Enduring endurance: a durability deep dive

Historically, sports scientists have assumed there are three primary drivers of endurance performance- VO2max, lactate threshold, and economy. VO2max is an indicator of the maximum amount of oxygen that can be used to produce energy; the more energy that can be produced, the higher the speed that can be sustained. Lactate threshold is a measure of what fraction of that oxygen can used sustainably to create energy without the accumulation of (fatiguing) lactate in muscles. Economy meanwhile indicates how efficiently that energy can be used to generate athletic motion.

Not so straightforward

It all seemed pretty straightforward; athletes need to be able to use oxygen to produce a lot of energy, sustain as high a percentage of their maximal energy output as possible, and be able to use that energy as efficiently as possible. The athlete that had the best combination of those three factors would win. However, built into this paradigm is the assumption that these physiological traits are static throughout a race or competition – ie an athlete’s VO2max is the same before and after, their lactate threshold is the same before and after, and their economy is the same before and after. Unfortunately, this assumption is false.

Initially suspected by intuitive coaches, and recently confirmed by scientific research, there is a fourth component that is critical for successful endurance performance. That component is the athlete’s durability, or their ability to maintain robust physiological function (VO2max, lactate threshold, and economy) throughout a race or competition. [Ed – see this article by Andrew for a more in-depth discussion about the concept of durability]

As durability has gained more and more research attention, initial answers have prompted more questions which have led to rapid changes in our understanding of durability. One of the key findings is that ‘durability’ doesn’t describe one single physiological process. In fact, it describes at least four, all of which emerge in unique situations for unique reasons. These different types of durability are all relevant, but they may be more or less relevant to different athletes, depending on the competitive demands of their chosen sport and events. Understanding these different types of durability will lead to better research and better practical application, helping coaches and athletes more effectively solve specific training and performance problems.

New durability research

A recent review paper by four leading scientists has sought to clarify the different types of durability⁽¹⁾. They defined four different types of durability (see figure 1 for a visual representation), explained the likely physiological causes of a loss in durability for each type, and provided training implications for each. Let’s take a look at each in greater detail, as well as potential training applications to limit losses of performance.

Basic durability

The authors defined basic durability as the ‘deterioration in physiological characteristics over time during. prolonged exercise’. The specific focus here is on a loss of the ability to deliver oxygen to the tissues and use oxygen to create energy. This tends to be most expressed during long-duration events, which consistently place a large demand on physiological function for an extended period of time. This is very similar to the original definition of durability expressed as a reduction in VO2max, lactate threshold, or economy over the course of an exercise bout.

From a physiological perspective, there are several potential causes of a loss in physiological function. The authors cite a gradual increase in heart rate, neuromuscular fatigue, glycogen depletion, slow increases in oxygen demand, decrease in threshold, or a loss of economy as factors.  Of course, these can all interact to produce an overall loss of performance.

From a training perspective, enhanced durability may be one of the reasons that strength training counter-intuitively enhances long duration performance. As strength training is performed at high intensities for very brief durations, it differs significantly from long-duration performance, yet it has been shown to improve endurance performance. By strengthening the muscles, athletes may not improve performance directly but improve the ability to sustain a given performance level.

Beyond the impact of strength training, exposure to long-duration training bouts is likely to enhance durability. While long runs or similar activities may not necessarily be the most efficient way to enhance metabolic function, they often remain in training program as coaches and athletes recognize that they do ‘something’. That ‘something’ is likely enhancing the ability to maintain performance over very long durations!

Figure 1: The four elements of durability