Durability: the forgotten ingredient in endurance performance

If you want to perform well in endurance events, there are three main parameters you need to improve: your aerobic capacity (how much oxygen you can consume), your threshold (ie the percentage of your aerobic capacity you can sustain) and your efficiency (so that you can create more speed from the same amount of energy – also known as ‘economy’). That’s all it takes, right? Well actually, no. There’s emerging evidence that another important aspect of performance is highly influential^(1,2).

The durability concept

While the ‘big three’ of endurance performance do really matter, the problem is that they may not be static qualities. In other words, aerobic capacity, threshold and efficiency may change with fatigue. And the degree to which they change may differ significantly from individual to individual. Thus, the ‘durability’ or ‘physiological resilience’ of an athlete can be a key performance determinant.

Let’s say we have two athletes with identical physiological profiles in terms of threshold, aerobic capacity, and economy who decide to race a marathon. Over the course of the race, one of those athletes experiences a 3% drop in their physiological profile and the other athlete only experiences a 2%. One athlete is athlete is more ‘durable’ than the other, and is going to run faster as a result.

Training for durability

This is a critical insight because it provides a broader perspective as to what determines endurance performance, and what must be done to build endurance performance. Not only is it important to develop the foundational physiological capacities, athletes must be able to sustain these capacities at as high a level as possible throughout their races. And while the training that improves threshold, aerobic capacity, and economy may positively improve durability, more specialized training may be necessary for those individuals that have a particular deficit in this quality. Simply being aware of its importance is the starting point for enhancing performance.

While the concept of durability is pretty intuitive, and most athletes have experienced a lack of durability firsthand, there’s not much scientific research quantifying the impact of durability.  And there’s less information as to what physiological factors may be influencing durability. And as with all anecdotally witnessed phenomena, research can be a powerful strategy for gaining greater insight and understand into what is going on. Fortunately, a group of Finnish researchers have taken a big step forward in demonstrating that durability is real and relevant.

New research

In this study, a group of 31 recreational athletes participated in two testing sessions that were completed on two separate days⁽³⁾. As one of the important questions was whether there are sex-based differences in durability, 15 females and 15 males participated so that any differences could be observed. All the subjects were training at least 250 hours per year and were free of chronic health conditions or injuries.

The testing days were scheduled as close together as possible to minimize the impact of any changes in performance status. On the first testing day, the subjects performed a maximal incremental treadmill test. The purpose of this test was to identify the metabolic capacities of the subjects, including the first lactate threshold (the exercise intensity at which blood lactate concentrations begin to rise significantly above resting levels) and maximal aerobic capacity. The measured first lactate threshold was then used as the intensity for the durability run during the second testing session.

The second testing session formed the meat of the study. Its main focus was a 90-minute run performed at the first lactate threshold intensity, as determined by the incremental treadmill test. Prior to the run, the subjects performed a submaximal threshold test and a 10/5 reactivity test. As the name implies, the submaximal threshold test was performed at lower intensities to minimize the impact of fatigue from the testing itself. This allowed the researchers to determine if there were any changes in the lactate threshold following the 90-minte run.

In addition to the submaximal threshold test, the subjects also performed a 10/5 reactivity test, consisting of ten repeat jumps as fast and as high as possible. The height of the jump and the amount of time spent on the ground was measured was measured for each jump and the best 5 jumps were used for analysis. This test basically measured the subjects’ ability to quickly create force to propel themselves through space. The same testing battery was performed after the 90-minute run to determine whether any changes in neuromuscular function occurred because of fatigue accumulated during the run.

Testing was also performed throughout the 90-minute run itself. Every 15 minutes, blood lactate sampling was performed, and respiratory gases were collected. Lactate sampling was performed to measure any shifts in energy production over the course of the test. Respiratory gases were collected to determine energy expenditure, as well as the respiratory exchange ratio. This ratio measures the relationship between oxygen and carbon dioxide exchange, which reflects the contribution of carbohydrate and fat metabolism.

Finally, heart rate was also monitored throughout the run to see if the heart rate increased throughout the run. The heart rate data was also used to calculate ‘detrended Fluctuation Analysis alpha 1’, or DFA-a1, which is a measure of heart rate variability.  While heart rate variability is often thought of as one measure, there are many different ways to calculate it, with each calculation providing slightly different information. DFA-a1 is associated with the aerobic threshold, with lower values indicating that intensity is approaching the threshold. This can be useful as a real-time, non-invasive way to measure threshold.

What they found

The researchers found clear durability effects, with the lactate threshold decreasing 5.8% in females and 5.3% in males – see figure 1. [Ed - in other words, over the period of the test, the workload at which first lactate threshold is reduced, meaning that while an athlete might start exercise at an intensity that doesn’t produce a rise in blood lactate, after a certain period of time, he/she might hit that threshold, causing a rise in lactate, even though the exercise intensity remains unchanged.]This physiological parameter therefore is not fixed in time and can be significantly influenced by fatigue, even if exercise is performed at relatively low intensities. Interestingly, the only physiological variable that was correlated with the change in lactate threshold was DFA-a1. As this measurement is thought to be associated with the aerobic threshold, this finding is not entirely surprising. The greater the change in threshold, the more DFA-a1 dropped (more on this later).

Throughout the test, heart rates increased on average by 5.9% in females and 5.5% in males. Similarly, oxygen consumption increased as well. In other words, the physiological cost of exercise increased despite no change in the speed at which the subjects were running. Over the course of the run, the respiratory exchange ratio decreased, indicating an increase in fat burning. When looking at the jump testing data, there were no differences between before and after measures. Jump height, contact time, and average power were unchanged. This indicates that changes in threshold were likely not occurring at the neuromuscular level, and were more likely related to metabolic or cardiovascular changes.

Figure 1: Durability of first lactate threshold over time