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 (V̇E – the volume of air breathed in a minute), frequency of breathing (FR), and depth of breathing (VT) 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 V̇E 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 V̇E 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 V̇E at VT1 was generated by more rapid, shallower breathing after prolonged exercise. So, contrary to our hypothesis (we thought FR at VT1 was most likely to stable with prolonged exercise), it seems that V̇E is the most promising metric in this space!
Figures showing similar rates of ventilation (V̇E) at VT1 before and after prolonged exercise, with increased breathing frequency (FR) and decreased tidal volume (VT) at VT1 after prolonged exercise. We think that makes V̇E a good candidate for real-time monitoring for use in within-session intensity regulation and training load monitoring.
Before we get too carried away, we should acknowledge that we can’t yet measure V̇E accurately and non-invasively in field settings, during real-world training. In the study, we measured V̇E 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 V̇E, but these data are an exciting start!
Endure IQ's weekly Brew Up newsletter brings you Dr. Dan Plews' weekly top 5. Everything from the latest science and tech, to must read books/podcasts, and weekly workouts.