The Effect of Hypoxia on the Respiratory Compensation Point and Critical Power

Many studies were done investigating the effects of hypoxia on Respiratory Compensation Point (RCP) and Critical Power (CP). However, no studies were done to investigate the relationship between RCP and CP in hypoxic condition, which are thought to have a common underlying mechanism. The aim of this study is to examine the effects of hypoxia on the relationship between CP and RCP. Eleven well-trained male cyclists volunteered in this study (mean ± SD; age 27 ± 6.6 years, height 179 ± 7.5 cm, body mass 78.5 ± 7.9 kg). Participant age was (18-35 years), training history was a minimum of 2 years of cycling training history, with 7 hours per week average training volume, free from injury or illness. Participants attended three times to perform incremental exercise tests on an electronically braked cycle ergometer. Experimental protocol was done once in normoxia (NORM: FiO2 of ≈0.155), and once in normobaric hypoxia (HYPO: FiO₂ ≈0.155). Testing order of conditions was single-blinded, randomized and counterbalanced. Results showed that at GET and RCP, there was a significant main effect of HYPO on power, VO2, VCO2, RER, %SpO2, and VO2peak (P< 0.05). Furthermore, CP was significantly correlated to RCP, and revealed a strong effect size (r² = 0.93, P <0.001). In HYPO, CP was also not significantly different to RCP (P =0.12), whereas the correlation between CP and RCP remained significant yet there was only a small effect size (r² = 0.32, P =0.07). We concluded that the relationship between CP and RCP in HYPO was mildly disassociated, which indicates that despite the likelihood that a common underlying mechanism exists, there are also differences in these mechanisms, and therefore caution should be used when using RCP as a proxy for CP, especially in hypoxia.