Critical Torque Is a Phase Transition, Not a Threshold
by George Blackwell
In a previous newsletter, I discussed using both physiological and ‘race pace’ data to prescribe training sessions in an optimal way (rather than choosing one or the other, as they both have limitations). This newsletter, although grounded in cycling rather than running, presents a similar conundrum in the philosophy of our training methods and how we conceptually view ‘thresholds’ and ‘zones’.
The premise of a new piece of research was to explore the physiological responses to exercise just underneath and just over critical power (or critical torque as referenced in this study, but the two terms mean the same thing). Pethick et al. hypothesised that, if critical power can be conceptualised as a sudden threshold, there should be no gradual transition from heavy to severe intensity domains (this can be below critical power to above critical power, below LT2 to above LT2, or from Z4 to Z5, take your pick, it means the same thing!)
However, the present study demonstrated that the Critical Torque / Power did not represent a distinct and clear change in physiological response. Specifically, when exercise was performed above the estimate of Critical Torque (~107% of CT), most of the observed responses seemed to be in the heavy-intensity domain, or Z3-Z4, and still considered ‘steady state’. This was the case for the majority of participants, with at most only 3-5 participants showing a clear ‘severe-intensity response’. The confidence limits associated with critical torque seem to reflect a state of physiological uncertainty, where different system states can coexist and the responses observed cannot be predicted i.e., a continuous phase transition. This implies that the responses to exercise “at” critical power (i.e., within the 95% confidence limits of CT/CP) are unpredictable.
So, what does this mean from a training prescription perspective?
Unfortunately for the many runners / cyclists / triathletes that enjoyed the specificity of your Critical Power ‘value’ or clear differentiation between zones, it might not be an effective way of training. To ensure consistent physiological responses, this study recommends that exercise should be performed outside the confidence limits associated with your Critical Power estimate, to ensure you are getting the physiological stimulus desired. The tough bit, as the study also highlights, is that there will be a variation in your critical power on a day-to-day basis as well, before accounting for the 95% confidence limits of the value.
All in all, you want to keep your hard stuff hard, and your easy stuff easy, and try to be less concerned with the specificities of exact watts and thresholds. Instead, think about getting the stimulus you need to help you make progress!
References:
Pethick, J., Winter, S.L. and Burnley, M., 2020. Physiological evidence that the critical torque is a phase transition, not a threshold. Medicine and science in sports and exercise, 52(11), p.2390.