Uncovering the Secrets of Fatigue resistance

Imagine two runners who, during laboratory tests, exhibit the same VO2 max, running economy, and lactate threshold values. These three variables are key components in the "standard model" of endurance performance, allowing for the prediction of marathon times through an equation. In such a scenario, which of the two runners is likely to emerge victorious in the race?

The question may appear hypothetical, but it holds practical significance. In elite sports, athletes typically exhibit comparable values across all three parameters. Should they lack in one area, they make up for it in the others. Despite this, laboratory tests struggle to accurately predict success. For instance, the cyclist with the highest VO2 max failed professionally, and the top marathoners selected by Nike for the Breaking2 event, such as Eliud Kipchoge, did not display exceptionally high lab results. It is evident that there is a crucial element absent from the equation.

In recent years, researchers have focused on identifying a key factor referred to as "the fourth dimension" in the endurance equation. This factor goes by different names such as durability, physiological resilience, and fatigue resistance (the term I originally used in 2021). It gauges how much your physiological indicators change during a race. For instance, your VO2 max may be 70 ml/kg/min at the race's beginning, but could decrease to 65 ml/kg/min after running 20 miles. If your counterpart begins with the same values but maintains a VO2 max of 67 ml/kg/min after 20 miles, they will outperform you.

Factors that may influence fatigue resistance.

There is uncertainty regarding the factors influencing fatigue resistance, leading to a surge in research on the subject. Two recent papers shed light on this by indicating that both pre-race activities and actions during the race can influence fatigue levels.

The initial research, conducted by Runar Jakobsen Unhjem from Nord University in Norway and published in the Scandinavian Journal of Medicine & Science in Sports, involved a comparison between trained runners and "active adults." The runners, who had been training for approximately ten years, typically covered a little over 40 miles per week and achieved 10K best times of 33 minutes for men and 38 minutes for women. On the other hand, the active adults engaged in recreational sports but limited their endurance training to no more than two days a week. Both groups underwent a series of VO2 max and running economy tests before and after running for an hour at a moderate pace equivalent to 70 percent of their individual VO2 max.

In each group, the decline in VO2 max and running economy was more pronounced following an hour of running, particularly in the active adults. Running economy, which indicates the energy required to maintain a specific pace, escalated four times faster in the active adults compared to the runners. While the active adults experienced a 5.0 percent reduction in VO2 max, the runners only saw a 1.2 percent decline, which was not deemed statistically significant.

This implies that the active adults needed to exert more effort to maintain the same speed, even though their ability to work was diminishing. Initially, both groups were working equally hard at 70 percent of their maximum oxygen consumption (VO2 max) after rest. However, the data presented indicates that towards the end of the activity, the active adults were pushing well beyond 75 percent of their maximum, whereas the trained runners were hardly increasing their effort.

Graph showing results of fatigue resistance study (Photo: Scandinavian Journal of Medicine & Science in Sports)

The key point to remember is that fatigue resistance is not solely determined by genetics, which is a positive revelation. Through training, it can be enhanced – although the most effective type of training for this purpose is yet to be identified.

The second study, conducted by a multinational research team led by James Spragg from the University of Cape Town in South Africa and published in the European Journal of Sport Science, examined the impact of different in-race efforts on baseline values. Spragg, who was also involved in the 2021 study , Focused on fatigue resistance in professional cyclists following energy expenditures ranging from 1,000 to 3,000 kilojoules. However, there is a significant distinction between expending 3,000 kilojoules steadily and incorporating surges, hill-climbs, and similar variations.

The recent study examined the endurance of 14 professional cyclists after expending 2,000 kilojoules through two different methods. In one scenario, they maintained a consistent low-intensity pace below their critical power. In the other scenario, they engaged in 5 eight-minute intervals of intense riding above critical power, followed by sufficient easy riding to reach a similar total workload. As expected, the higher-intensity intervals led to a more significant decline in their baseline physiological metrics, particularly impacting their sprinting performance. These findings indicate that sudden bursts of speed during a race or ride are likely to diminish your ability to finish strongly.

According to the specific outcomes, Spragg and his team categorise the cyclists as fatigue-resistant, semi-fatigable, or fatigue-sensitive, with three, four, and seven riders, respectively, fitting into these groups. Previous studies based on real-world data from professional cyclists indicate that, under similar conditions, fatigue-resistant cyclists are the ones who emerge victorious in races. Therefore, what factors influence the classification of individuals into these categories?

Additional research has associated poor fatigue resistance with inadequate fuelling, however, in this particular study, all participants consumed 90 grams of carbohydrate per hour. It is possible that there are metabolic variations in how individuals utilise their carbohydrate and fat stores.

The latest findings indicate that maintaining a consistent pace and avoiding unnecessary accelerations could be beneficial. Similar to the comparison between well-trained runners and active individuals, training could play a role in addressing this issue. Previous findings from professional cyclists indicated that increased training intensity was linked to improved fatigue resistance. Essentially, the mystery of unlocking the fourth dimension remains unsolved, but we are starting to uncover some clues.