Why sprinters peak at night and marathon runners don’t


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Most athletics world records are set in the evening; most road racing world records are set in the morning. This is not a deep physiological enigma, it is just a reflection of great track encounters and road races. For mass participation endurance events, in particular, the anticipated start times are largely dictated by the need to close the streets and the desire to avoid the hot weather, not when the human body is ready. for maximum performance.

Yet even when the usual logistical constraints were rejected for Eliud Kipchoge’s less than two hour marathon attempts, they still opted for early morning starts. Was it a mistake, or at least a missed opportunity? The answer, according to a new review in Medicine and science in sport and exercise by researchers at Harvard Medical School and the University of Basel, is not as clear as one might think.

Conventional wisdom regarding circadian rhythms and physical performance is that you are at your best in the late afternoon or early evening, with a typical peak hour a few hours past 6 p.m. The usual explanation is that this is when your body temperature is highest. , generally having risen nearly two degrees Fahrenheit from its morning nadir. A warmer body means looser muscles, faster metabolic reactions, and faster transmission of nerve signals. Among the indirect evidence of this effect: when researchers from the always mild climates of Guadeloupe attempted to reproduce these results, they found no influence of time of day on muscle power, possibly because subjects were hot all the time.

But there are a lot of caveats and additional factors to consider. Is your body clock itself peaking at some point, or is it a function of how long you’ve been awake or the last time you ate? Several studies have shown that if you shift your sleep-wake cycle by a few hours, you also shift the timing of your peak performance by a few hours, suggesting that the external rhythms of daily life are important. Then there’s the matter of individual variation: it seems unlikely that early risers and night owls will peak at the same time.

All of these potential confounders are the reason Harvard’s Raphael Knaier and his colleagues decided to put together as much data as possible in a large meta-analysis. They ended up with a total of 63 relevant articles, but inconsistencies in what was tested and how the data was presented meant they could only combine 29 of the studies in their meta-analysis. They divided these studies into four categories: jump height, anaerobic power (tested in a 30-second cycling sprint), grip strength, and endurance exercise (tested in a time trial, a shuttle run or a VO2 max test).

Results for the top three were more or less as expected: “strong evidence” that jump height and anaerobic power peak between 1:00 pm and 7:00 pm, and “some evidence” that grip strength achieved. peak between 1:00 p.m. and 9:00 p.m. For example, here is the data for jump height from various studies. Solid lines indicate a statistically significant effect of time of day, while dotted lines indicate a non-significant effect; thicker lines indicate studies with a larger sample size; and darker lines indicate studies with lower risk of bias in design and analysis.

(Photo: Medicine and science in sport and exercise)

Although there is some scattering, the trend for best results in the late afternoon is very pronounced, with lots of thick, dark, un-dotted lines. In comparison, see the results of the endurance tests:

Diurnal variation graph according to the time of day: time trial and shuttle race
(Photo: Medicine and science in sport and exercise)

Most of these studies fail to find a significant difference. To be fair, it’s harder to recruit people to run a 5K series than to get them to do a bunch of jumps, but while the lack of statistical significance is a consequence of the small sample sizes, the actual size any difference also appears to be small to nonexistent compared to those observed for jump height and other parameters. Interestingly, endurance is the only category of testing where you might expect a higher core temperature to be a hindrance rather than a help, as overheating is a limiting factor during strenuous exercise.

Even a meta-analysis like this cannot tell us much about the reasons for the differences in time of day, whether it is mainly body temperature, time of wakefulness, daylight or other factors. One of the points made by Knaier and colleagues in their discussion is that future studies need to better capture individual results and individual differences, rather than just overall averages. Ultimately, the goal is to find ways to shift the time of peak performance or minimize the time of day effects, but it’s mostly guesswork until you find it. which actually causes the effect.

We can draw one conclusion: despite the general trend for peak performances in the late afternoon, Eliud Kipchoge’s decision to aim for the under-two in the morning does not look like a mistake. When I asked the scientists at Breaking2 why they made this decision, they focused on balancing the theoretical advantage of a steadily dropping nighttime temperature against the practical inconvenience of having to find what to eat all the time. the day before a marathon. The meta-analysis suggests these are the right factors to consider, because for endurance, circadian rhythms don’t seem to matter much after all.

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