Our Research Updates: Why Durability Should Be a Focus in Endurance Training

Those of you that read our blogs, or subscribe to our monthly Training Science Summaries, will know that we often write about ‘durability’. Durability has been one of our key research areas over the last few years, and something that is seeing a lot of attention in our field by other researchers, too. I think durability has massive implications not only for endurance performance, but also for the day-to-day decisions we make in endurance training.

In this blog, I’m going to provide an update on the durability research – what we know, what we don’t know, and what the implications are for athletes. However, I’ll before we get into durability, I’ll discuss physiological profiling.

Physiological Profiling and Endurance Training

I often have my athletes perform incremental exercise tests. With an incremental exercise test, we can estimate the athlete’s overall aerobic capacity or V̇O₂max, how economical their movement is, and their speed or power output at the so-called ‘thresholds’. We can also look at their capacity to use fats as energy sources during exercise in these assessments (1, 2). Therefore, these incremental exercise tests give us the athlete’s physiological profile for exercise. These physiological profiling variables are useful for assessing an athlete’s performance capabilities, monitoring progress, and programming and monitoring training.

For example, the ‘thresholds’ are particularly useful. The thresholds are the workloads at which two breakpoints in an athlete’s physiological responses to exercise occurs. According to a three-zone model of exercise physiology, these are the ‘moderate’, ‘heavy’, and ‘severe’ intensity domains, and these intensity domains are characterised by quite distinct responses and different levels of stress (3–5). Therefore, knowledge of what speeds, power outputs, and heart rates these thresholds occur at for an individual athlete is useful for planning training. Optimising training is, after all, all about stress management – too little training stress and the athlete will be undercooked, too much training stress and the athlete will be overcooked.

In the example below, an athlete’s blood lactate concentration was measured at a range of power outputs during an incremental cycling test. The two ‘thresholds’ were identified as ~235 W and ~340 W, and these were used to generate three training zones – the moderate-intensity zone below 235 W, the heavy-intensity zone between 235 and 340 W, and the severe-intensity zone above 340 W.

Notice how the blood lactate concentration starts to rise above the baseline value at ~235 W, and how it rises much more steeply at ~340 W.

 The Durability Problem

We use these thresholds to write training sessions. With this athlete, we might set them off on a four-hour, low-intensity, low-stress ride with a power cap at ~225 W, to ensure they remain at a low intensity throughout. However, a crucial weakness with the translation of these physiological profiling variables – which are generated in incremental exercise tests lasting ~20-30 minutes – is that they are not static during the 3, 4, 5-hour rides that they are applied to.

For example, we now have a series of studies showing that power output at the first (6–8) and second (9–12) thresholds decline over time during prolonged training. These studies have shown that the degree to which these thresholds decline is highly-individual, with some individuals showing massive declines in threshold power after a couple of hours, and others much smaller changes. Therefore, in our 2021 article in Sports Medicine, we defined durability as an athlete’s resilience to the negative effects of prolonged exercise on their physiological profiling characteristics (13).

Therefore, if our athlete in the example above – who hit their first threshold at ~235 W in their incremental cycling test – had pretty modest durability, that threshold might have dropped to ~215 W after two hours, ~190 W after three hours, and ~155 W after four hours. Therefore, if they managed to do their four hours at ~225 W, they’d find themselves crossing from the moderate-intensity zone to the heavy-intensity zone. If accumulating time in the heavy domain is not the goal of the session, this may be problematic, or at least undesirable. Heavy-intensity exercise has a different recovery timeframe than moderate-intensity exercise (14, 15). In this example, the athlete may be getting more stress than was intended with the session, and also more than would be appreciated in training load calculations like TSS.

In contrast, another athlete, who has the same initial threshold power of ~235 W, may be able to maintain their threshold power much better – it’s still at ~235 W after two hours, and then ~228 W after three hours and ~220 W after four hours. This athlete is therefore more durable, and, when performing the same four-hour ride at the same power, accumulates much less time in the heavy-intensity domain.

Notice how the less durable cyclist (left) accumulates more time in the heavy-intensity domain than the more durable cyclist (right), despite the cyclists having the same threshold power at baseline.

We Think Durability Confers a Performance Advantage

It might seem obvious, but we’re pretty convinced that being more durable – better able to sustain power or speed at thresholds during prolonged exercise – confers a performance advantage for endurance athletes. In the example above, imagine how much tougher four hours at 220 W would be for the less durable athlete than the more durable athlete.

A couple of our lab studies support this. For example, we recently had thirteen cyclists perform incremental tests – to determine the first threshold power – and 5-minute performance tests when fresh, and, on a separate day, after 2.5 hours in the saddle (7). This study was led by one of our recent Masters students, Kate Hamilton. Unsurprisingly, Kate found that power at the first threshold and 5-minute power were lower after prolonged cycling. She also found that these effects had a strong relationship with each other – so those that saw their threshold power reduced the most, the least durable cyclists, also saw the largest reductions in 5-minute performance. She also found massive variability in the effects of prolonged cycling, with some cyclists seeing quite modest reductions in threshold power after 2.5 hours (~10 W or less), and others losing more than 30 W. One cyclist dropped almost 150 W in their 5-min performance test!

The relationship between the effect of prolonged cycling on power at the first threshold (ΔVT₁) and 5-min performance in Kate’s study (7).

Our other lab study which suggests more durable athletes have a performance advantage was led by then-PhD student Gabriele Gallo (8). Gabriele was interested in the time-course of the loss of power at the first threshold during prolonged cycling, so he recruited 12 cyclists for one long day of testing. His protocol involved an incremental test to determine first threshold power, which required ~30 minutes, followed by 30 minutes of steady of pedalling. He then repeated the incremental test, to determine what effect an hour in the saddle had on first threshold power. He repeated this protocol – alternating ~30-minute incremental tests and 30 minutes of steady pedalling – until the cyclists were unable to keep going. This gave him a measure of first threshold power every hour, and a measure of exercise capacity – how long the cyclists lasted. Gruelling stuff!

Gabriele’s study gave a couple of interesting insights. The first was that the effect of prolonged cycling on power at the first threshold was non-linear – which means power at the first threshold didn’t decline at all after an hour, a little after two hours, and then declined much more rapidly. That’s interesting, and suggests we need not worry too much about these shifts in our profile when programming and monitoring training lasting an hour or so, if you’re reasonably well-trained.

The second interesting insight was that exercise capacity had a reasonably strong relationship with durability – so, the more durable cyclists could keep going for longer than the less durable cyclists. That’s not surprising, but is nice to see in the data.

The relationship between durability – measured here as time at which first threshold power had dropped by 5% (Δ5%VT₁) and time-to-task failure (TTTF), or exercise capacity in Gabriele’s study (8)

How to Improve Durability

Given the importance of durability for performance, it’s important to figure out ways of improving it. To do that, we need to consider what causes the reduction in threshold power we see with prolonged exercise.

We don’t yet have a clear picture of this. There is some evidence to suggest the depletion of our muscle glycogen fuel stores during prolonged exercise contributes (16), and this fits with Gabriele’s data showing a non-linear time-course (8) – maybe we hit some kind of glycogen depletion ‘threshold’ before threshold power drops. Carbohydrate ingestion during exercise seems to help preserve threshold power (11), but this effect is probably more related to liver glycogen and blood glucose, given carbohydrate ingestion has often been shown to have no effect on the rate at which you burn muscle glycogen (17). Interestingly, Gabriele’s study found that cyclists who burned fat at higher rates were more durable (8), which adds up given that accessing your fat stores during exercise takes pressure off liver and muscle glycogen.

Part of the reason for the loss of threshold power during prolonged exercise is loss of movement economy (6) Studies in runners (18) and cyclists (20) have reported that the same level of metabolic work – the energy expenditure taking place in your mitochondria – results in less speed or less power after prolonged exercise. However, the specific mechanisms for this are a little unclear.

We also don’t yet have a clear picture – from the research at least – on how to train to improve durability. There simply aren’t any studies out there. Based on my experience, I think it’s pretty obvious that to improve durability you need to include long sessions in your plan, and to do some work under fatigue. I think there’s a good chance that working at steady intensities off-the-back of high-intensity intervals improves durability – as you are training your body to maintain work while also fatigued. I do a session that involves eight repeats of 1 min of hard work, followed by 4 min right around the first threshold.

We’re sure to see a raft of studies looking at the best methods to improve durability in the next few years.

Durability and Training Programming and Monitoring

We also think durability – the loss of power at thresholds during prolonged exercise – has big implications for training programming and monitoring. Take the example of the highly-durable and less-durable cyclists above, who have the same baseline thresholds. The long ride at 220 W will have a markedly different impact on them – with the less-durable cyclist accumulating a lot of stress and likely needing a lot of recovery following the session. Therefore, we need to consider these effects when writing prolonged sessions.

We’ve done a bit of research looking at what tools you can use during prolonged exercise to track how your thresholds are changing. If you had something that could accurately tell you where you are with respect to your thresholds while out for a ride or a run, you’d be able to make real-time decisions about speeding up or slowing down, to stick to your training plan.

In our first study, we simply looked at whether your heart rate thresholds change during prolonged exercise (6). When we do incremental tests and identify the power output or running speeds at the thresholds, we also look at what heart rate those thresholds occurred at. You might, for example, identify your first threshold as ~235 W, with a heart rate of ~150 bpm. That might allow you to programme your long ride with a heart rate cap of ~150 bpm, rather than a power cap of ~235 W. That’s good old-fashioned heart rate training, which has been around longer than power meters.

The trouble is our heart rate thresholds are moving targets, too. We found that while power at your thresholds decreases during prolonged exercise, heart rate at your thresholds increases (6). So, if your threshold is initially ~235 W and ~150 bpm, after a couple of hours it may be ~215 W and ~160 bpm. Heart rates are definitely a useful guide, but relying on heart rate alone to regulate intensity is a bit tricky.

We also had a look at ventilation thresholds (19). Your ventilation rate is the Litres of air you’re breathing per minute. As technologies are emerging that may allow us to see ventilation rates during training, we thought this was worth a look. We found that your rate of ventilation at threshold remained pretty stable during prolonged cycling, which suggests it might be a useful tool for monitoring and regulating training.

To be honest, we think accurately estimating how much your threshold has changed in real-time during training sessions using data from practical, wearable technologies is likely to require integration of multiple signals, such as heart rate and ventilation rate, and maybe even things like breathing frequency and heart rate variability. We’re working on that right now – as being able to accurately regulate training intensity in accordance with thresholds in real-time, and accurately quantify the training load of long training sessions, is something we see as useful for athletes and coaches in endurance sport.

How Can I Monitor Durability Right Now?

Acknowledging that we don’t yet have perfect tools for assessing durability in real-time during prolonged exercise, below are three methods you could use to get insights into your durability:

1. Heart rate drift during prolonged sessions. The more your heart rate drifts upwards for a given speed or power during prolonged exercise, the more your thresholds are likely to have shifted. It’s likely that seeing less heart rate drift is a sign of improved durability.
2. Performance tests. Whilst performance assessments won’t tell you how much your thresholds have shifted, you can get insights into how resilient you are to the effects of prolonged exercise by comparing performance outputs when fresh, and after a long session. So, on separate days in the same training week, you might do a performance test – maybe a 5-minute effort or a 2-km run – when fresh, and then again after a long session. You could repeat these tests after a block of training, to see how much your performance has improved when fresh, and how much your performance has improved when fatigued.
3. Lab tests. If you have access to a physiology lab, you could book in for two assessments – one as normal, and one after a long session.

Summary

It’s not about what you can do for five minutes, it’s about what you can do for five minutes, after five hours. Durability – or how resilient you are to the effects of prolonged exercise – is something that we as endurance athletes should consider in performance and in training. The thresholds we use to assess our performance level and programme and monitor training decline during prolonged exercise, which limit the application of these physiological profiling data to the real-world of endurance sport. We need to better understand the mechanisms behind durability, and why one athlete is more durable than another, such that we can develop strategies to improve it, given it’s a key performance determinant.

 References

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