How is Your Durability? Why understanding athlete durability is a key metric for Ironman Success

As endurance athletes, we know that getting enough volume into our training programme is critical in our preparation for an event, and we know that to get enough volume without becoming overreached and overtrained, we need to manage and regulate our training intensities effectively (4, 6). We might therefore plan and programme-specific long-duration training sessions designed to generate low physiological stress within our programme (7). Ideally, we'll use physiological profiling numbers – specifically, knowledge of the power output, running speed, and heart rate at the first threshold or moderate-to-heavy intensity transition – to programme and regulate these sessions (5). For example, suppose we determine that our first threshold occurs at 240 W. In that case, we might cap our long, low-intensity weekend ride at, say ~230 W. We also use this number to help quantify things like training load and how much weekly volume was completed within a specific zone.

This is a generally quite effective approach. However, as applied exercise physiologists running these tests, we are quite aware that they are typically conducted in fresh, well-rested athletes and last little more than ~25 min. We then analyse the data and provide recommendations for training sessions that may last three hours or more. If our first threshold is estimated at 240 W when fresh, is 230 W still below the first threshold in the second, third, and fourth hour of a training session?

As we all intuitively know, it's probably not; 230 W feels a lot harder after four hours than it did after 20 minutes. Physiologically speaking, it would make sense that your thresholds would drift downwards over time during prolonged exercise; in fact, this has already been shown for the second threshold, or heavy-to-severe intensity transition (1–3), and we have discussed how resilient an athlete is to the effects of prolonged exercise on their physiological profile refers to their 'durability'. However, the effect of prolonged exercise on the first threshold has not previously been studied, which is what we set out to do in our recent study, just published in the European Journal of Applied Physiology (8). This effect has strong implications for training; failing to account for the likely deteriorative effects of prolonged exercise on our thresholds risks an inadvertent upward drift in relative intensity and, therefore, the generation of more stress in training than we are after in a low-intensity, low-stress session.

What did we do?

In our study, which was led by our recent Masters graduate Julian Stevenson, we estimated the moderate-to-heavy intensity transition using both the first ventilatory threshold (VT₁) method and using blood lactate concentrations before and after two hours of constant-power cycling (initially at 90% of VT₁). So, the second assessment took place after 2.5 hours of exercise (i.e. the first test, then the two steady hours). We had fourteen well-trained cyclists do the study (13 males, 1 female), and the assessment took place after an overnight fast.

                    
Figure 1. Our study design – we measured the first threshold (VT₁ and LoglogLT) using an incremental exercise 'step' test in cyclists before and after a long bout of exercise.

 What did we find?

Our study yielded three main findings:

1. Unsurprisingly, power output at the moderate-to-heavy intensity transition, whether identified as VT₁ or using blood lactate data as LoglogLT, declined with prolonged exercise by an average of ~10%. So, to continue our example, if the threshold occurred initially at 240 W, it might have declined to ~216 W after the prolonged exercise. This would mean that 225 W was initially in the moderate intensity domain, below the first threshold, but in the heavy domain, above the first threshold, after 2.5 hours of cycling. Notably, however, there was marked variation in the magnitude of the reduction in power output between athletes, suggesting that 'durability' is an individual trait.
2. Secondly, by using regressions of the power output vs energy expenditure relationships in the pre-and post-exercise assessments, we found that the reduction in power output at the threshold was explained by both reduced cycling efficiency – how effectively we translate metabolic work into mechanical work on the bike (i.e. Watts) – and a per se reduction in metabolic energy expenditure at the threshold. These findings give us insight into the mechanisms that underpin durability and will help us in our future work.
3. Thirdly, we also estimated the heart rate associated with the threshold in both assessments. We routinely do this in our profiling assessments, so athletes have two numbers to use when regulating training intensity (i.e. power/speed and heart rate). We found that the heart rate associated with the new threshold at the lowered power output increased following prolonged exercise. So, if an athlete's threshold initially occurred at 240 W and 150 beats^.min^-1, it might have been 216 W and 160 beats^.min^-1 following the prolonged exercise. This shows that the cardiovascular drift we expect during exercise – i.e. increase in heart rate at a given power/speed – is proportionally greater than the downward intensity domain (or threshold) drift we observed. This might mean we can tolerate a little increase in heart rate during exercise and remain at a low physiological intensity.

Conclusions and practical applications

Okay, so what does this all mean? We are excited that our work will drive the research in this space forward, but for practitioners and athletes, we feel that there are a couple of key takeaway messages:

1. Power output at the first threshold decreases during prolonged exercise, and the magnitude of the decrease varies between athletes; we think this means that assessments of 'durability' are warranted (perhaps with testing performed when fresh and then on a separate occasion after a long duration training session?) and should be considered in training intensity regulation and load monitoring.
2. If we are going out intending to perform a low-stress training session, we need to add a 'buffer' within our power output cap to ensure we don't drift above the first threshold; we can, however, allow a small increase in heart rate over time and expect to remain below the first threshold.

Endure On!

N.B. For a short video summary of the study, check out this video with Endure IQ Physiologist Ed Maunder.

References

1. Clark IE, Vanhatalo A, Bailey SJ, Wylie LJ, Kirby BS, Wilkins BW, Jones AM. Effects of two hours of heavy-intensity exercise on the power–duration relationship. Med Sci Sports Exerc 50: 1658–1668, 2018. doi: 10.1249/MSS.0000000000001601.
2. Clark IE, Vanhatalo A, Thompson C, Joseph C, Black MI, Blackwell JR, Wylie LJ, Tan R, Bailey SJ, Wilkins BW, Kirby BS, Jones AM. Dynamics of the power-duration relationship during prolonged endurance exercise and influence of carbohydrate ingestion. J Appl Physiol 127: 726–736, 2019. doi: 10.1152/japplphysiol.00207.2019.
3. Clark IE, Vanhatalo A, Thompson C, Wylie LJ, Bailey SJ, Kirby BS, Wilkins BW, Jones AM. Changes in the power-duration relationship following prolonged exercise: estimation using conventional and all-out protocols and relationship with muscle glycogen. Am J Physiol - Regul Integr Comp Physiol 317: R59–R67, 2019. doi: 10.1152/ajpregu.00031.2019.
4. Granata C, Jamnick NA, Bishop DJ. Training-induced changes in mitochondrial content and respiratory function in human skeletal muscle. Sports Med 48: 1809–1828, 2018.
5. Maunder E, Seiler S, Mildenhall MJ, Kilding AE, Plews DJ. The importance of ‘durability’ in the physiological profiling of endurance athletes. Sports Med 51: 1619–1628, 2021. doi: 10.1007/s40279-021-01459-0.
6. Seiler KS. What is best practice for training intensity and duration distribution in endurance athletes? Int J Sports Physiol Perform 5: 276–291, 2010.
7. Seiler S, Haugen O, Kuffel E. Autonomic recovery after exercise in trained athletes: Intensity and duration effects. Med Sci Sports Exerc 39: 1366–1373, 2007. doi: 10.1249/mss.0b013e318060f17d.
8. Stevenson JD, Kilding AE, Plews DJ, Maunder E. Prolonged cycling reduces power output at the moderate-to-heavy intensity transition. .