Is ventilation a useful marker for use in within-session intensity regulation? Our recent durability study suggests it is

As readers of our blogs will know, I am involved in research with colleagues at AUT in New Zealand on ‘durability’. We defined durability as the time of onset and magnitude of deterioration in physiological profiling characteristics – such as the ventilatory and lactate thresholds that mark the boundaries between intensity domains – over time during prolonged exercise (4). More simply, physiologically and perceptually, a 300 W effort when 20 min into a session is not the same as a 300 W effort when 200 min into a session. An athlete’s durability refers to how big the effect of those 200 min is.

We published a study last year that found an ~10% reduction in power output at the first ventilatory threshold (VT₁) following 150 min of moderate-intensity cycling (9). VT₁ is used as a marker of the transition between moderate and heavy intensity exercise. I use it as the upper boundary of “Zone 2”, and encourage my athletes to perform the bulk of their training at intensities below VT₁, because exercise below VT₁ is of relatively low physiological stress (7) and requires much shorter recovery periods (8), and thus this approach facilitates accumulation of a large overall training volume (6). I therefore have my athletes tested in physiology labs, such that I can identify their VT₁ and programme their training accordingly.

Our study showed that if an athlete’s power output at VT₁ when fresh was ~200 W, it might fall to ~180 W after 150 min of moderate-intensity exercise (at least in the protocol we used). However, we found that some athletes fell to ~160 W, whilst others maintained VT₁ at values very close to their fresh numbers. As virtually all physiological profiling assessments performed in labs are with athletes in a well-rested, fresh state, our study demonstrated the quite profound limitations of applying data from these assessments to long-duration training sessions in the real world!

Take the example I gave above – someone who hit VT₁ at 200 W when fresh, and 180 W after 150 min of moderate-intensity exercise. A cautious coach sending them out for a long ride designed to be of low physiological stress might encourage them to cap their power output at 190 W, which is comfortably below the VT₁ number they obtained in a physiology lab profiling assessment. You can see here that, from 150 min onwards, we know that 190 W is actually above VT₁, and so quite a different stimulus than originally intended.

Figure showing decreased power output at VT₁ after prolonged exercise. This demonstrates what we all already knew – physiologically speaking, a given Wattage is harder late in a long training session.

This effect – the reduction in power output at VT₁ with prolonged exercise, which we also see for critical power, or the heavy-to-severe intensity transition sometimes called the ‘anaerobic threshold’ (1–3) – calls for identification of a (physiological) marker that can be used to determine if an athlete is above or below a threshold in real time. Really, we need a marker that holds constant relative to thresholds during prolonged exercise, such that an athlete can see it and easily determine if they are above or below a particular threshold. That would be useful for within-session intensity regulation – staying within the target ‘zone’ – and also when quantifying training load. At present, a 300 W effort after 20 min contributes just as much to training load scores as a 300 W effort after 200 min.

It would be convenient if heart rate did this for us, as heart rate is easy and cheap to measure accurately, and can be seen in real time. In our study, we quantified heart rate as well as power output at VT₁. Unfortunately, we found that the heart rate associated with VT₁ increased over time during prolonged exercise. So, if your VT₁ initially occurred at ~200 W with a heart rate of 140 b^.min^-1, it might have been ~180 W with a heart rate of ~150 b^.min^-1 after prolonged exercise. That doesn’t mean that heart rate is useless for within-session intensity regulation and training load monitoring – it’s definitely not – but it does make interpretation of heart rates in the third, fourth, fifth hour of a session a little tricky to interpret. 
Figure showing, despite the reduction in power output, increased heart rate at VT₁ after prolonged exercise. This implications for the application of heart rate threshold data to within-session intensity regulation and training load monitoring in prolonged exercise. 

What we did in the study

The study we’ve just published was a re-analysis of the data we’ve just described, only we quantified the rate of ventilation (V̇_E – the volume of air breathed in a minute), frequency of breathing (F_R), and depth of breathing (V_T) associated with VT₁ in the incremental tests performed before and after 150 min of cycling (10). We thought that these ventilatory parameters might be good threshold markers, as they are highly sensitive to exercise intensity and metabolic stress (5).

The rate of ventilation at VT₁ was similar when fresh and after 150 min of exercise

We found that V̇_E at VT₁ was pretty stable over time, and not statistically different, with mean values of 72 L^.min^-1 initially and 69 L^.min^-1 after prolonged exercise. To us, this makes V̇_E a pretty compelling candidate marker for use in within-session intensity regulation and training load monitoring, as values derived from well-rested lab-based profiling tests do seem to hold over time during prolonged sessions.

Interestingly, we found that this pretty stable rate of V̇_E at VT₁ was generated by more rapid, shallower breathing after prolonged exercise. So, contrary to our hypothesis (we thought F_R at VT₁ was most likely to stable with prolonged exercise), it seems that V̇_E is the most promising metric in this space!

Figures showing similar rates of ventilation (V̇_E) at VT₁ before and after prolonged exercise, with increased breathing frequency (F_R) and decreased tidal volume (V_T) at VT₁ after prolonged exercise. We think that makes V̇_E a good candidate for real-time monitoring for use in within-session intensity regulation and training load monitoring.

Applications

Before we get too carried away, we should acknowledge that we can’t yet measure V̇_E accurately and non-invasively in field settings, during real-world training. In the study, we measured V̇_E using a metabolic cart. Metabolic carts are typically confined to laboratories.

However, we are firm believers that if a solid use-case is established, the sports technology industry can and will follow with products to fill the void. We acknowledge that we need to do a lot more work before we can confidently say how real-time V̇_E, but these data are an exciting start!

Endure on!

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. 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.
5. Nicolò A, Marcora SM, Bazzucchi I, Sacchetti M. Differential control of respiratory frequency and tidal volume during high-intensity interval training. Exp Physiol 102: 934–949, 2017. doi: 10.1113/EP086352.
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. Stanley J, Peake JM, Buchheit M. Cardiac parasympathetic reactivation following exercise: Implications for training prescription. Sports Med 43: 1259–1277, 2013. doi: 10.1007/s40279-013-0083-4.
9. Stevenson JD, Kilding AE, Plews DJ, Maunder E. Prolonged cycling reduces power output at the moderate-to-heavy intensity transition. Eur J Appl Physiol 122: 2673–2682, 2022. doi: 10.1007/s00421-022-05036-9.
10. Stevenson JD, Kilding AE, Plews DJ, Maunder E. Prolonged exercise shifts ventilatory parameters at the moderate‑to‑heavy intensity transition. Eur J Appl Physiol 2023. doi: 10/1007/s00421-023-05285-2.