RILEY, M. S., and C. B. COOPER. Ventilatory and gas exchange responses during heavy constant work-rate exercise. Med. Sci. Sports Exerc., Vol. 34, No. 1, 2002, pp. 98–104.
Purpose: At constant work-rates below the gas exchange threshold (V̇O2θ), V̇O2 normally achieves steady-state values within 3 min, whereas at heavier work-rates, V̇O2 may continue to rise. The V̇O2 response to heavy exercise can be described by a three-exponential model with the slow phase usually commencing 2–3 min after the onset of exercise. The aim of our study was to estimate precisely the V̇O2, V̇CO2, V̇E and fC required for above-V̇O2θ exercise from the relationship of the specific variable to work-rate below V̇O2θ and to compare this with the actual value achieved.
Methods: Nine cyclists performed five constant work-rates of 8 min duration, four below V̇O2θ (40, 80, 120, 160 W) and one midway between V̇O2θ and V̇O2max (295 ± 34 W). The V̇O2, V̇CO2, V̇E and fC were averaged for the final 2 min of each below-V̇O2θ test and were found to be linear with respect to work-rate (average r2 >0.95). Variables for the above-V̇O2θ work-rate were predicted by extrapolation and compared with the actual measured values at the end of the exercise bout.
Results: V̇O2 exceeded the predicted value by 0.48 ± 0.21 L·min−1 (12.4 ± 5.1%), V̇CO2 by 0.78 ± 0.26 L·min−1 (23.2 ± 7.2%), V̇E by 40.3 ± 16.3 L·min−1 (51.0 ± 23.1%), and fC by 12.2 ± 12.5 beats·min−1 (8.8 ± 9.3%), all P < 0.0001 except fCP < 0.02, paired t-test. The point at which V̇O2 during above-V̇O2θ exercise exceeded the predicted value (145.7 ± 64.9 s) agreed with the point at which the slow component of V̇O2 began, as determined by nonlinear regression analysis (131.5 ± 44.3 s, P = NS, ANOVA).
Conclusion: There is an excessive metabolic response to heavy exercise over and above that predicted by extrapolation from light-moderate exercise and this excess V̇O2 approximates on average to the slow phase of a three-compartment exponential model.