The relationship between critical speed and the respiratory compensation point: Coincidence or equivalence

It has previously been suggested that the respiratory compensation point (RCP) and critical speed (CS) parameters are equivalent and, therefore, like CS, RCP demarcates the boundary between the heavy- and severe-intensity domains. However, these findings are equivocal and therefore must be interpreted cautiously. Thus, we examined the relationship between CS and RCP across a wide range of subject fitness levels, in an attempt to determine if CS and RCP are equivalent. Forty men and 30 women (age: 23.2 ± 2.5 year, height: 174 ± 10 cm, body mass: 74.1 ± 15.7 kg) completed an incremental and four constant-speed protocols on a treadmill. RCP was determined as the point at which the minute ventilation increased disproportionately to CO₂ production and the end-tidal CO₂ partial pressure began to decrease. CS was determined from the constant-speed protocols using the linearized 1·time⁻¹ model. CS and RCP, expressed as speed or metabolic rate, were not significantly different (11.7 ± 2.3 km·h⁻¹ vs. 11.5 ± 2.3 km·h⁻¹, p = 0.208; 2.88 ± 0.80 l·min⁻¹ vs. 2.83 ± 0.72 l·min⁻¹, p = 0.293) and were significantly correlated (r² = 0.52, p < 0.0001; r² = 0.74, p < 0.0001, respectively). However, there was a high degree of variability between the parameters. The findings of the current study indicate that, while on average CS and RCP were not different, the high degree of variability between these parameters does not permit accurate estimation of one from the other variable and suggests that these parameters may not be physiologically equivalent.