lucia et al. (10) is also very high from a theoretical viewpoint. It is known that muscle efficiency during whole-body movements such as cycling is ∼30% (2,12). The measurement of GE, however, is a whole-body measurement including other energy costs such as resting metabolic rate (∼4 kJ·min−1) that cannot be attributed to power output. GE is therefore likely less than 28% (as reported for one cyclist).
The cyclists in the study by Lucia et al. (10) rode at an average workload of 385 W or 23.1 kJ·min−1. With a GE of 24.5%, this means that their energy expenditure was 94.3 kJ·min−1. If we assume an average energy equivalent of 20.9 kJ·L−1 O2 (for RER = 0.90), they must have consumed 4.51 L O2·min−1 to ride at 385W, well below that predicted by well-established O2-W regression equations (1,7,13) but higher than the reported values by Lucia et al.
One explanation for high GE is a systematic error in the measurement of O2. A low O2 overestimates GE. The authors used an automated breath-by-breath system (CPX/D; Medical Graphics; St. Paul, MN). A recent comparison of the CPX/D with automated Douglas bags revealed a significant underestimation (10.7–12.0%) of O2 at workloads between 100 and 300 W (6). Interestingly, correction of Lucia et al.’s data for such an underestimation would result in GE in the normal range 20–22%.
Given that 1) the GE reported are outside the normal range, 2) the values reported are high from a theoretical viewpoint, 3) there seem to be calculation errors, and 4) that problems with the gas analysis equipment used in this study have been observed, it is likely that there is an error in the reported GE values. If, however, these values are correct, then some extremely interesting physiological adaptations may exist that require further study.
David T. Martin
Christopher J. Gore
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