Jogging versus sprinting Where's the proof

Endurance: what is the performance limit?

Natural limit: Researchers have found out why endurance athletes can't break new speed records all the time. As their study shows, there seems to be a physiologically determined performance limit. With longer-lasting activities, the metabolism slows down at some point and levels off at a constant level. This is apparently never higher than 2.5 times the individual basic metabolic rate.

Whether Tour de France or Ironman: Sports competitions like these prove time and again what impressive endurance the human body is capable of. But where is the limit? Even the best athletes cannot set new records indefinitely over long distances, it seems. Is there a natural limit that determines the maximum amount of energy we can burn over the long term - and thus also what we are able to achieve?

Endurance athletes in view

To find out, Caitlin Thurber from Hunter College in New York and her colleagues have now devoted themselves to one of the toughest endurance races in the world: In the Race Across the USA, participants run the 4,800-kilometer route from Huntington Beach in California to Washington D.C. That means they run a marathon almost every day for 20 weeks.

The scientists collected data on the daily energy consumption of the athletes during the race in 2015. They also analyzed previously published information on the metabolism of participants in other endurance sporting events - from the Tour de France to the trekking race through the Arctic.

Uniform upper limit

The evaluations showed: The same pattern was shown in all longer-lasting sports competitions. While the athletes' metabolism was at full speed at the beginning and they burned a relatively large number of calories, the energy consumption curve sagged over time and then remained at a constant level for the rest of the event.

The upper limit thus reached seemed to be a maximum of 2.5 times their basic metabolism rate for all athletes, as the researchers report: "We have not found a single case in which the curve goes beyond this limit." lasted a day, according to them, however, a different picture emerged. "With such shorter activities, there seems to be a different performance limit."

Limit not only in sports

According to the scientists, these results indicate that the organism downregulates the metabolism at a certain point in order to achieve a sustainable level. “We can only sprint 100 meters, but jog for miles. This relationship also applies to metabolism, ”explains Thurber's colleague Herman Pontzer.

Because the pattern now observed was shown in very different sports, the scientists believe: There is a maximum of exertion that is valid regardless of the activity that a person can maintain over the long term - even in areas beyond sport.

Gut as a limiting factor

For example, this limit also seems to apply to the exertion of pregnancy. As the researchers found, the maximum sustained energy consumption in the athletes was only marginally higher than the metabolic rates observed in pregnant women. The same mechanism that prevents Ironman participants from reaching new record times may also affect how big a baby can get in the womb.

But which physiological factors determine the performance limit of our metabolism? Thurber and her colleagues found evidence that, among other things, the digestive process is a limiting factor. "Our digestive tract can only use a limited amount of calories effectively per day," says Pontzer.

Carved in stone?

In the future, further studies will have to show which other factors play a role in this connection - and whether the limit documented by the researchers really cannot be broken. “I see this as a challenge for all elite endurance athletes. Perhaps one of them will manage to exceed this limit and show us that we have overlooked something, ”concludes Pontzer. (Science Advances, 2019; doi: 10.1126 / sciadv.aaw0341)

Source: AAAS / Duke University

June 7, 2019

- Daniela Albat