by Arnie Baker, M.D.
Power is the rate of work. Power meters provide the best measure of muscular work. Power on a bicycle is measured in watts. Power over time, or work, is measured in kilojoules.
Power is a measure of workout intensity. Its key features are:
- Absolute, objective measure
- Race predictor
- Not affected by confounding variables. Unlike speed, for example, it is unaffected by environmental conditions such as wind or elevation change.
- Effort sensitive
- Quantify and document current and past fitness
- Compare fitness to others
- Quantify and document fitness changes that have occurred with training, overtraining, overuse injury, or traumatic injury
- Quantify total work, total work during intervals or stratify work performed at various intensities
- Quantify the demands of events
- Quantify and compare work performed with confounding variables—such as varying grades, wind conditions, temperatures, and group vs. solo riding
- Provide training intensity targets
- Encourage athletes to ride harder, or easier
- Provide training guidelines in rehabilitation
- Predict performance in training and events
- Show the decrease in ability with exposure to altitude—and the improvement that comes with acclimation13
- Show the decrease in ability with exposure to heat, humidity, or cold—and the improvement that comes with acclimation
- Demonstrate changes in power with hydration status or fatigue
- Suggest a check for medical causes of decreased power
- Demonstrate the aerodynamic savings of equipment
- Demonstrate the aerodynamic savings of position
- Demonstrate improved power with changes in position
- Demonstrate the value of drafting, especially to “slow learners”
- Give a measure of calories burned
- Provide immediate and reviewable feedback about pacing
- Provide athletes with biofeedback and quicker appreciation of perceived exertion
- Show that performance (power) may be fine even though feeling tired or otherwise “slow”
- Help motivation
- Help confidence
Training Load and Race Predictor
Power measurement is the gold-standard measure of absolute workload. Power is what gets you down the road. (Wind resistance, rolling resistance, and gravity hold you back. Formulae exist for predicting performance based on power.)
Hill climbing ability correlates well to aerobic power output divided by weight. Time trialing ability correlates well to aerobic power divided by frontal area or drag. Sprinting ability correlates well to anaerobic power.
While other measures of intensity, such as perceived exertion or heart rate, can provide relative measures of individual workload intensity, they do not predict performance.
Power-based testing is easy. Testing can help evaluate the effectiveness of training. Like VO2 max testing, power testing is valuable in predicting race performance. Unlike VO2 max testing, power testing can be portable and need not require a physiology lab.
Immediate and Effort Sensitive
The first thing most riders notice is that power readings change dramatically and immediately even in the course of what might have been thought of as hard steady efforts.
Heart rate is a physiologically smoothed function. Riders used to looking at heart rate values know that if they surge or relax somewhat during the course of a time trial, heart rate may change only a few beats. As Figure 13 shows, power changes may be striking.
Power measurement—traditionally available on laboratory ergometers—has been available on new- generation portable consumer devices for more than a decade.
Durability was initially a problem for some units. This has improved.
Electronic stationary trainers generally measure power as the rear wheel turns a resistance device.
Force-measuring devices that can be installed at the pedals, crank, bottom bracket, chain, or rear wheel axle are available. These allow riders to measure power on the road, trail, or track.
Some devices may add up to half a pound in weight and so fractionally worsen uphill performance.
Some devices that purport to measure power output do not—they impute it from speed, gradient, and rider weight. Such units are useful in that they provide a measure of relative workload intensity under identical conditions—for example, climbing a steady grade with no wind.
To evaluate workload, you ideally want to measure power at the pedal.
The further away from the pedal you measure, the greater the mechanical power losses.
For example, you would expect power at the pedal to be greater than power measured in turn at the crank, the bottom bracket, the chain, and the rear wheel.
Although such power losses may amount to as much as 10%, they are generally less.
Accounting for such losses may be important in evaluating the same athlete on different devices, or in comparing different athletes on different devices.
Overall Validity and Reliability
Some devices give quite valid (or accurate) readings. Accurate units tend to be expensive ($500 to $2,000+).
Some devices may give consistent values with the same workload—though the absolute values may not be accurate. In other words, they give repeatable results but are not calibrated correctly.
Such units can provide valuable information about relative training intensity during workouts for an individual, but are not useful in estimating absolute fitness relative to others.
Response to Peaks
Power output varies considerably at different points in the pedal stroke. Some devices can measure changes in small fractions of a pedal stroke.
Power measuring devices smooth this measurement to give useable averages.
Power measuring units have an easier time measuring steady-state values than peaks.
Some devices smooth or lag so much that they miss the peak outputs of surges or sprints.
Some devices allow the user to choose the degree of smoothing. This useful function shows peaks with minimal smoothing; it gives insight into overall-ride power output, including pacing, with more smoothing.
Training hours and mileage are commonly used as measures of training volume. Total work may be a better measure of training stress.
Power is the rate of performing work. Many devices can compute the work accomplished over a period of time. This is commonly reported in kilojoules.
A joule is one watt of power for one second. There are 3,600 seconds in an hour. One kilojoule equals 1,000 joules. Therefore, averaging 100 watts of power for one hour yields 360 kilojoules of work.
Figure 23 on shows an SRM-brand crank-measuring power report that includes total work.
Rule of Thumb—Close Enough
Since a kilojoule equals 0.24 calories and since the body is about 24% efficient in converting energy to muscular work, kilojoules of work provide a good estimate of calories burned.
That is to say if your total ride work is 1,200 kilojoules, you have also burned about 1,200 calories in producing that work.
For most riders, the body is closer to 22% efficient, about 10% less than the 24% quoted above. If your total ride work is 1,200 kilojoules, you have burned about 10% more calories, or about 1,320 calories.
Three Ways to Use Power
Riders and coaches use power meters in three general ways:
- During a workout
- Download and analysis of a single workout
- Analysis of multiple downloads
“Want to make a room full of coaches burst out laughing? Make a joke about not downloading files or (gasp!) not reviewing them. Power-meter software is critical for taking full advantage of the big picture that power data paints.” –Frank Overton, VeloNews, Vol. 35, No. 1 January 2, 2006.
Power During Efforts
This is what most riders and coaches initially examine.
What power can be sustained during a climb, time trial, or other interval?
What happens during racing? What kinds of efforts are required? Are those efforts simulated in training?
Determination is made crudely while riding, or more precisely with computer software of downloads.
Watts Per Kilogram
Just as absolute heart rate is of limited importance for most riders (percentage of maximum heart rate is a more useful statistic), absolute power output is less relevant than power per unit of mass, that is, watts per kilogram. (Metric units are used more frequently for this statistic, although some use watts per pound. A kilogram equals 2.2 pounds.)
A 90-kilogram (200-pound) rider will generate roughly twice the power of a 45-kilogram (100-pound) rider to ascend at the same speed. Watts/kilogram (pounds) will be roughly the same.
Power monitoring provides a much better measure of pacing performance than heart rate. A declining heart rate usually indicates declining power. However, heart rate may remain high, or even increase, though power declines, as Figure 18 shows.
Power in Different Positions
Within limits, power is generally greater the more open the hip angle and the greater the leg extension. However, a high saddle position compromises leg speed.
Time trialists often prefer a high-saddle position that allows a more open hip angle. Power when riding bent over on aerobars is lower than when upright. Aerodynamics more than compensates for this power loss. Remember, in flat-terrain time trialing, power per frontal area/drag coefficient is a better performance predictor than power per mass (watts per kilogram).
Since most riders cannot calculate their frontal area and do not have access to wind-tunnel testing, power is not the most helpful measure of performance. Elapsed time, speed, or cadence in a specified gear on a controlled course provides a better indicator in assessing the relative effectiveness of time-trial positions.
Power vs. Time
The shorter the interval, the more average power can be generated for the interval.
Figure 19 shows how values rapidly decline from short, pure anaerobic efforts to a slow decline for aerobic work.
Power Ranges for Athletes
Riders of different abilities have substantially different power outputs.
This is in contrast to the percentage of maximum heart rate at which riders can time trial—this value is similar for men and women, young and old, beginning racers and professionals.
For example, most riders are able to complete 3- to 5-minute intervals at 90% of maximum heart rate. Beginners may perform these intervals at 100 watts. Professionals at 500 watts.
Power range standards for elite riders, stratified by age and sex, are available.
Values based on athletes I have coached are listed in the appendix starting on page 213. Values according to Andrew Coggan are graphed in Figure 20.
Relative to their overall ability, strong sprinters put out relatively more power for short periods of time than relatively strong time trialists.
Work Time at Different Power Levels
Just as users of heart rate monitors are used to looking at time spent in different heart rate zones, power meter programs can give the same information.
However, training time in specific power zones only tells part of the story. Training at 300 watts for 15 minutes continuously is different from training at 300 watts for three minutes five times.
Some programs use three-dimensional displays to illustrate this.
Power Per Heart Beat
Some power-based computer programs can estimate how incremental power changes relate to heart rate.
Over time, this information can help give a measure of conditioning and evaluate the effectiveness of training. As fitness im proves, under similar conditions (fatigue/recovery, nutritional, environmental), incremental power increases require fewer incremental heart beats.
Figure 23 shows an SRM report that estimates the heart rate change with each incremental one-watt increase in power. Dividing this value into one estimates the power output gain for every one-beat increase in heart rate.
Power at Fixed Heart Rate
Some power-based computer programs can compute or impute how much power is generated at a specific heart rate—say 150 beats per minute.
Over time, this information can help give a measure of conditioning and evaluate the effectiveness of training. As fitness improves, under similar conditions, more power can be generated at a given heart rate.
Figure 23 shows an SRM report that estimates power output at a heart rate of 150 beats per minute. In this example, it was 149.5 watts.
Training: Power-Based Intervals
As stated above, the shorter the interval, the more average power can be generated for the interval.
The maximum average amount of power that can be generated for a given time is sometimes termed critical power (CP). For example, the maximum average amount of power than can be generated for 60 minutes is sometimes referred to as CP60.16
Power ranges for ABC high-intensity workouts are large—given the great variations in individual fitness.
One goal of training is to increase the power that can be generated for any specific length interval. This is illustrated in Figure 24.
Basing Training Intensity on Threshold Power
One method of prescribing power-based interval intensity is to target values based on threshold power.
Using threshold power to determine training intensities for various length intervals means that multiple testing sessions for different interval lengths are not necessary.
There is a physiologic basis for this approach.
For example, some coaches target power outputs for 3- to 5-minute intervals at 110% of 10-mile or 20-kilometer average time-trial power.
If you can time trial for 10 miles or 20 kilometers at 300 watts, you might target to perform 3- to 5-minute intervals at 330 watts.
This approach has pitfalls, especially for intervals of one minute or less.
Although power level multipliers for intervals of specific durations have population averages or norms, these do not apply to the individual.
Consider that although the average bicycle racer may be able to average 4 times more power for a 5-second interval than for a 10-mile time trial, the sprint specialist will have a higher multiple.
Basing Training Intensity on Max Ramped Power
This is a variation of the above. Similar to a VO2 max test, this is a progressive test until exhaustion.
After a warm-up, power is ramped up—say 20 watts per minute—until failure.
Percentages of maximum ramped power are used to determine training intensity levels.
This method has pitfalls similar to basing intensity on threshold power.
Basing Intervals on Trial and Error
In some ways, basing training intensity on threshold or max-ramped power is like basing weight-room workouts on 1-rep max. Many strength and conditioning coaches specify, for example, 10-rep workouts at say 80% of 1-rep max. Determining 1-rep max generally is for experienced lifters only.
From a practical point of view, many who strength train simply determine, though trial and error over a few sessions, how much they can lift for 10 repetitions.
In the same way, adapting to 3- to 5-minute intervals over several sessions, and determining watts goals through trial and error is just as valid a method as arbitrarily trying to work out at 110% of time-trial threshold.
As alluded to above, determining anaerobic efforts based on time-trial threshold is fraught with error, as riders have more or less anaerobic vs. aerobic power.
Although testing critical power for many interval lengths has theoretical merit, in practice as fitness increases, these values change.
In practice, riders must determine their anaerobic power targets through trial and error, based in part on number of repetitions, recovery interval length, and coaching philosophy.
Targeting power based on trial and error has another advantage: It is simpler than performing multiple, repeated tests and calculating percentages.
Predicting Individual Form
Manipulation of training volume and intensity is used to improve training and peak for events.
Software can be used to analyze past training load and predict an individual’s form.
Such software requires using a power meter on every ride. Cross training confounds the program.
An intuitive rider or coach can often predict performance as well as such programs, or better.
Power vs. Other Intensity Measures
As shown in Figure 26 and Figure 27, under controlled circumstances, other measures of intensity can give the same, similar, or additional information.
- Power-measuring devices provide an immediate indication of absolute workload.
- Watts per kilogram is generally more useful than absolute power.
- Power-based testing is useful. Like other measures of fitness, it is contextual, most accurate under controlled conditions.
- Power-based training can be effective in training many fitness systems.
- Unless one knows at what percentage of possible power one is riding, power does not provide a measure of relative individual exercise effort.
- In practice, I use power-measuring devices to evaluate current fitness, estimate what is possible, and to help motivation and pacing.
- I generally fall back to the oldest methods of measuring intensity—perceived exertion and instinct—in determining how much work to perform.