Relative Proximity of Critical Power and Metabolic/Ventilatory Thresholds: Systematic Review and Meta-Analysis

Critical power (CP) has been redefined as the new 'gold standard' that represents the boundary between the heavy- and severe-exercise intensity domains and hence the maximal metabolic steady state (MMSS). However, several other "thresholds", for instance, the maximal lactate steady state [MLSS], ventilatory thresholds [VT₁, VT₂] and respiratory compensation point [RCP]) have been considered synonymous with CP.

This study aimed to systematically review the scientific literature and perform a meta-analysis to determine the degree of correspondence/difference between CP and MLSS, VT₁, VT₂ and RCP.

A literature search on 2 databases (Scopus and Web of Science) was conducted on October 2, 2019. After analyzing 356 resultant articles, studies were included if they met the following inclusion criteria: (a) studies were randomized controlled trials, (b) studies included interrelations between CP and VT₁, VT₂, MLSS, RCP. Articles were excluded if they constituted duplicate articles or did not meet the inclusion criteria. Nine studies met the inclusion criteria and were included in this meta-analysis. This resulted in 104 participants. A random effects weighted meta-analysis with correlation coefficients was used to pool the results.

The pooled correlation coefficient of CP and all thresholds analyzed was r = 0.73 (p > 0.00001). The subgroup analysis for each threshold with CP demonstrated significant correlation coefficients of r = 0.80 (95% CI [0.40; 1.21], Z = 3.90, p = 0.0001) for CP & RCP; r = 0.77 (CI 95% = [0.36; 1.18], Z = 3.71, p = 0.0002) for CP & MLSS; r = 0.76 (CI 95% = [0.31; 1.21], Z = 3.32, p = 0.0009) for CP & VT₁. However, CP & VT₂, r = 0.39 (CI 95% = [- 0.37; 1.15], Z = 1.01, p = 0.31) were not significantly correlated. Despite the significant correlations between CP and VT₁, MLSS and RCP these variables and VT₂ under- (VT₁, 30%; MLSS, 11%) or over-estimated (RCP, 6%; VT₂, 21%) CP.

Regardless of the presence of significant correlations among CP and ventilatory or metabolic thresholds CP differs significantly from each. Thus, logically, if CP represents the best estimate of the heavy-severe exercise intensity transition none of the thresholds considered (i.e., VT₁, VT₂, MLSS, RCP), at least as determined in the studies analyzed herein, should be considered synonymous with such.