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Most people who do hard interval sessions will have noticed this mystery: why does the second or third interval usually feel easier than the first one? I always figured it had to do with “getting into the rhythm” or something along those lines. Whatever the reason, Pete Sherry — my main training partner for 2002-2004 — and I eventually decided that we’d run 2x400m in ~72 sec a few minutes before every workout, in the hopes of making the first interval feel easier. Our impression was that it worked, and we started doing it before races too.
It turns out there’s plenty of physiology behind this. If you suddenly start running at a hard pace, with no warm-up, it takes a while before your body can adjust to start delivering oxygen to your muscles at its maximum possible rate. That’s one of the reasons VO2max tests take 10-12 minutes, rather than simply involving a short, all-out sprint. It takes time for the blood flow to your muscles to increase, and for the enzymes that extract oxygen from the blood and oxidize fuel to ramp up their activity levels. A good warm-up gets this ramp-up process over with, allowing your body up to deliver more oxygen to muscles right from the start of the workout or race, and reducing the temporary oxygen debt.
Still, most people warm up with gentle jogging, flexibility drills, and some short sprints. But how about including a six-minute “hard” effort (above lactate threshold but below VO2max pace), about ten minutes before the start of your race or workout? Would that “prime” your oxygen kinetics even more? The challenge is as follows: a sustained burst of hard exercise (above threshold) definitely improves how quickly your body can process oxygen once the actual race starts; this effect can last for a half-hour or more. If you exercise too hard, on the other hand, you deplete your anaerobic energy stores (phosphocreatine), and metabolites build up in your muscles that may slow you down. Numerous experiments over the past decade have found conflicting results: depending on the precise details of the duration, intensity and recovery time following the “priming” burst, performance either increases, decreases, or stays the same.
A new cycling study just posted online at Medicine & Science in Sports & Exercise, from Mark Burnley’s group at Aberystwyth, adds some more data on finding the right balance. They used a six-minute priming bout, 10 minutes before the “race” — a formula that other studies have found to be effective. For intensity, they compared “heavy” (about 25% of the way between threshold and VO2max power) and “severe” (about 63%) priming bouts. The findings: “heavy” priming boosted oxygen kinetics and significantly increased time-to-exhaustion in tests ranging from ~2-10 minutes. “Severe” priming also boosted oxygen kinetics, but didn’t increase time-to-exhaustion, suggesting that the downside of depleted anaerobic reserves outweighed the benefits of more aerobic energy available early in the test.
So what does this mean in practical terms? It’s hard to know how generalizable this protocol is, but I’d say it’s worth experimenting with some sort of extended surge ~10 minutes before the end of your warm-up. If you’re doing a six-minute effort, it looks like you should aim just above your threshold. I know quite a few runners who have incorporated similar but shorter surges of ~1-2 minutes into their warm-up routine. There may be a good argument for runners to stick to shorter surges, since the impact of leg-pounding is a bigger factor than it is in cycling. In that case, you may be able to get away with a higher intensity. But so far I don’t think the research has answered that question — for now, it’s trial and error.