Ventilatory and gas exchange responses during heavy constant work-rate exercise.

PURPOSE: At constant work-rates below the gas exchange threshold (VO(2 theta)), VO(2) normally achieves steady-state values within 3 min, whereas at heavier work-rates, VO(2) may continue to rise. The VO(2) 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 VO(2), VCO(2), VE and f(C) required for above-VO(2 theta) exercise from the relationship of the specific variable to work-rate below VO(2 theta) and to compare this with the actual value achieved.

METHODS: Nine cyclists performed five constant work-rates of 8 min duration, four below VO(2 theta) (40, 80, 120, 160 W) and one midway between VO(2 theta) and VO(2max) (295 +/- 34 W). The VO(2), VCO(2), VE and f(C) were averaged for the final 2 min of each below-VO(2 theta) test and were found to be linear with respect to work-rate (average r2 >0.95). Variables for the above-VO(2 theta) work-rate were predicted by extrapolation and compared with the actual measured values at the end of the exercise bout.

RESULTS: VO(2) exceeded the predicted value by 0.48 +/- 0.21 L x min(-1) (12.4 +/- 5.1%), VCO(2) by 0.78 +/- 0.26 L x min(-1) (23.2 +/- 7.2%), VE by 40.3 +/- 16.3 L x min(-1) (51.0 +/- 23.1%), and f(C) by 12.2 +/- 12.5 beats x min(-1) (8.8 +/- 9.3%), all P < 0.0001 except f(C) P < 0.02, paired t-test. The point at which VO(2) during above-VO(2 theta) exercise exceeded the predicted value (145.7 +/- 64.9 s) agreed with the point at which the slow component of VO(2) 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 VO(2) approximates on average to the slow phase of a three-compartment exponential model.