We’ve previously discussed the idea of “threshold intensity”. You may wish to review that article before you read this one. Simply put, it refers to the exercise intensity above which working just a little bit harder means having to stop a lot sooner. What people seem to have problems with is the actual cause of this sudden onset of fatigue when they go above critical power (CP, which is the cause of the ‘threshold’ you feel). It involves something called the W’ (pronounced “W prime”), and it is the subject of my latest scientific paper which you can now read in Medicine and Science in Sports And Exercise. We’ll discuss that in this article.
To fully grasp what we’re talking about, it is important for you to understand the difference between work and work rate. Work is how much stuff you do. Work rate is how fast you do it. If you run a mile, you have done 1 mile of work. However, you may have run it at 5 minutes per mile or 10 minutes per mile. In other words, you can think of the distance as the work, and the speed you do it at as the work rate. If we’re talking bikes, work is equal to joules. Power is equal to joules per second. (A watt is simply equal to 1 joule per second. If you are riding at 200 watts, you are spending 200 joules per second.)
With me so far? Ok. Here’s the part that may seem a little tricky. Once you go above CP, the amount of additional work you can do becomes fixed. This amount of additional work is called the W’ (formerly known as the ‘anaerobic work capacity’, or AWC). You can use it quickly (by going really hard), or you can use it more slowly (by limiting your surges above CP), but either way, once you use it all up you are going to have to slow down, or maybe puke in the bushes. You won’t be able to get above CP again until you take some recovery.
A good way of thinking about it is like this: Imagine you unplug your iPhone and start doing a bunch of stuff. The battery starts running down. So, you plug it back in and it starts to recharge. Now, it doesn’t recharge right away…it takes a little while. How fast the recharge happens depends on how much you keep using it. If you are trying to watch ESPN SportsCenter at the same time…recharge will be very slow. On the other hand, if you stop using it, it will recharge much more quickly.
What we did was this. We made some cyclists ride above CP by a fixed amount (e.g. unplug the iPhone and start using it a lot). We then gave them some recovery (e.g. to recharge the battery a little), and then we made them repeat, over and over again until they had to quit (e.g. they ran the battery down to zero and the iPhone shut off). We did this experiment a few times, at different recovery power levels. What we wanted to find out was how quickly the athletes recharged their W’, and if recovery was dependent upon how hard or easy the recovery is. We formulated a model based on calculus which allowed us to calculate the speed of the recharge.
What we found was that when athletes dropped their power far below CP during recovery (e.g. rode at 20W, or just used the iPhone a little) they recharged their W’ at a particular speed. In contrast, when they recovered at a harder power output (e.g. rode at 100W, or kept using the iPhone a lot), they recovered much more slowly. It turns out that the relationship is defined by the difference between CP and the recovery power. Thus, a person with a CP of 300W will recover more quickly when they drop to 20W than a person with a CP of 200W. In the former case, the person is recovering 280W below CP. In the latter case, the person is recovering only 180W below CP. This helps explain why great athletes seem to recover so quickly.
To give you some ballpark figures, let’s take a pretty average athlete with a CP around 220W who has completely used up their W’. Assuming recovery at 20W, it will take them roughly 380s to recover about 60% of their W’. To totally recharge the W’, the athlete will have to tool along at 20W for about a half hour! Now imagine that same athlete is trying to recover at around 150-175W. It may take them much, much longer!
Hopefully, a little light bulb went on over your head. This is the reason why doing a lot of surges above CP (i.e. ‘burning matches’ in cycling parlance) is such a bad idea. You are using up precious W’, and W’ takes a VERY long time to recharge. For triathletes, if you come into T2 with a low “W’ balance” remaining in the tank, you are going to feel terrible and run quite poorly. For cyclists, if you come into the final 1-2k of a race with a low W’, you won’t be able to make the final break or be a contender in the final sprint.
The actual physiological reasons behind all this this are actually a bit complicated, and are not entirely clear as of yet. We’ll discuss those in a later article. However, the above examples should at least give you some practical ideas for how to better pace your training and racing.
By way of closing, I need to thank my co-authors Prof. Andy Jones, Dr. Anni Vanhatalo, and Weerapong ‘Tony’ Chidnok at the University of Exeter for their assistance with this work. Science of this quality is never a solo enterprise, and the paper this article is based on would never have come to fruition without their effort.
Keep on training smart, everyone!
