The physiology of the ending kick

I used to see my finishing kick as a sign of toughness. Nobody overtook me in the final stages of a race, I would tell myself, because nobody wanted it more than me.

But over time I started to see it from a different perspective. No matter how a race went, whether it was fast or slow and whether I was way ahead or way behind, I would always be able to sprint the last quarter mile or something. Why did I still have the energy to sprint even if I was dropped by the leaders? Shouldn’t I have used this energy to avoid dropping at all? Eventually, my kick became a source of frustration. I tried to ride so hard that I didn’t want a kick, but I almost never made it.

As a result, I’ve always been intrigued by attempts to unravel the physiology and psychology of the finishing kick. The most recent addition: a study in medicine and science in exercise and exercise led by University of Calgary graduate student Rafael Azevedo under the direction of Juan Murias and Guillaume Millet’s Brain and Body Before and After the Finishing Kick.

Some important background: I keep coming back to a study by researchers Ross Tucker, Michael Lambert and Tim Noakes from the University of Cape Town in 2006, in which stimulation patterns were analyzed in a century of world record achievements for men over distances between 800 and 10,000 meters . As I discussed in more detail here, they observed a remarkably consistent U-shaped stimulation template for 800 meter races that has a quick start, a smooth middle, and a quick finish, as shown in this graph:

(Figure: International Journal of Exercise Physiology and Performance)

The presence of a finishing kick, even in elite athletes running at world record pace, reflected a hardwired tendency to maintain physiological reserve during intense workouts. In other words, it was evolution rather than cowardice that made me hold back the energy for a sprint.

This comprehensive explanation makes sense intuitively, but it has proven more complicated than expected to decipher what goes on inside your body at different stages of a race. To that end, Azevedo’s new study included 12 male volunteers doing a series of 4K bike time trials. The trials lasted a little over six minutes on average and, as expected, followed a U-shaped stimulation profile with a quick start, an even middle, and a finish sprint. According to a mathematical analysis, the quick start took an average of 827 meters and the target kick started with a lead of 410 meters.

After some training attempts, subjects completed three experimental attempts in random order: one where they were stopped after the quick start, a second where they were stopped after the steady section, and a third where they continued the end. Once they stopped, they were put through a series of tests to assess fatigue in a number of ways. The measurements used force sensors that are attached to the pedals of the bicycle – a crucial detail as fatigue subsides within a few seconds. In previous experiments, subjects were removed from their bikes and then strapped into a separate device to measure fatigue. So this is an important technical innovation.

The simplest way to measure muscle fatigue is by maximal voluntary contraction: you ask the subject to contract the relevant muscle (in this case the quads) as hard as possible. You can also break it down into two sub-components using more sophisticated techniques. “Central Fatigue” is how much the signal from the brain to the muscles has decreased; “Peripheral fatigue” is how much weaker the muscle fibers themselves are when you stimulate them with electricity. The researchers took all three measurements.

The results showed a rapid increase in fatigue during the initial quick start: the maximum voluntary contraction decreased 23 percent, the central fatigue was 8 percent, and the peripheral fatigue was 40 percent. Then things stabilized: at the end of the steady phase, which makes up about 70 percent of the entire race, all three fatigue marks remained essentially unchanged compared to shortly after the quick start. But after the finish sprint, the tiredness increased again, for example to 34 percent for a maximum voluntary contraction.

In other words, muscle fatigue doesn’t build up in a nice straight line. After the initial excitement of the start, we reach a sustained pace that hardly seems to affect muscle function. The feeling that your gelled legs couldn’t take another step after a race is created almost entirely by the finish sprint, not the miles that preceded it. One way to understand this is critical speed (or, equivalently, critical power), a concept that I have written about at length a few times recently. Your critical speed is essentially the threshold for what is metabolically sustainable. You can run above critical speed for a while, but you’re using your limited reserves of anaerobic capacity – and by the time they’re done, you’re boiled.

If you were to ride a race at a perfectly steady pace, you would gradually use up your anaerobic capacity and hit zero as you cross the finish line if you judge it correctly. In contrast, most of us start using some of our anaerobic capacity. (There may be good physiological reasons for this, since a quick start will increase your oxygen processing capacities more quickly.) Then we will reach a pace relatively close to critical performance, only very slowly nibbling at anaerobic capacity. Then, when we get closer to the goal, we consume everything with a splendid sprint.

In the Azevedo data, for the middle part of the race, the cyclists adjusted to a pace that was barely above critical power, meaning they used most of their anaerobic capacity at the start and end. The big question is whether this approach is not optimal. That is certainly my intuition. When Joshua Cheptegei broke the 5,000 meter world record last summer, I argued that the wavelight stimulation lights blinking around the perimeter of the track at a perfectly steady pace helped Cheptegei by enabling him to set the smoothest world record ever . It has to be more efficient, right?

But it may not be as obvious as I thought it would be. As early as 2013, in a study from Andy Jones’ laboratory at the University of Exeter, different stimulation strategies were compared in three-minute bike tests: the typical U-shape at your own pace, a comprehensive effort from the beginning and a steadiness pace process. This is what these stimulation patterns looked like, with the workload above the critical performance (ie the anaerobic capacity) highlighted in gray. Panel A is an incremental test to exhaustion, B is complete from the start, C is evenly stimulated, and D is self-pacing.

Finishing-Kick-Chart-2.jpg(Illustration: Medicine & Science in Sport & Exercise)

The numbers indicate the total anaerobic capacity indicated by the shaded areas and there are no significant differences between them. Uniform stimulation resulted in an anaerobic capacity of 12.9 kJ; The self-pace with a finishing kick was 12.8 kJ. However, there is a huge difference in how these different strategies feel. The closer you are to depleting your anaerobic capacity, the worse you will feel. “My interpretation / guess,” Jones told me via email, “is that athletes have learned, or intuitively know, that a stimulation strategy with an end thrust will produce the same performance outcome as other strategies BUT that the same performance can be achieved.” with less pain for most of the race! The athletes will end up being just as crazy, but this middle section won’t be quite as unbearable if they implement an end-sprint strategy. “

That’s an interesting idea. And it would explain why U-shaped stimulation patterns are so ubiquitous among even the tallest runners in the world. It has always confused me that a seemingly sub-optimal stimulation strategy could produce so many world records. Even if we had to be careful, you would still expect world records to be set if athletes accidentally started too fast, when a finishing kick was really that bad for performance.

On the other hand, as Ross Tucker noted, the pace of world records seems to be steadily increasing. The gains from smoothing your stimulation may be small, but at this level you need to look for every possible edge. Personally, though, I find Andy Jones’ reasoning very tempting – because if the U-shaped pace doesn’t cost you anything, I can again see my finishing kick as a sign of pride rather than a sign of shame.

If you’d like to learn more about Sweat Science, visit me on Twitter and Facebook, subscribe to the email newsletter, and read my book Endure: Mind, Body, and the Strangely Elastic Limits of Human Achievement.

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