The Respiratory Compensation Point is Not a Valid Surrogate for Critical Power

It is unclear whether the respiratory compensation point (RCP) may be used as a valid surrogate for critical power (CP). Accordingly, we sought to determine the measurement agreement between the CP and the RCP obtained during incremental cycling of varying ramp slopes.

Eleven recreationally active men completed three separate ramp-incremental cycling protocols, where the work rate increment was slow (SR, 15 W·min), medium (MR, 30 W·min), or fast (FR, 45 W·min). The RCP was obtained using the ventilatory equivalent for CO2 output method. The CP was determined via Morton's model for ramp-incremental exercise. The assumption that the RCP and the CP occur at equivalent external work rates 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 protocol, separately.

The external work rate corresponding to the RCP increased with increases in the ramp-incremental slope (P < 0.05). The RCP values in MR (268 ± 37 W) and FR (292 ± 41 W), but not SR (243 ± 35 W), were different (P < 0.05) from CP (247 ± 43 W). The degree to which the relationship between the CP and the RCP approximated the line of identity was relatively poor for SR (CCC = 0.73 and RMSE = 28 W), MR (CCC = 0.63 and RMSE = 36 W), and FR (CCC = 0.42 and RMSE = 55 W).

Our data confirm that the external work rate associated with the RCP is labile and that these power outputs display poor measurement agreement with the CP. Taken together, these findings indicate that the RCP does not provide an accurate estimation of CP.