051 The Red Line: Critical Power and the Limits of Severe Exercise Tolerance

Vital with Paul Weber

The fatigue mechanisms that produce exhaustion depend on the exercise intensity.
Exercise Intensity Domains
Moderate – below lactate threshold
Heavy – above lactate threshold, below critical power
Severe – above critical power
Almost everything you do in CrossFit occurs in the severe intensity domain.
Below critical power, there are stable levels of muscle PCr, blood lactate and VO2.
Above critical power, you’ll deplete PCr and progress towards your maximum lactate tolerance and VO2max.
Each of these exercise intensities are sustainable for different amounts of time.
Duration
Moderate – ultra-endurance events lasting longer than 2 hours
Heavy – half-marathon and marathon, many cycling time trials, 10k swimming
Severe – 2-40 minutes*
*It is important to note that most events considered “endurance” e.g. 5k or 10k running occur in the severe intensity domain. Athletes who train for these events still spend 80% of their training time in the moderate and heavy intensity domains.
*It is also important to note that even at severe intensity performed to exhaustion, energy production is still 40-50% oxidative.
*In trained athletes, both lactate threshold and critical power occur at higher exercise intensities (measured as a percentage of VO2max) because the athletes’ oxidative metabolism has been trained.
Exhaustion in each domain results from a combination of central and peripheral fatigue.
Fatigue Mechanism
Moderate – primarily central fatigue, disruptions to brain neurotransmission resulting in a loss of ‘drive’ to exercise (also injury, overheating)
Heavy – both central and peripheral – depletion of muscle glycogen
Severe – primarily peripheral fatigue – derangements of muscle metabolic homeostasis (accumulation of H+, ADP, Pi)

Because the intensity in CrossFit is so great, CrossFit results in severe intensity, intermittent exercise.
When you’re doing CrossFit, you’re constantly taking mini-breaks. This makes most workouts a series of work:rest scenarios.
The amount of work that you can do in the severe intensity domain is your W’ (pronounced “W Prime”).
It is utilized at different rates depending on the power output.
When you are doing thrusters, or wall balls, or rowing at a high intensity, you are depleting your W’.
When you take a break or transition to the next movement, you start recovering W’.
How fast you recover W’ is one of the most important physiological attributes to CrossFit performance, along with your W’ itself.***
***W’ does not recover as fast as you can deplete it. This is one of the reasons anaerobic work:rest ratios are so low (1:5+).
***When performing multiple severe intensity bouts, W’ will recover slower and slower the more times you deplete it. This is why CrossFit workouts often result in decreasing power output over their durations, despite athletes’ pacing strategy.
Your W’ and W’ Reconstitution are related or strongly related to measures of aerobic fitness, such as VO2max and Critical Power.