Critical power: Over 95 years of “evidence” and “evolution”

Dear Editor, We thank Broxterman et al. for their interest in our recent article concerning the Critical Power/Critical Speed (CP/ CS) concept.1 The authors endeavored to emphasize the “foundational physiological underpinnings of CP/CS,” mainly based on2 (1) CP/CS is a boundary that demarcates the heavy from the severe intensity domain because it is coincident with the upper intensity at which oxygen uptake (VO2) can be sustained in a steadystate condition (i.e., maximal metabolic steadystate, MMSS), and (2) CP occurs at ~7%– 11% higher power output (PO) than maximal lactate steadystate (MLSS), it represents a unique work rate differentiated from MLSS, and it is more appropriate than MLSS to constitute the MMSS. These assertions disagree with previous studies suggesting that PO at MMSS occurs near or at MLSS.3– 5 As we have recently brought up,6 the statement that CP is coincident with MMSS2 is based on a landmark study5 in which CP practically corresponded to MLSS. To base the claim that CP/CS is an integral and unique biological concept, distinct from MLSS, that occurs at MMSS, mainly on one study in which both MLSS and CP variables were coincident, is misleading. This fact itself also disrupts one of the main “physiological foundations” of CP/CS (i.e., the abovementioned point 2). A recent study conducted by Iannetta et al.7 showed that MLSS occurs in close proximity to MMSS. The authors compared the concordance between CP and MLSS with MMSS in 10 recreational cyclists. From this study,7 we analyzed the reported average values of VO2 and blood lactate concentration ([La]b) measured every 5 min during a 30 min constant exercise at MMSS intensity. Figure 1 illustrates the substantial agreement (r = 0.986) existing between the changes in average [La]b and the corresponding changes in average VO2 while exercising at MMSS. This figure indicates that when an increase in VO2 during constant exercise at MMSS occurs, a proportional and concomitant increase in [La]b is observed (~1 mmol L−1 per 100 ml min−1 of VO2). Interestingly, this relationship also shows that when there is a “perfect” O2 steadystate (delta VO2 = 0), there is no change in [La]b (bintercept = 0). The high degree of concordance between changes in VO2 and changes in [La ]b agrees with previous classical studies,3,4 which indicated that “the highest VO2 at which a steadystate can be attained coincides with the highest work rate at which [La]b does not continue to raise. ”4 Therefore, if (1) MLSS is coincident with MMSS, and (2) CP is 7%– 11% higher than MLSS, then, contrary to what Broxterman et al. asserted, CP cannot be a physiological boundary demarcating intensity domains based on VO2 stability. Given the high degree of concordance observed in Figure 1 between changes in VO2 and changes in [La ]b at MMSS, the next stage should be to define the time intervals and VO2 or [La ]b cutoffs to be used interchangeably to determine VO2 steadystate or MLSS. However, to our knowledge, there is no consensus upon this topic. For example, with regard to [La]b cutoffs that should be used to determine MLSS, the most widely accepted definition is the upper intensity at which a change of [La]b ≤1 mmol L−1 in the last 20 min of a 30 min constantPO trial occurs.2,7 Nevertheless, since the analytical error of [La]b measurements is ~3% (e.g., <0.2 mmol L −1 for a [La]b of 4 mmol L −1),8 it is likely that a [La]b cutoff increase of, for example, ≤0.5 mmol L−1 (twice higher than the analytical error), rather than ≤1 mmol L−1, would determine MLSS with a higher precision. In such a case, the currently most accepted definition of MLSS probably slightly overestimates the true MLSS or MMSS. It is therefore advisable to establish a basic consensus on the definition of MMSS and of its very much associated, almost coincident, MLSS. Broxterman et al. claimed that the duration of the exercise trials used to determine CP/CS are specifically recommended (between 2 and 15 min),2 even though they did not always fulfill this criterion. In a recent paper written by one of them, the duration of the longest trial used to analyze 10 000 m running performance was near 27 min.9 In another recent study, Jones and Vanhatalo10 used a longest trial of near 40 min to analyze marathon performance. Since, as we have shown,1:(1) CS determined by the hyperbolic model, decreases as the duration of the longest trial is increased, and (2) CS corresponds to ~96%– 99% of the speed sustained during the longest trial