Physiological evidence that the critical torque is a phase transition, not a threshold

INTRODUCTION: Distinct physiological responses to exercise occur in the heavy- and severe-intensity domains, which are separated by the critical power or critical torque (CT). However, how the transition between these intensity domains actually occurs is not known. We tested the hypothesis that CT is a sudden threshold, with no gradual transition from heavy- to severe-intensity behavior within the confidence limits associated with the CT. METHODS: Twelve healthy participants performed four exhaustive severe-intensity trials for the determination of CT, and four 30-min trials in close proximity to CT (one or two SE above or below each participant's CT estimate; CT - 2, CT - 1, CT + 1, CT + 2). Muscle O₂ uptake, rectified electromyogram, and torque variability and complexity were monitored throughout each trial, and maximal voluntary contractions (MVC) with femoral nerve stimulation were performed before and after each trial to determine central and peripheral fatigue responses. RESULTS: The rates of change in fatigue-related variables, muscle O₂ uptake, electromyogram amplitude, and torque complexity were significantly faster in the severe trials compared with CT - 2. For example, the fall in MVC torque was -1.5 ± 0.8 N·m·min^-1 in CT - 2 versus -7.9 ± 2.5 N·m·min^-1 in the lowest severe-intensity trial (P < 0.05). Individual analyses showed a low frequency of severe responses even in the circa-CT trials ostensibly above the CT, but also the rare appearance of severe-intensity responses in all circa-CT trials. CONCLUSIONS: These data demonstrate that the transition between heavy- and severe-intensity exercise occurs gradually rather than suddenly.