Paper Review: Do Training Zones Change With Fatigue?
The Study:
Prolonged cycling reduces power output at the moderate‑to‑heavy intensity transition
Quick Synopsis
This study looked at whether the first ventilatory (VT1) is reduced following several hours of Zone 2 riding. VT1 is a key metabolic transition point that defines the top end of Zone 2 in a 6-7 zone system, or the top end of Zone 1 in a 3-zone polarised model. Having a high VT1 is a strong marker of endurance.
Methods
This study involved 14 endurance trained cyclists and triathletes (13 male), VO2peak 59.9 ± 6.8 mL/kg/min, training volume 9 ± 3 h/week.
Participants completed a ramp test, then rode for 2H @ a Zone 2 intensity (90% of VT1), and then completed another ramp test. The tests were terminated after a fixed duration, and thus participants were not required to continue to the point of exhaustion in either ramp test.
For each ramp test, the first ventilatory (VT1) was determined from expired gas.
Key Findings
The researchers found a statistically significant reduction in power at VT1 after 2H of Zone 2 riding. In other words, these athletes had a reduction in their Zone 2 power after 2H of riding. The reduction in power averaged 21W, which was a 10% decrease.
This drop was partly, but not completely explained by a drop in cycling efficiency (i.e. an increase in the amount of energy required in order to produce power on the bike). It appears that this was a result of a switch to increased fat utilisation over time, which is a less efficient, but more sustainable way to produce power compared with carbohydrate oxidation.
Strengths & Limitations
There are some limitations in relation to the statistical analyses used. We won’t go into the details, but the key take-home is that the methods used can give rise to a higher than expected rate of ‘false positive’ findings, so we need to interpret the results with some caution.
The athletes included in this study were all moderately-trained athletes. We suspect that well-trained cyclists would have a smaller drop-off in VT1 power, and that less well-trained cyclists would have a higher drop-off.
Key Take-Aways
1. Fatigue-resistance testing is important
This study provides data to support the use of ‘fatigue resistance’ testing in cycling, as suggested in our musings above. How well you can maintain your VT1 power, or produce supra-threshold power outputs after several hours of riding is very important across a range of cycling disciplines.
2. Monitor heart rate and breathing to avoid unplanned fatigue
This study also suggests that, if you’re riding for an extended period, you may inadvertently cross VT1 into Zone 3 if you’re only riding to power, and not paying attention to other physiological markers such as heart rate and breathing.
In some instances, stepping into Zone 3 is ok and indeed might be a goal of the session.
However, if it’s important that a session doesn’t result in a high level of metabolic/physiological stress or fatigue, it might be worth also keeping an eye on heart rate and breathing rate, and making sure these parameters aren’t rising significantly.
In this study, the 21W drop in VT1 power was associated with an increase in heart rate of around 9bpm on average, which should give you an idea of the magnitude of HR drift you might be looking for. However, interestingly, three of the 14 participants actually had a drop in heart rate as the ride went on, so this variable clearly isn’t always reliable..! We suggest making sure your breathing rate also always remains very comfortable (so that you could hold a conversation easily), if you’re looking to minimise fatigue.
