Accumulated oxygen deficit (AOD) is widely regarded to be the most theoretically acceptable measure of anaerobic capacity (Saltin, 1990). A primary source of error in the AOD method, however, is failure of the oxygen uptake (Ω_O2)-power regression to predict accurately the energy demand of more intense exercise. An increase in fat oxidation during submaximal exercise is a common response to endurance training (Kiens et al. 1993). In accordance with the higher O₂ cost of fat metabolism relative to carbohydrate, increased fat oxidation during exercise will increase Ω_O2 (Heigenhauser et al. 1983). Thus we hypothesised that the gradient of the Ω_O2-power regression and the O₂ demand predicted for a supra-Ω_O2,max power output would be higher in elite rather than club-level athletes.

Following ethics committee approval and written informed consent, 34 elite (GB National Squad) and 19 club-level oarsmen participated in the study (mean ± S.D. Ω_O2,max 6.35 ± 0.64 and 5.56 ± 0.57 l min^-1; body mass 92.8 ± 5.7 and 85.6 ± 6.3 kg, respectively). Tests were administered on a modified air-braked rowing ergometer (Concept IIC, Nottingham, UK). The test system (Avicon II, Berlin, Germany) incorporated a load cell for force measurement and a rotary transducer for the determination of stroke length. All participants performed a discontinuous incremental protocol that involved 30 W increments every 4 min and an increase in stroke rate of 2 strokes min^-1 with each increment. Ventilatory and gas exchange parameters were measured breath-by-breath (Mijnhardt Oxycon Champion, Bunnick, Holland), and Ω_O2 during the 4th minute of each incremental stage was used to determine an individual Ω_O2-power regression. The regression was based on an average of four sub-lactate threshold determinations ranging from 59 ± 6 to 78 ± 7 % Ω_O2,max. Correlation coefficients (r) for individual regression equations averaged 0.996 ± 0.005 for all groups and were accepted as valid predictive equations (standard error of the estimate = 0.039 ± 0.032 l min^-1). The amount of fat oxidised during sub-lactate threshold exercise intensities was estimated from measures of Ω_O2 and Ω_CO2 using stoichiometric equations (Péronnet & Massicotte, 1991).

The gradient of the Ω_O2-power regression was greater in elite compared with club-level oarsmen (14.7 ± 2.4 vs. 13.4 ± 1.8 ml min^-1 W^-1, respectively; P ▓le│ 0.05, independent samples t test). As a result, the predicted O₂ demand at 438 W (the mean maximum power output sustained for 4 min) was also higher in elite than club-level oarsmen (6.85 ± 0.60 vs. 6.62 ± 0.44 l min^-1, respectively; P ▓le│ 0.05). The change in excess O₂ cost due to fat metabolism relative to the change in power output over sub-lactate threshold exercise intensities yielded a gradient equal to 0.13 ± 0.02 ml min^-1 W^-1, which accounted for 9.7 % of the difference in Ω_O2-power slopes between the groups. The remainder of the difference in Ω_O2-power slopes was due to between-group differences in body mass. In conclusion, results of the present study suggest that greater rates of fat metabolism at lower intensities in elite oarsmen can contribute to differences in Ω_O2-power slopes determined across the same relative range of exercise intensities.