Energy Expenditure of Constant- and Variable-Intensity Cycling: Power Meter Estimates ERIC C. HAAKONSSEN 1,2 , DAVID T. MARTIN 1 , LOUISE M. BURKE 1 , and DAVID G. JENKINS 2 1 Australian Institute of Sport, Belconnen, AUSTRALIA; and 2 School of Human Movement Studies, University of Queensland, Brisbane, AUSTRALIA ABSTRACT HAAKONSSEN, E. C., D. T. MARTIN, L. M. BURKE, and D. G. JENKINS. Energy Expenditure of Constant- and Variable-Intensity Cycling: Power Meter Estimates. Med. Sci. Sports Exerc., Vol. 45, No. 9, pp. 1833–1840, 2013. Purpose: The objective of this study is to compare the effects of constant- and variable-intensity cycling on gross efficiency (GE) and to compare estimates of energy expen- diture (EE) made using indirect calorimetry (CAL) with estimates derived from commercially available power meters. Methods: Nine national team female road cyclists completed a GE test (GE test = 4 min at approximately 45%, approximately 55%, approximately 65%, and approximately 75% maximal aerobic power (MAP)) before and after 10.5 min of either constant- (CON)- or variable- (VAR)- intensity cycling averaging approximately 55% MAP. GE measured before, after, and during CON and VAR cycling was compared. Total EE (kJ) for 10.5 min of VAR cycling was estimated using indirect CAL and compared with estimates on the basis of mechanical power [Schoberer Rad Messtechnik (SRM)] using the group mean GE, each athlete’s mean GE, and each athlete’s power to GE regression. Results: There was no effect of VAR on GE tests (P = 0.74). GE reduced from 19.1% T 0.4% (mean T SE) during the pretrial GE tests to 18.7% T 0.4% during the posttrial GE tests (P G 0.05) in both conditions. Differences in GE (mean T SD) measured during CON (18.4% T 1.6%) and VAR cycling (18.6% T 1.1%) were trivial (P = 0.28). SRM-based estimates of EE were most accurate when using individual athlete’s power GE regression using Pre- and Post-VAR GE test data combined ($ X ; (%) T 90% CI, 0.3 T 0.8; R 2 0.98, P G0.001). Group mean estimates were within approximately 1% of CAL, although individual errors were approximately 11%. Conclusion: Findings support the use of calibrated power meters for estimating cycling EE. For trained female road cyclists, total mechanical work (kJ) multiplied by 5.3 (GE = 19%) provides a valid estimation of total EE during variable-intensity cycling G75% MAP, although determining each athlete’s GE improves accuracy greatly. Key Words: COMPETITIVE CYCLISTS, CYCLING POWER METERS, ENERGY EXPENDITURE, FEMALE TRIAD, GROSS EFFICIENCY I n cycling, it is desirable for athletes to optimize the power they can produce relative to their body mass. In- creased body mass increases the cyclist’s energy demand to overcome rolling resistance and vertical displacement, and when associated with increases in frontal area, it will increase aerodynamic drag (30). This has implications for flat road races, time trialing, and climbing performance. Because there is an inverse relationship between mass and acceleration, an increased body mass will also impair sprint performance in cycling. Partly due to these power-to-mass relationships, competitive cyclists frequently manipulate body composition to maximize performance. Martin et al. (26) have shown that internationally competitive Australian National Team female cyclists tend to be leaner and almost 3 kg lighter than less successful cyclists. Reducing energy intake (EI) to reduce body mass causes a reduction in energy availability (EA), where EA = EI j exercise energy expenditure (EE). For elite female road cyclists who undertake high volumes of training while trying to maintain a high power-to-body mass ratio, there is a risk of exces- sively reduced EA (G30 kcalIkg FFM j1 Id j1 ), which is known to compromise bone status and reproductive health (23). In this population, exercise typically represents the largest component of total daily EE (37). Therefore, accu- rate measures of exercise EE may be important in guiding appropriate levels of EI to support safe body composition manipulation strategies. SPECIAL COMMUNICATIONS Address for correspondence: Eric C. Haakonssen, M.Sc., Department of Physiology, Australian Institute of Sport, PO Box 176, Belconnen, ACT 2616, Australia; E-mail: eric.haakonssen@ausport.gov.au. Submitted for publication September 2012. Accepted for publication February 2013. 0195-9131/13/4509-1833/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE Ò Copyright Ó 2013 by the American College of Sports Medicine DOI: 10.1249/MSS.0b013e31828e18e6 1833 Copyright © 2013 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.