# Critical Power: an overview

Back in 1965, Critical Power was defined in a seminal paper by Monod and Scherrer as a power level that ‘could be sustained for a very long time without fatigue’. It has subsequently been shown that it is not as simple as these authors made it out to be, and that 60 minutes is probably the maximum duration that Critical Power can be held. However, it has been shown to have a high correlation with VO2max, at one time the ‘gold standard’ measure of endurance (aerobic) fitness. Critical Power is measured in watts which is a work rate commonly used in an exercise context when athletes exercise on an ergometer (a piece of apparatus which simulates a sport’s movement patterns and which typically measures work performed in joules or kilojoules and the work rate in watts. A rowing ergometer is an example of this).

Critical Power was described as the ‘upper limit of aerobic capacity’, a definition which implies an intensity similar to what was commonly employed in the 1980s, the Anaerobic Threshold. This concept has now been shown to be a complete misnomer, being neither anaerobic nor a threshold, and hence its use has all but disappeared in the literature and in the sports training environment.

Critical Power Models**
**Many research groups have examined Critical Power in efforts to determine its physiological characteristics with clarity, and several different models have been proposed. We at TPE Sports Coaching have used the linear model which enables Critical Power determination from two (or three) relatively simple time trials, and which yields information which can be used to monitor progress as a training program progresses. The diagram shown here illustrates the application of the same data to two of the models used to calculate Critical Power. The top section of the diagram illustrates the hyperbolic model where power values (y-axis) are plotted against time (x-axis). This is the model most commonly employed in research projects and requires that athletes complete at least three time trials. Critical Power is determined as the ‘asymptote’ or the power value at which the curve levels out. The value for Anaerobic Work Capacity (AWC) is the area under the curve formed from the data.

The lower section of the diagram, a linear model, plots work (y-axis) over time (x-axis) and Critical Power is the value for the slope of the line formed from the data points. The AWC is determined as the point (intercept) where that straight line intersects the y-axis. In the example shown, Critical Power = 168 watts and AWC = 160 Kj. Because a straight line can be determined from only two points, two time trials can be performed, though three will provide a more reliable result. Research has shown that two time trials can be reliable provided the duration of the trials is between 1 and 10 minutes, and if the trials were 5 minutes or more apart (Housh et al. 1990).