Your Heart Rate is Probably Wrong

(Is your heart rate data helping you reach your peak, or holding you back?)

For decades, heart rate data has been the “go to” for athletic training. Heart rate promises a simple and inexpensive window into our bodies' cardiovascular system. With the explosion of wearable tech like smartwatches and smart rings, constant heart rate data is now readily available, though its accuracy remains a point of contention. As an elite cyclist, sport scientist and coach, I've seen both the high standard of lab based heart rate analysis to the wild west of wrist worn sensors. Between significant daily fluctuations and inherent sensor inaccuracies, the question remains: is your heart rate data a valuable tool or a training liability?

The Hidden Flaws: Why Your Heart Rate Monitor Isn't Always Your Best Training Guide

Using heart rate as a training metric offers a theoretically sound approach to gauge exercise intensity. By directly reflecting the cardiovascular system's response to exertion, heart rate provides a quantifiable measure of physiological stress. The long term monitoring of heart trends can also be used to evaluate fitness adaptations and also as an indicator of overtraining and fatigue.

The inexpensive nature of heart rate sensors has long been a driving selling point for athletes. Particularly for those who are just getting into training. For example, a Garmin heart rate chest strap retails for about A$125. Additionally, anyone with a modern smart/sport watch will have a photoplethysmogram sensor built in to measure heart rate.  Another widely used training tool for athletes is the power meter. While these are extremely precise for training, they come at much steeper cost with the more cost effective options still around A$1000. This difference in cost explains why heart rate remains a prevalent training tool, particularly for new athletes

For training we want a consistent and reliable tool to measure our exertion during efforts. Striving for 100% accuracy isn't necessary. It wouldn’t matter if your heart rate sensor measures ten beats per minute (bpm) lower than your actual heart rate as long as it measures ten bpm lower every time. Heart rate suffers from numerous external factors which makes reliability very difficult.

(Target Analogy to Understand Accuracy and Reliability - credit Scienceready)

Your hydration level significantly affects heart rate. Dehydration induces elevated heart rate (Heaps et al., 1994), compromising the reliability of heart rate data for measuring exertion. This effect is compounded by several factors; natural dehydration during exercise, particularly exacerbated by elevated temperatures, fluctuating pre-training hydration levels due to daily life and variable sweat rates across different training sessions and environmental conditions.

Heart rate lag is another well known limitation (Jagoda et al., 2014). What’s lesser known is that this lag time varies based on cardiovascular fitness. Jagoda et al. (2014) demonstrated that athletes with greater cardiovascular fitness display an increased heart rate lag during the initial phase of exercise. This is attributed to a dominant parasympathetic influence, a hallmark of cardiovascular adaptation. In elite athletes, this translates to a more delayed sympathetic activation (Jagoda et al., 2014). Changes in heart rate lag, notably with cardiovascular fitness changes, challenge the consistent reliability of long term heart rate data.

Another significant challenge presented by heart rate lag arises during high intensity interval training. The delayed heart rate response fails to accurately represent the true exertion of these efforts. For instance, in a Tabata protocol (20 seconds on, 10 seconds off, repeated), an athlete cannot pace using heart rate. Initially, the lag prevents the heart rate from reflecting the actual intensity of the work phase. Subsequently, the brief ten second rest periods prove insufficient for the heart rate to decrease and accurately depict the recovery phase. Pacing a neuromuscular sprint power session based on heart rate data demonstrates the mismatch between the metric and the activity. The limitations of heart rate during high intensity intervals are demonstrated by the data below, which were taken from one of my athletes during a Tabata workout.

Cardiac drift imposes a significant limitation to heart rate pacing, particularly during long sessions. Cardiac drift is the gradual increase in heart rate due to constant exercise intensity caused by dehydration, body temperature increase, and blood distribution changes (Wingo et al., 2020). Consequently, athletes attempting to stay within a specific heart rate zone may mistake the rising heart rate as an indication that they are working harder. 

(The onset of cardiovascular drift - credit Sport Performance Bulletin)

Another external factor is in your coffee (caffeine). Caffeine, a widely used psychoactive substance. It works by blocking adenosine receptors, leading to stimulation of the release of adrenaline and resulting cardiac stimulation. This leads to an immediate increase in heart rate and blood pressure (Ribeiro & Sebastião, 2010).

Stress, be it from physical or emotional stress, brings on a sequence of physiological events that profoundly affect heart rate. The body's "fight or flight" mechanism initiated by the sympathetic nervous system, sends out hormones like adrenaline and cortisol. These stimulate an increase in heart rate and blood pressure and prepare the body to act. This increase in heart rate is in response to perceived threats or challenges. Athletes experiencing high life stress must be aware of heart rate changes during training, when stress hormones increase baseline heart rate they may overestimate responses to effort.

(The heart rate sensor has green and infrared LED modes. - credit CultofMac)

Chest strap heart rate sensors detect electrical activity through a band that wraps around your chest. These are considered the best option for measuring heart rate outside of a lab setting and are superior to wrist based photoplethysmogram (PPG) sensors (Pasadyn et al., 2019).

Wrist based PPG devices face a few challenges. Accurate estimation of heart rate at high intensities is a real drawback for PPG sensors, rendering them effectively useless for pacing of high intensity intervals and related data analysis (Reddy et al., 2018). Compounding this weakness, Reddy et al. (2018) noted that PPG sensors are also inaccurate while experiencing dynamic movements. This is a critical concern when athletes are training during off road and uneven terrain sports since such movement is prevalent. Finally, A systematic review of ten studies by Koerber et al. (2023), involving 469 subjects, showed a troubling inconsistency. Studies present conflicting results with some showing a clear reduction in accuracy for darker skin tones, while others found no significant difference. The inconsistency represents a significant drawback in using PPG based sensors, particularly in striving to treat all athletes fairly and with accuracy.

The inherent inconsistencies of heart rate data, stemming from the many internal and external factors, serve as strong evidence against its sole use in training analysis. Observing athletes' tendencies to compare heart rates on platforms like Strava, leading to potentially unwarranted feelings of accomplishment or inadequacy, underscores this concern. With the heart rate carrying such a large genetic component, comparisons can be highly misleading. 

However I do believe that using heart rate can be a valuable tool to learn how the above factors influence their heart rate and performance. By consistently tracking heart rate responses, athletes can gain a deeper understanding of how various training factors influence both their physiological strain and overall performance. This long term use of heart rate tracking ultimately refines their ability to accurately interpret internal signals and correlate them with objective data.

While not without its own limits, heart rate is a useful tool to develop self awareness. Rather than aim for absolute accuracy, athletes may focus on detecting personal patterns and what influences the response of their own heart rate. The long term approach allows more personalised interpretation of the data and uncovering patterns that can be applied to alter training and enhance performance. Through becoming more aware at interpreting the signals their body is providing, athletes can overcome the constraints of raw data and gain a better understanding of their own physiology. So keep recording, because often it’s the more, the merrier when it comes to data collection.

References

Heaps, C. L., González-Alonso, J., & Coyle, E. F. (1994). Hypohydration causes cardiovascular drift without reducing blood volume. International journal of sports medicine, 15(2), 74–79. 10.1055/s-2007-1021023

Jagoda, A., Myers, J. N., Kaminsky, L. A., & Whaley, M. H. (2014). Heart rate response at the onset of exercise in an apparently healthy cohort. European journal of applied physiology, 114(7), 1367–1375. 10.1007/s00421-014-2867-0

Koerber, D., Khan, S., Shamsheri, T., Kirubarajan, A., & Mehta, S. (2023). Accuracy of Heart Rate Measurement with Wrist-Worn Wearable Devices in Various Skin Tones: a Systematic Review. Journal of racial and ethnic health disparities, 10(6), 2676–2684. 10.1007/s40615-022-01446-9

Pasadyn, S. R., Soudan, M., Gillinov, M., Houghtaling, P., Phelan, D., Gillinov, N., Bittel, B., & Desai, M. Y. (2019). Accuracy of commercially available heart rate monitors in athletes: a prospective study. Cardiovascular diagnosis and therapy, 9(4), 379–385. 10.21037/cdt.2019.06.05

Reddy, R. K., Pooni, R., Zaharieva, D. P., Senf, B., El Youssef, J., Dassau, E., Doyle iii, F. J., Clements, M. A., Rickels, M. R., Patton, S. R., Castle, J. R., Riddell, M. C., & Jacobs, P. G. (2018). Accuracy of Wrist-Worn Activity Monitors During Common Daily Physical Activities and Types of Structured Exercise: Evaluation Study. JMIR mHealth and uHealth, 6(12). 10.2196/10338

Ribeiro, J. A., & Sebastião, A. M. (2010). Caffeine and adenosine. Journal of Alzheimer's disease, 20(1), 3-15. 10.3233/JAD-2010-1379

Wingo, J. E., Stone, T., & Ng, J. (2020). Cardiovascular Drift and Maximal Oxygen Uptake during Running and Cycling in the Heat. Medicine and science in sports and exercise, 52(9), 1924–1932. 10.1249/MSS.0000000000002324