Anyone who has raced Kona knows the moment. You get off the bike on the Queen K and there's still a marathon to go, out to the Energy Lab and back. Which raises a question I keep coming back to: if you're already fading at the back end of the bike, are you automatically less durable on the run? To explore it, I analysed the race files of four athletes who raced Kona in 2026 (Athletes B to E), plus earlier Kona races where we had them.

My 2018 race: a rock steady bike

My own 2018 race shows what holding the back end of the bike can look like. My bike decoupling was -0.6%, with no drift onset at all, and 259 W normalised power (FTP then was 370 w, so a easy bike). My last quarter power was 6.6% higher than the first (242 to 258 W). Then came a 2:50:56 marathon, with -0.6% run decoupling. Race conditions are also undeniably playing a big part.

What is durability?

In 2021 we defined durability as the time of onset and magnitude of deterioration in physiological characteristics duri...

It’s not what you can do for 5 minutes, it’s what you can do for 5 minutes after five hours. As endurance athletes, we know that our endurance – our physiological and psychological ability to endure, to keep going as the hours tick up in our event – is fundamental to our performance.

‘Durability’ is an emerging concept in exercise physiology, and, more importantly, we think, a critical factor in endurance training and performance. In our 2021 paper, we defined durability as an athlete’s resilience to the effects of prolonged exercise on their physiological profiling characteristics – the thresholds we use to inform pacing strategies, programme training sessions, and monitor training load (1). Subsequently, durability has been proposed as a crucial factor in endurance performance, as it defines, physiologically at least, our ability to stave off the upward creep in effort required at a given speed or power output as exercise progresses (2). As readers of our blogs, or our monthly Train...

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 ...

Effective endurance training is all about stress management, and the balance between stress and recovery. Individual training sessions generate physiological stress – elevations in temperature, heart rate, and stress hormones, changes in pH, depletion of fuel stores, and so on. The stress of individual training sessions is what triggers the adaptive response we look for with training, and this occurs during recovery (1). However, too much physiological stress with inadequate recovery will eventually lead to fatigue, overreaching, and maladaptation, as anyone who has dug themselves a hole with a hard training block can testify.

Therefore, when planning a training programme, and then regulating and monitoring that training programme, we are managing the balance between stress and recovery. We want to ensure there is enough stress to stimulate adaptation, but not so much that we become fatigued, overreached, and burned out. We want to make sure our recovery processes are geared up to ens...

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Fat oxidation, and fat oxidation rates get talked about a lot in the world of endurance sports and triathlon. Of course, being able to break down fats for use as fuel during prolonged exercise is an important determinant of performance in ultra-endurance events like Ironman. Our other fuel source – carbohydrates – are relatively limited and are depleted during prolonged or intense exercise. Being able to use fat effectively reduces the rate at which we burn through our carbohydrate stores, and thus delays the fatigue associated with depleted carbohydrate stores, or ‘hitting the wall’.

Given that having a robust capacity to make use of fat stores during exercise is important, I quantify it in the lab with the athletes I work with. We can measure fat oxidation rates during exercise using a technique called indirect calorimetry, which involves the collection of expired air, and we typically do this across a range of exercise intensities in what’s called an incremen...

There is increasing interest in the effect that ketones might have on the performance of athletes. We know that by consuming a diet very low in carbohydrates – less than 50 grams per day or so – we can enter ketosis or increase the circulating concentration of ketones in the blood. That’s why these diets are called ‘ketogenic’ (1, 8). In recent years, supplements have been developed that allow us to increase our circulating ketone concentrations independently of our habitual diet. These are the so-called ‘exogenous ketone supplements’ (9).

There are various types of exogenous ketone supplements, including ketone esters, ketone salts, and ketone precursors (that are converted into ketones in the body). Ketone salts combine ketones with sodium, potassium, calcium, or magnesium, and, predictably have the potential to provide a very high salt load. Ketone salts tend to provide a modest elevation in blood ketone, specifically D-βHB, concentrations.

Ketone esters on the other hand are boun...

Exogenous ketones have generated a lot of buzz in the endurance sports community over the last few years. Exogenous ketones are not to be confused with the ketones that circulate in the blood after adaptation to a very low carbohydrate, high fat, ketogenic diet; exogenous ketones are instead ingested in a supplement like a sports drink in much the same way that carbohydrates might be. There are a number of different types of exogenous ketones (6), with ketone monoesters seeing the most attention.

It was initially thought that, again like carbohydrates in sports drinks, taking on exogenous ketones during exercise might provide us with an alternative fuel source to help power long duration exercise and reduce use of our finite glycogen stores. There has been some indication that exogenous ketones do help us reduce glycogen use during exercise (2), although probably not via direct burning of the ketones to supply energy; the effect instead likely relates to some kind of signalling respon...

Those of us on social media can’t get away from the hype and discussion around so-called “Zone 2” training. Like with many discussions on social media, you might easily be mistaken in thinking that so-called “Zone 2” training is a brand-new and somehow magical training construct and the only way to achieving your endurance training goals. Whilst that is not the case, training in “Zone 2” is one of the core pillars of my training philosophy when working with endurance athletes and long-distance triathletes in particular.

In this blog, I have tried to clear up some of the confusion around “Zone 2” training and offer my rationale for why accumulating a large volume of low-intensity training is important.

What do we mean by “Zone 2”?

Before discussing why so-called “Zone 2” training might be important, we need to nail down what we mean by Zone 2. In zone 2 training, athletes and coaches use a five-zone model of exercise intensity. Zone 1 is very easy, recovery intensities, and the trans...

We have discussed in detail that having a robust capacity to use our fat energy stores to support exercise is important for long-distance triathlons. That is because our stored fat is effectively unlimited in the context of exercise, even very long-duration exercise in very lean athletes. In contrast, our stored carbohydrate reserves are finite and will be depleted to very low concentrations after exercise of sufficient length and intensity. To put this in perspective, a lean, 70-kg triathlete with 10% body fat has at least enough energy in their 7 kg of stored fat to complete more than Ironman triathlons back-to-back (3). Of course, those are theoretical Ironmans; the point I am making here is that whilst depletion of stored carbohydrates can lead to the fatigue we recognise as ‘hitting the wall’, we don’t slow down in an Ironman because we run out of fat energy (4).

As an exercise physiologist working in applied practice, I regularly test the fat oxidation capacities of endurance at...

Sooner or later, as an endurance athlete, you will come across a discussion of ‘lactate’, whether you are also an exercise physiologist or not. Blood lactate concentrations are routinely measured in endurance athletes, and so-called ‘lactate thresholds’ are used to inform training decisions. Some athletes and coaches even spot-test blood lactate concentrations during exercise to regulate intensity during training. And this is becoming more popular than ever – word on the street is in Kona this year, just about every athlete stopped to take lactate. Of course, this has been massively popularized by the Norwegian program's success, and rightly so! They are doing some amazing things!

However, I believe many people misunderstand what lactate is and isn’t and what blood lactate responses to exercise can and can’t tell you. So, in this blog, I will briefly summarise some of the fundamental physiology of lactate metabolism. Brace yourself for some physiology!

Lactate has been studied by exe...