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Heart Rate in Cycling Training: Limitations and Best Practices
Heart rate (HR) is a commonly used indicator to assess exercise intensity during cycling training. However, many factors influence HR, which can lead to misinterpretations and limit its accuracy. We discuss phenomena such as HR drift and inertia, as well as external parameters that affect it. We propose solutions to optimize its use.
By Guillaume Judas – Photos: depositphotos.com
About forty years ago, heart rate monitors for exercise revolutionized training in sports, and cycling in particular, for the first time. And it took the appearance and then the democratization of power meter devices a few years later to further refine and personalize the preparation of racers and sportive riders. Even today, many practitioners are content with heart rate measurement, which is widely available and common on most bike computer/GPS units on the market.
Monitoring heart rate during exercise allows for optimizing the time spent in certain intensity zones, for faster and more targeted progress. Heart rate monitoring also proves very useful for managing energy intake, limiting drifts related to dehydration, anticipating illness or a heart problem, or complying with doctors’ instructions based on age or medical history.
However, heart rate measurement has certain limitations. And in some cases, a simple HR reading is not relevant for indicating exercise intensity.
External Factors Influencing HR
Several external and physiological parameters can modify HR for the same exercise intensity:
- High temperatures: significant heat complicates thermoregulation, forcing the heart to beat faster to maintain blood circulation, which increases HR.
- Dehydration: fluid loss reduces blood volume, forcing the heart to accelerate to ensure stable cardiac output.
- Altitude: at high altitude, lower oxygen availability forces the body to increase HR to meet energy needs.
- Fatigue: a state of general fatigue, whether physical or mental, disrupts the adaptation of HR to effort, making data less predictable.
HR Inertia: A Delay in Adaptation
HR inertia corresponds to the time delay required for HR to adapt to a sudden increase in effort. This delay is particularly problematic during rapid changes in intensity.
Example: during an interval where power goes from 150 W to 350 W, HR does not instantly jump from 130 bpm to 195 bpm. It increases gradually, which can distort the assessment of effort.
Consequences: for short, high-intensity intervals (e.g., 30 seconds at maximal aerobic power, or MAP), HR does not have time to reach its peak. Thus, even if the cyclist reaches their VO2 max (maximal oxygen consumption), HR remains an unreliable indicator. For long intervals (e.g., 8 minutes at threshold), trying to quickly reach a target HR can lead to starting too hard, at the risk of not sustaining the effort.
Cardiac Drift: A Progressive Increase in HR During Effort
Cardiac drift refers to the gradual increase in HR during an effort, even when intensity, measured by power (in watts), remains constant. This phenomenon is particularly visible during prolonged or repetitive sessions, such as intervals.
Example: during an interval session, you might produce 290 W for an HR of 180 bpm at the beginning of the session. At the end of the session, for the same HR of 180 bpm, power can drop to 275 W due to fatigue. This shows that, as fatigue sets in, the same HR corresponds to a lower intensity.
Consequences: relying solely on HR to determine the intensity zone can be misleading, as it does not accurately reflect the actual power produced.
How to Use HR Effectively?
Given these limitations, here are recommendations for optimally integrating HR into cycling training:
- At high intensity: prescribing training based solely on HR is imprecise. For greater reliability, it is recommended to couple HR with perceived exertion (RPE), which allows for subjective assessment of intensity, and/or to use a power meter, which directly measures the power produced and remains the most reliable tool for managing intensity.
- At low intensity: HR is a good indicator in endurance zones, where cardiac drift is less pronounced. It ensures that the effort remains within the target zone.
- Physiological analysis: HR remains a valuable tool for analyzing physiological responses to training, such as the body’s adaptation to effort or recovery. It helps understand how the cyclist reacts to different workloads.
Essential, but Not Always Sufficient
Although heart rate is a useful and, in our opinion, indispensable indicator for monitoring health, it should not be used alone, especially at high intensity, due to drift, inertia, and the many external factors that influence it.
For precise training, combining HR with a power meter and perceived exertion is the best approach. At low intensity or for fairly long efforts, HR remains a reliable and accessible tool, while its overall analysis allows for a better understanding of physiological adaptations. By integrating these principles, you can optimize your training and achieve your goals with greater precision.
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