Estimating the maximal metabolic steady state using critical power: assessment of the adequacy of different models

It has been advocated that critical power (CP) should be considered the gold standard to determine the maximal metabolic steady state (MMSS). However, the choice of the model affects the estimation of CP, previous research reporting differences of up to 28% between the lowest and highest CP estimates. The purpose of this thesis was to investigate which of the models, exponential (CPexp), 3-parameter hyperbolic (CP3-hyp), 2-parameter hyperbolic (CP2-hyp), linear (CPlinear), and inverse of time (CP1/time), estimates MMSS best, defined by the maximal intensity at which an oxygen uptake (V ̇O2) steady state is still achievable. Eleven male participants (Age: 31 ± 11 years, Body mass: 70.5 ± 5.6 kg) performed three time-trials (12-, 6-, and 3-min long) to determine CP from the five models. On three subsequent visits, participants cycled for 30-min, or until task failure, at the CP estimated by each model. CPexp estimated the highest CP (303 ± 69 W), followed by CP1/time (272 ± 66 W), CPlinear (270 ± 64 W), CP2-hyp (266 ± 65 W) and CP3-hyp (262 ± 63 W). V ̇O2 stabilised at a significantly lower value than peak V ̇O2 (V ̇O2peak) during exercise at CPlinear, CP2-hyp, and CP3-hyp (94 ± 5%, P = 0.041; 87 ± 4%, P < 0.001; 86 ± 4%, P < 0.001, respectively). CPlinear had a mixed individual response, 7 out of 11 participants failing to attain a V ̇O2 steady state. V ̇O2 stabilisation was not significantly different to V ̇O2peak during exercise at CPexp and CP1/time (98 ± 2%, P = 1.000; 94 ± 6%, P = 0.130, respectively). Rate of perceived exertion significantly increased over time during exercise at CP1/time (P < 0.001) and CPlinear (P = 0.006) but was unchanged between minute 15 and end-exercise during CP2-hyp (P = 0.762) and CP3-hyp (P = 0.569). Lactate increased significantly in the last 10, 15, and 20 minutes of the exercise for all models. No model had an increase of ≤ 1 mmol · L-1 from minute 10 to 30. These results suggest that CP2-hyp or CP3-hyp should be favoured when CP is used to assess MMSS.