Equivalence between FTP and RCP: Assessment of association, agreement and sex effect

Functional Threshold Power (FTP) is widely used for performance evaluation and training prescription in endurance sports. Although FTP is strongly associated with power at the Respiratory Compensation Point (RCP), its interchangeability remains debated. This study examined the relationship and agreement between FTP and RCP in amateur triathletes and tested whether sex influences this relationship.

Forty-nine amateur triathletes (male and female) participated. Each athlete completed a maximal cardiorespiratory cycling test and an FTP test, separated by at least 72 h. Linear regression models were used to test the association between FTP and RCP. A regression model including the interaction term (FTP × sex), equivalent to an ANCOVA framework, was applied to assess sex effects on the FTP-RCP relationship. Agreement between methods was assessed using Bland-Altman analysis. Two-way analysis of variance (ANOVA) was performed to examine the effects of sex, test type, and their interaction on absolute power (W), power relative to total body mass, and power relative to lean mass.

FTP and RCP were strongly correlated (r = 0.916; p < 0.001). The interaction term FTP × sex was not significant (β = -0.135, p = 0.524), indicating that the regression slopes do not differ between males and females. Thus, the relationship between FTP and RCP was statistically similar across sexes. However, Bland-Altman analysis revealed wide limits of agreement, indicating poor interchangeability between FTP and RCP at the individual level.

FTP is strongly associated with RCP, and the relationship is independent of sex. Despite this association, FTP and RCP are not interchangeable measures. Caution is warranted when using FTP as a direct surrogate for RCP in training prescription and performance assessment.