Factors Affecting Power and Heart Rate

The Problem with Heart Rate

When heart rate monitors were introduced for consumer use, the main goal was to increase training efficiency. By measuring heart rate, a better understanding of the impact and intensity of a workout on the body could be gained. Prior to heart rate monitors, speed was used as an indicator of workout intensity, which was not very accurate due to external circumstances playing a significant role. However, heart rate is also an indirect measure of workout intensity, as it is the body's response to effort. Unfortunately, numerous factors can influence heart rate, such as stress, caffeine, altitude, recovery, sleep, and time of day. All these factors have an impact on heart rate.

To illustrate this further, let's consider an example of a rider with a Functional Threshold Power (FTP) of 250 W. When this rider cycles at a power of 200 W, which is below their maximum lactate steady state (MLSS), they can sustain this intensity for a long duration. However, if the same rider is at a high altitude of 2500 meters above sea level, their heart rate will be higher at the same power. This is because, at high altitude, the air pressure is lower, resulting in lower oxygen uptake in the lungs. Consequently, the body has to work harder to supply the muscles with the same amount of oxygen. Additionally, fatigue and caffeine consumption can also affect heart rate.

The Effect of Blood Volume on Heart Rate

The relationship between power and heart rate can be disrupted by various factors. For instance, if you haven't cycled for a while, your heart rate is likely to be high during the initial rides. This is because, when you are fresh and well-rested, your heart has to pump harder due to the rapid decrease in blood volume following a period of inactivity. However, this doesn't mean that heart rate is entirely useless, and relying solely on power to determine workout intensity is not recommended.

The Body's Response to Exercise

Training with a heart rate monitor provides an objective measurement of the body's response to exercise. Combining heart rate and power can reveal how the body reacts to exercise and what it is capable of (power). It becomes even more valuable if your subjective perception during a workout aligns with the numerical data displayed on your bike computer. For example, if your heart rate is elevated during an extensive endurance training session and you don't feel great, it could indicate an underlying illness or insufficient nutrition. Conversely, if your heart rate is slightly higher than normal and you have had a few days of rest, it may signify that you are well-rested.

Considering Power and Heart Rate

The complete picture of workout intensity can be obtained by considering power, heart rate, and your subjective feeling. Even power can vary depending on environmental and situational factors on a given day. However, when all external factors remain consistent, such as performing your indoor workout one hour after dinner each week, it provides a reliable means to measure your workout load and track progress. It's important to note that your FTP will not change significantly from day to day. Therefore, resist the temptation to increase the intensity of a training session solely based on a slightly lower heart rate. While a lower heart rate at the same power level could indicate progress, it could also be influenced by other factors. Similarly, a higher heart rate at a certain power level should not be immediately concerning. Instead, observe long-term trends in your workouts to determine if you can deliver more power at a specific heart rate, indicating a positive training effect. Always listen to your body and avoid drawing conclusions based solely on a single workout, as heart rate cannot be directly compared to power.