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What are Normalized power, Intensity
Factor (IF),
and Training Stress Score (TSS)
By. Andrew R. Coggan, Ph.D.
One of the first things that catches the attention of any beginning power
meter user is how variable, or "jumpy", their power output tends to
be. This is largely due to the constantly changing resistances (e.g., small
changes in elevation, gusts of wind) that must be overcome when cycling
outdoors. Because of this variability, training with a power meter is not
directly comparable to training using a heart rate monitor. In particular, it
is
very difficult (as well as counterproductive) to try to keep power constantly
within a certain range, or zone, at all times during a training session. Just
as
importantly, this variability means that the overall average power for a ride
or
part of a ride is often a poor indicator of the actual intensity of the
effort.
This is especially true for races, since power can vary dramatically from one
moment to the next as, e.g., a rider first tries to conserve energy and then
attacks.
To account for this variability, CyclingPeaks uses a special algorithm to
calculate an adjusted or normalized power for each ride or
segment
of a ride (longer than 30 seconds) that you analyze. This algorithm is
somewhat
complicated, but importantly it incorporates two key pieces of information: 1)
the physiological responses to rapid changes in exercise intensity are not
instantaneous, but follow a predictable time course, and 2) many critical
physiological responses (e.g., glycogen utilization, lactate production,
stress
hormone levels) are curvilinearly, rather than linearly, related to exercise
intensity, By taking these factors into account, normalized power provides a
better measure of the true physiological demands of a given training session -
in essence, it is an estimate of the power that you could have maintained for
the same physiological "cost" if your power output had been
perfectly
constant (e.g., as on a stationary cycle ergometer), rather than variable.
Keeping track of normalized power is therefore a more accurate way of
quantifying the actual intensity of training sessions, or even races. For
example, it is common for average power to be lower during criteriums than
during equally-difficult road races, simply because of the time spent
soft-pedaling or coasting through sharp turns during a criterium. Assuming
that
they are about the same duration, however, the normalized power for both types
of events will generally be very similar, reflecting their equivalent
intensity.
In fact, normalized power during a hard ~1 hour long criterium or road
race will often be similar to what a rider can average when pedaling
continuously during flat 40k time trial - the normalized power from mass start
races can therefore often be used to provide an initial estimate of a rider's
threshold power (see below).
Although normalized power is a better measure of training intensity than
average power, it does not take into account differences in fitness within or
between individuals. CyclingPeaks therefore also calculates an intensity
factor (IF) for every workout or time range analyzed. IF is simply the
ratio of the normalized power as described above to your threshold power
(entered under "Athlete Settings" at your "Athlete Home").
For example, if your normalized power for a long training ride done early in
the
year is 210 W and your threshold power at the time is 280 W, then the IF for
that workout would be 0.75. However, if you did that same exact ride later in
the year after your threshold power had risen to 300 W, then the IF would be
lower, i.e., 0.70. IF therefore provides a valid and convenient way of
comparing
the relative intensity of a training session or race either within or between
riders, taking into account changes or differences in threshold power. Typical
IF values for various training sessions or races are as follows:
Typical IF values for various training sessions or races are as
follows:
- Less than 0.75 recovery rides
- 0.75-0.85 endurance-paced training rides
- 0.85-0.95 tempo rides, aerobic and anaerobic interval workouts (work and
rest periods combined), longer (>2.5 h) road races
- 0.95-1.05 lactate threshold intervals (work period only), shorter
(<2.5
h) road races, criteriums, circuit races, longer (e.g., 40 km) TTs
- 1.05-1.15 shorter (e.g., 15 km) TTs, track points race
- Greater than 1.15 prologue
TT, track pursuit, track miss-and-out
Note that one particularly useful application of IF is to check for
changes in threshold power - specifically, an IF of more than 1.05 for a race
that is approximately 1 hour in duration is often a sign that the rider's
threshold power is actually greater than that presently entered into the
program. Thus, by simply examining a rider's IF for various events during
the course of a season, increases or decreases in threshold power can often be
revealed without the need for frequent formal testing.
While exercise intensity is clearly an important factor in determining the
type and magnitude of physiological adaptations to training, exercise
frequency
and duration - which together determine the overall training volume - are
important factors as well. However, there is obviously an interaction between
training intensity and volume, i.e., at some point as intensity goes up volume
must come down, and vice-versa, or else an you will become overtrained. To
quantify the overall training load and hopefully help avoid such a situation,
CyclingPeaks uses your power data to calculate a training stress score
(TSS) for every workout, and provides a graphical summary of your
recent
TSS on your Athlete Home page. TSS, which is modeled after Dr. Eric
Bannister's
heart rate-based training impulse (TRIMPS), takes into account both the
intensity (i.e., IF) and the duration of each training session, and might be
best viewed as a predictor of the amount of glycogen utilized in each workout.
Thus, a very high TSS resulting from a single race or training session can be
used an indicator that one or more days should be scheduled. For example,
while
individuals will tend to differ in how much training they can tolerate,
depending on their training background, natural abilities, etc., the following
scale can be used as an approximate guide:
The following scale can be used as an approximate guide:
- Less than 150 - low (recovery generally complete by following day)
- 150-300 - medium (some residual fatigue may be present the next day, but
gone by 2nd day)
- 300-450 - high (some residual fatigue may be present even after 2 days)
- Greater than 450 - very high (residual fatigue lasting several days
likely)
As well, the cumulative TSS per week or per month can be used help identify
the maximum intensity and volume of training that still leads to improvements,
rather than overtraining.
By allowing convenient tracking of normalized power, IF, and TSS for each
workout and over time, CyclingPeaks provides both individual athletes and
coaches a powerful tool for analyzing the enormous amount of data gathered by
training with a power meter. The results of such analyses can then serve as
the
springboard for improvements in training and, ultimately, race performance.
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