Power and Heart Rate Relationships

Edmund R. Burke, Ph.D.

Just as heart rate monitors improved training performance and feedback in the 1990's, power meters are pushing the theories and performance of endurance cyclists into new territory. The biofeedback that power meters provide brings another dimension to training and racing, if the information is interpreted properly. However, blending heart rate with power makes for a more comprehensive and effective system of monitoring your training and racing program.

In cycling power is expressed as "Watts". Power is the rate of work being completed. If you move from one point to another, against the resistance of rolling friction, gravity, and/or aerodynamic drag, you will do a certain amount of work. How fast you do that work is called power. Work is defined as the product Force times Distance. If you lift or move a 5-kilogram weight (1 kg = 2.2 pounds) over a 2 meter distance you would perform 10 kg meters of work, or 10 kgm.

Power = Work divided by Time. One Watt is defined as 6.12 kgm per minute or 0.102 kgm per second. Given enough time on the bike any cyclist could complete 2,000 kgm of work. However, only a few highly trained cyclists could perform that work in 60 seconds. Which would equal 33.33 kgm per second or 327 Watts. If several months latter they complete the task in 55 seconds they would be producing 356 Watts. They are now more powerful.

So, for example, if you climb to the top of a hill you'll do a lot of work. The work it takes to get to the top of that hill will be the same no matter how fast or slow you climb. However, if you climb that hill twice as fast as the time before, you'll have exerted twice the amount of average power as the time before. You can increase power by either increasing your cadence, gear or by pushing harder on the pedals while riding your bike.

In terms of useful cycling information, power is much more valuable than speed and distance. Variations in speed are difficult to evaluate. In fact, maintaining a certain average speed may be an inefficient use of energy, especially if one does not know the power required to maintain that speed. From the power calculation you get an accurate picture of how efficient your body is performing. Whether racing or training, you can compare one ride to another using power as the basis.

Why Power and Heart Rate Training?

So how does the added element of power monitoring play into training? Power output does not replace heart-rate measurement, but is intended to be used in conjunction with it. The main benefit is that it allows you to actually measure workload rather than make a guess at it based sorely on heart rate, which can be affected by temperature (heat or cold) and other factors.

Power is a direct reflection of the exercise intensity, whereas heart rate responds to changes in power. Power, therefore, will change rapidly and a power curve will show considerable variation. Heart rate, on the other hand, is relatively stable because it takes a while to adapt heart rate to the changing demands (increased or decreased power output).

Within any given ride you can easily evaluate performance based on power output. At any point in the ride you can know if they are performing at, below or above previous best performance.

Focus on power for intervals (up to 20 minutes for example), hill training, sprint training, and all anaerobic workouts. Often in these efforts heart rate lags behind the intensity and the resulting change in heart rate. For example, at the start of an interval, heart rate takes several seconds, or even minutes to catch up with energy demands being expended. During a short sprint it will never give an accurate measurement of the effort.

Heart rate along with power is best used for steady state training particularly that done below the lactate threshold. It is especially effective during long, aerobic rides and for recovery workouts.

For training or racing, you can compare one ride to another by measured power and heart rate. Within any given ride a you can easily evaluate performance based on power output. At any point in the ride they can know if you are performing at, below or above previous best performance. Applying each of these intensity monitoring systems has the potential to dramatically improve training and racing. While using a heart rate/power system you will see significant improvement in race performance when you begin training with power.

Training With Power and Heart Rate

One can now base endurance or aerobic workouts on power levels and heart rate zones. If you are making the same power at a lower heart rate than during previous endurance efforts over the same terrain, you can measurably tell that your fitness has improved. Likewise, if you're making less power at the same or higher heart rate, it is a sign you are losing fitness.

If your heart rate is higher than normal, but you can't ride at your target power output? Incomplete recovery from previous workouts is likely to blame. Also, you can monitor recovery time from an effort of a certain intensity, like during intervals. Equal power output followed by a more quickly dropping heart-rate shows improved fitness. Combine this information with distance, speed, cadence and other information and you have a complete training monitor.

Power is the new kid on the block to help you more accurately measure cycling intensity. Along with heart rate it accurately measures your performance and helps prevent you from overtraining and under training. In a future column I will share with you some ideas of how coaches and cyclists are using power and heart rate in designing training programs.

Table. Summary of approximate physiological test values for male competitive cyclists during a maximal stress test on a bicycle ergometer

Category	VO2 Max Ml/kg/min.	Peak Power Output Watts	Power to Mass Watt/kilogram
Competitive	60 - 65	300 - 350	4.5 - 5.0
Sub Elite	66 - 70	375 - 410	5.3 - 5.7
Elite/Pro	71 - 80	>430	6.0 - 6.5