Critical Power
By Rich Strauss

Before you read this article, please first read Alternative Methods of Determining Intensity and Critical Speed.

Determining training intensity by means of Critical Power is very similar to Critical Speed. However, it is only applicable to a bike with a power-measuring device, such as a PowerTap, or Computrainer.

By using velocity to determine power output, Critical Speed is an indirect measure of the work performed by a cyclist. Velocity on a bike is greatly affected by the environmental variables of wind and grade. But power is a direct measurement of intensity.

Power = force x velocity

In this formula, force is the amount of force that is applied to the pedals during the pedal stroke and velocity is leg speed, or cadence. According to this formula, riding 20.5 mph at 80 rpms on a trainer requires the same amount of power as riding 20.5 mph at 100 rpms. In the first example, more force is applied to each pedal stroke, but at a slow velocity (rpms). In the second example, less force is applied to each pedal stroke, but the velocity is higher.

For these reasons, power is an absolutely clear window on intensity. 250 watts is 250 watts, whether it is ridden on the flats or on a hill. Train at greater and greater watts and you will be able to produce more and more watts. This increased ability to put out higher watts yields an increase in velocity. You go faster. Studies have shown that using power as a measurement of intensity achieves excellent results.

Testing Critical Power

Critical Power must be determined using a power measuring device, such as a Computrainer or Powertap. Just as with Critical Speed, testing conditions should be repeatable from test to test, and for this reason the Computrainer is an excellent testing tool. The variables of wind, grade, etc. are eliminated.

How: Critical Power time trials are conducted for the same time lengths as Critical Speed: CP.2, CP1/6/12/30/60/90/180. Fortunately, just as with Critical Speed, the slope of the power curve for Critical Power begins to flatten as the intensity becomes more aerobic. For this reason, CP 60/90/180 can be extrapolated by extending the slope at CP 30. Therefore it is only necessary to perform CP testing for CP.2 through CP 30. After warming up, perform a maximum effort time trial for the prescribed time.

Because CP is a measurement of the average power for the length of the time trial, pacing is critical. You don't want to go out too fast, only to blow up, or go out too slow and have something left over at the end.

When: During a recovery week, before beginning your next training phase. You want to repeat testing conditions from test to test. Potential variables include nutrition, hydration, recovery, sleep, time of day, temperature, etc.

What do you do with CP values?

After determining your CP values in your recovery week, you then use these values to determine training intensities for the following training phase. During CP training sessions, workouts are performed according to prescribed goal power averages, described as CP values. For example, you might do 2 x 15 min at CP 30. If your CP 30 value is 250 watts, you would maintain an average power output of 250 watts for each 15 minute interval. Heart rate is then used as a supplementary method of determining intensity, not the exclusive method.

Your CP values will improve as your fitness improves. Because Critical Power is determined at end of each training phase and then applied to the following training phase, they ensure that you are always training at the proper intensity, given your current level of fitness.

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