Riders in indoor cycling classes spent a 50-minute session with their hearts working at an average of about 81 percent of maximum, and peaking near 94 percent, while their legs produced only 110 to 120 watts on average, about half of their maximal aerobic power. That gap between what the heart reports and what the pedals measure is the finding of a study published online October 10, 2026, in the International Journal of Sports Science and Coaching by four researchers at the University of León in Spain.

What the study measured

The research team recorded heart rate and pedaling power in 11 trained men across 130 real, instructor-led indoor cycling classes, the kind of group spin session offered at gyms across the country. Heart rate served as the measure of internal load, meaning the strain on the cardiovascular system. Power, in watts, served as the measure of external load, meaning the mechanical work actually done on the bike. The authors also compared the power that a standard heart-rate-to-power regression (the PWC-170 method) would have predicted against the power the bikes recorded.

The two measures did not line up. Heart rate averaged roughly 81 percent of maximum across a session and reached about 94 percent at its peak, numbers that would place most of a class in vigorous territory by heart rate alone. Mean power sat around 110 to 120 watts, which the authors put at roughly 50 percent of the riders' maximal aerobic power, and peaks rose only a little above that. Estimating power from heart rate overshot the measured figure by about 25 percent. The authors describe internal and external load in these classes as systematically decoupled and suggest that structured high-intensity intervals would bring the mechanical stimulus closer to the cardiovascular demand the class already produces.

How to read it

This is an observational study in a small, specific group: 11 men who were already trained, riding in classes rather than in a lab. The paper's metadata lists no funding source. Only the abstract could be opened for this article, so the full methods, the exact statistics and the limitations the authors name in their discussion could not be reviewed here. A result in trained men does not automatically describe what happens to a new rider, to women, or to anyone whose heart rate responds differently to heat, caffeine, sleep loss or medication.

The finding also does not say that spin classes fail to deliver a workout. A heart rate held at 80 to 90 percent of maximum for most of an hour is a real cardiovascular demand. What the study questions is the habit of using that number to judge how much mechanical work was done, or to compare a class against a structured interval session on a power meter.

Heart rate zones versus the guidelines

Federal physical activity guidance does not prescribe heart rate zones at all. The current Physical Activity Guidelines for Americans, published by the U.S. Department of Health and Human Services, ask adults for at least 150 to 300 minutes a week of moderate-intensity aerobic activity, with brisk walking as the standing example, plus muscle-strengthening work on at least two days a week. Intensity in that framework is about how hard the activity feels and how much energy it uses, not a watt target. By heart rate, a spin class as measured in this study would count toward the vigorous end of that range. By power, the same class looks like steady moderate work with a few short surges.

The American Heart Association publishes its own target heart rate guidance and physical activity recommendations on heart.org; those pages could not be opened while this article was being prepared, so their specific zone percentages are not reported here.

What this means for the bike

For anyone who tracks training with a chest strap or a watch, the practical question raised by the paper is which number to trust for which purpose. Heart rate still tells a rider how hard the cardiovascular system is working, which is what matters for the time-in-zone most fitness apps report. Power tells a rider how much work the legs produced, which is what matters for comparing sessions, tracking fitness over months and pacing intervals. In a class built around cadence drills, standing climbs and music, the two can drift apart by a quarter or more, according to this one study.

Many studio bikes now display watts alongside heart rate. The paper's authors point toward structured intervals as the way to close the gap. For a rider with a heart condition, on a beta blocker or any other medication that changes heart rate response, the question of which measure to pace by is one to bring to a clinician before the next class.

This article reports research and expert guidance. It is not medical advice. Talk to your clinician before changing medication, treatment or a medical diet.

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