What is the toughest endurance sport in the world? An analysis with thousands of athletes compares cycling, running, and cross-country skiing

The question has been generating debates among runners, cyclists, skiers, and triathletes for decades. What endurance sport is really the toughest? Now, an analysis led by double Olympic triathlon champion Alistair Brownlee has attempted to answer it using real data from thousands of athletes and hundreds of thousands of sessions recorded with sports devices.

The toughest endurance sport is not as easy to pinpoint as it seems

The study, conducted through the Terra platform, analyzed nine distinct modalities: running, road cycling, indoor cycling, cross-country skiing, alpine skiing, hiking, walking, backcountry skiing, and open water swimming. The goal was not to determine which sport is the most technical, dangerous, or mentally demanding, but to identify which places the greatest demand on the cardiovascular system during a typical session.

The results are clear in one aspect: running is the activity that keeps the heart rate highest and most sustained. Runners recorded an average heart rate of 145 beats per minute, the highest of all the analyzed disciplines, and also reached the highest peak rates.

According to Brownlee, the difference remains even when comparing athletes who practice multiple disciplines. Among users who recorded both running and cycling sessions, 93% showed higher cardiovascular intensities when running.

Cycling loses in intensity but gains in duration

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However, the conclusion changes when a key variable for any endurance athlete is introduced: time.

While the average running session lasted 41 minutes, the average outdoor cycling session reached 64 minutes. Hiking exceeded two hours, and alpine skiing also hovered around 129 minutes.

When combining intensity and duration to calculate the total cardiovascular load, the ranking transforms completely. Running no longer occupies the top position, and disciplines such as cross-country skiing, cycling, or even hiking generate an equal or greater physiological load during a complete session.

For Brownlee, this difference explains why it is so difficult to definitively answer the question. It all depends on whether effort is measured per minute or the accumulated load throughout the entire activity.

Indoor cycling surprises

Another of the most striking findings of the study directly affects cyclists. Indoor cycling recorded an average heart rate nine beats higher than outdoor cycling. The explanation is simple: on the trainer, there are no descents, traffic lights, traffic, curves, or moments of rest where heart rates can drop.

In fact, when analyzing the ability to maintain a constant effort close to maximum heart rate, indoor cycling appears practically tied with running and well ahead of road cycling.

And which one ultimately wins?

The answer depends on the methodology used.

If only cardiovascular intensity per minute is analyzed, running is the clear winner. No other discipline reaches such high average heart rates or maintains such elevated percentages of maximum heart rate for so long.

However, when researchers applied more advanced training load models, such as the TRIMP system commonly used in sports physiology, the result changed again. In that scenario, cross-country skiing appeared as the most demanding discipline, closely followed by running, while cycling occupied the next positions.

Beyond the debate about which is the toughest sport, the work leaves an interesting conclusion for cyclists. Running demands a more intense and sustained cardiovascular response, but cycling allows for accumulating a high physiological load for a longer time with less muscular and joint impact.

That is why many coaches use running as a complement to improve overall fitness, while cycling remains one of the most effective tools for developing large training volumes and aerobic endurance.

In other words, if the question is which sport makes the heart work the most minute by minute, the answer seems to be running. But if we talk about total training load, the discussion remains completely open.