Work Performed Above The Respiratory Compensation Point Is Not Equivalent To W′

Abstract

The hyperbolic power-time relationship for severe-intensity cycling exercise is defined by two physiological parameters: (i) the asymptote, critical power (CP); and (ii) the curvature constant, W′. Recently, we reported that the respiratory compensation point (RCP) displays poor measurement agreement with the CP. However, it is unknown whether the amount of supra-RCP mechanical work (RCP′) performed during ramp-incremental cycling is similar to that performed above the CP (i.e., W′). PURPOSE: We sought to determine the measurement agreement between W′ and RCP′ obtained during incremental cycling of varying ramp slopes. METHODS: Twelve male cyclists completed three separate ramp-incremental cycling protocols, where the work rate increment was slow (SR, 15 W[BULLET OPERATOR]min-1), medium (MR, 30 W[BULLET OPERATOR]min-1), or fast (FR, 45 W[BULLET OPERATOR]min-1). Initially, the RCP (adjusted for mean response time) was obtained using the ventilatory equivalent for CO method. To assess RCP′, we calculated the power–time integral between the RCP and the instantaneous power output observed at exercise termination for each ramp-incremental test, separately. W′ was determined via Morton’s model for ramp-incremental exercise. The assumption that W′ and RCP′ occur at equivalent kilojoule (kJ) values was assessed by one-way repeated- measures ANOVA and by evaluating the concordance correlation coefficient (CCC) and typical error (root-mean-square error [RMSE]) for each ramp-incremental test, separately. RESULTS:RCP′ decreased with increases in the ramp-incremental slope (P < 0.05). RCP′ in SR (21.5 ± 6.5 kJ), MR (16.8 ± 5.6 kJ) and FR (13.3 ± 4.3 kJ) were not different from W′ (15.7 ± 6.9 kJ). The degree to which the relationship between W′ and RCP′ approximated the line of identity was poor for SR (CCC = -0.09 and RMSE = 11.3 kJ), MR (CCC = 0.23 and RMSE = 7.5 kJ) and FR (CCC = 0.37 and RMSE = 6.5 kJ). CONCLUSION: Our data demonstrate that RCP′ is lower when the ramp-incremental slope is increased. Furthermore, despite occurring at similar kJ values, we observed poor measurement agreement between W′ and RCP′, as evidenced by the low CCC and the large RMSE values, irrespective of the ramp-incremental protocol. Together, these findings indicate that RCP′ obtained during ramp-incremental cycling is not equivalent to W′.