Active vs. passive warm-up

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What exactly is the purpose of a warm-up before exercise? According to a new study in the Journal of Strength and Conditioning Research, it’s:

to enhance physical performance, to reduce muscle soreness, and to prevent sports-related injuries by increasing the body temperature.

But if the main mechanism of the warm-up is literally to warm the body, could we accomplish the same thing by, say, sitting in warm water? That’s what this study tested: three different cycling tests (six minutes at 80% VO2max) after (1) no warm-up, (2) an “active” warm-up of 20 minutes easy cycling, or (3) a “passive” warm-up of soaking the legs in 39-C water for 20 minutes. The result: the active warm-up allowed subjects to use more oxygen (measured VO2) with less effort (lower HR), and possibly lower lactate accumulation (though the latter wasn’t statistically significant).

So what does this mean? It suggests that the benefits of a proper warm-up aren’t just the result of raising your temperature. Higher temperature does confer some benefits: for example, your muscles and tendons become more elastic, reducing the risk of injury. Nerve signals from brain to muscle are transmitted more quickly. The rate of metabolic reactions inside your cells speeds up by 13% for each degree C that the temperature increase.

But there are other benefits beyond temperature. Crucially, the active warm-up causes your blood vessels to dilate to speed the flow of oxygen to working muscles. When you start the main workout or race, the sudden increase in demand puts you into temporary oxygen debt, because your heart, lungs and muscle metabolism can’t respond instantly to the higher demand. If you’re properly warmed up, your systems are already partly ready for the increased demand (blood vessels dilated from the warm-up, heart rate already elevated, etc.), so they can deliver more oxygen than if they were starting cold. That means the short period of initial oxygen debt doesn’t last as long — and since aerobic metabolism is more efficient that anaerobic metabolism, it means that you’re more efficient overall.

The practical take-away: well, we all know that warm-ups (as opposed to sitting in a luke-warm bath) are important, so this doesn’t change anything. But there’s still lots of debate about exactly what a warm-up is supposed to do, and what the best way to do it is — hence all the posts about dynamic versus static stretching, for example. In the long run, figuring which parts of a warm-up really do boost performance will help us design better warm-up routines.