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The limits of endurance


Ultra marathons over many weeks and with impressive distances and similar events in this sector seem to push the limits of human performance ever further upwards. The participants cover long distances, sometimes over weeks, often under extreme climatic conditions. Always higher, faster, further seems to find its realization here as a motto.

But is there actually a physiological limit of endurance from which it's over at some point? This is what a study published in Science Advances magazine suggests. In this blog post you will find out exactly how this went, what the results were and what you can take away from it.



The partner study between Duke University and the University of Aberdeen examined a group of competitive athletes who participated in the 2015 Race Across the USA. The aim was to run a distance of 252 km per week over a period of five months. Initially, the BMR (Basal Metabolic Rate), i.e. the energy consumption of the body to maintain vital functions, was determined and the variation in total turnover over the duration of the race was measured using spirometry. The researchers then compared the data with other races. Their duration ranged from 0.5 to over 250 days.

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A clear trend was shown in accumulation. Graphically, the curve initially rose rapidly and sharply, but then declined again until it stabilized at a steady level. While the daily turnover was still very high at the beginning of longer races, it gradually decreased in negative proportion to the duration until it leveled off just below three times the BMR. The bodies of the test persons continued to reduce the total energy consumption despite the same distance.


In the next step, the study group drew on data from various overfeeding studies in which test persons were given higher amounts of energy in excess of the basal metabolic rate. From this, the conclusion could be drawn that the body is only able to effectively convert a maximum of 2.5 times the BMR into energy. If the daily turnover is higher, he is forced to use his own body mass to provide energy. He falls into a catabolic state where, in addition to breaking down adipose tissue and intramuscular glycogen stores, he also begins to break down muscle tissue.



The results allow for different theories. The most likely, also in the opinion of the investigative group, is the following:
The plateau, which resulted from the study, could represent a protective mechanism of the body in order not to literally "use itself up". By reducing the basal metabolic rate and an overall more economical energy balance, he can perform the exercise longer. At the same time, however, this probably also represents the absolute biological threshold for such long-term events that the body can afford. If this is forced to be exceeded, it inevitably ends in an exhaustion abort to protect one's own resources. However, as mentioned, this effect only occurs in competitions of exceptional duration. This was not observed in shorter races.



Not only do the results allow for several possible interpretations, the design of the study also raises inconsistencies. First of all, the very small sample group, which comprised five men and one woman, should be listed here. In addition, not all of this group finished the race. One competitor dropped out of the race due to exhaustion, while the other chose an alternative, longer route. Likewise, one participant with a backpack marched instead of actually running the route. The preliminary determination of the BMR was estimated using a formula without testing it for actual accuracy, which can cause large deviations from the actual actual value. Finally, spirometry, as a method for determining BMR, has often been criticized in the past for its inaccurate results.

Accordingly, the study results appear in a less clear light.

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Take home message

While the study can certainly provide a starting point for further investigation, no clear statement, if any, can be made from it on its own. More research with better design in this direction is definitely needed to gain better insights.


link to study

Extreme events reveal an alimentary limit on sustained maximum human energy expenditure


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