KOM Informatics

Critical Power (CP) is a theory in exercise physiology which holds that the boundary between
the heavy, sustainable, and the severe, unsustainable exercise domains can be determined
through one or more trial efforts and some statistics.

The theory has been around for a while. Proponents trace it's roots all the
way back to 1925 to a paper by Professor A.V. Hill, University of London called
The Physiological Basis Of Athletic Records"

Perhaps the most important boost to the theory in more modern times came from a paper called
"The Work Capacity Of A Synergic Muscular Group" by Monod and Scherrer published in 1965. There
have been many studies involving the concept since then. It's safe to say CP has
been extensively studied and refined.

The theory posits a linear relationship between the energy output of a cyclist and
the duration over which he or she is working. Under the two parameter model implemented
in KOM Informatics, a regression line using the trial efforts as inputs is calculated using the
ordinary least squares method. This method yields the slope and Y intercept of the equation
of the line plotting kilojoules on the Y axis and duration on the X axis. The intercept
equates to Anerobic Work Capacity (AWC). Once kilojoules and duration are known it's not difficult
to calculate watts for any second. The hyperbolic, smooth power duration curve that appears
in graphs of Critical Power is the end result of this process.

The CP 2 parameter model is generally held to be invalid at shorter durations where predicted power
approaches infinity. Similarly, the model predicts a asymptotic power level which can be maintained
for an infinite amount of time. Common sense would seem to indicate the theory should be
restricted to a more narrow time range.

So, how well does it actually work at predicting power at various durations? Dr. Andrew
M. Jones weighs in on the topic in his paper The maximal metabolic steady state: redefining the ‘gold standard’:
"the time to the limit of tolerance when subjects are asked to exercise at CP is
highly variable (e.g., range of approximately 15 to 40 min or occasionally up to ~60 min;
McLellan and Cheung 1992; Bull et al. 2000; Brickley et al. 2002; McClave et al. 2011;
Bergstrom et al. 2013), with the group mean physiological responses being characteristic of
either heavy-intensity (Poole et al. 1988, 1990; Wakayoshi et al. 1993) or
severe-intensity (Jenkins and Quigley 1990; McLellan and Cheung 1992; Brickley et al. 2002)
exercise."

Jones posits that CP shouldn't be understood as an absolute number but rather as a bandwidth
which can vary +_ 3-5% due to both standard error and inherent biological variability.

Jones also provides some recommendations based on the accumulated science, for the
number and duration of the trials.

With respect to duration: "It is essential that subjects give their maximum effort in each trial
and that cadence is consistent across all trials. Ideally the shortest trial should be 2–3 min
and the longest should be more than 10 but no longer than 15 min (Hill 1993; Vanhatalo et al.
2011a). It has been recommended that there should be at least a 5 min difference between the
shortest and longest trials (Bishop et al. 1998) but the goodness of hyperbolic fit is
improved by making the range of times to exhaustion as broad as possible (i.e., 8–12 min) within
the severe-intensity domain. The precise duration of the prediction trials is of secondary
importance to the attainment of VO2max, but it is unusual for VO2max to be attained if exercise
duration is shorter than 1–2 min or longer than 15–20 min (Hill et al. 2002; Vanhatalo et al.
2016). "

With respect to number of trials:
" In practice, 3-4 (Smith and Jones 2001; Brickley et al. 2002; Pringle and Jones 2002;
Dekerle et al. 2005; Black et al. 2015) or 5-7 (Hughson et al. 1984; Gaesser and Wilson 1988;
Poole et al. 1990; Bull et al. 2000; Vanhatalo et al. 2007) trials are commonly used. "

CP can be used for several different things:

Figure out target wattage for a non-standard interval duration.
A time trialer could use shorter tests to figure out a target wattage
for a longer TT effort
You could compare CP curves from different dates to get an idea about
changes in fitness
If you see an area in the CP curves which is above your maximal effort best this
could indicate an area which could use some improvement.
Identify strengths, weaknesses and areas for improvement
The KOM Informatics UI allows you to change the wattage of prediction trials.
This allows you to investigate "what if" scenarios involving changes to
one or more parts of the CP curve affecting other parts of the CP curve.
You could use your CP 60 (CP @ 60 minutes) number in place of FTP to calculate power training zones.
To be on the safe side we would use CP x .97, since for many it seems that CP is a little
higher then FTP