Stretching: what it does and doesn't do for runners

0:03 · The question, and the recruits' trial

Many runners stretch before or after a run, hoping to avoid injuries and run better. Trials have put that hope to the test, and here is what they found. One thousand five hundred and thirty eight army recruits. For twelve weeks, half of them stretched their legs before every training session. At the end, both halves had been hurt about as often. So what does stretching actually do for a runner? There is a real answer, and later in this video you will see who used the least oxygen, the stiff or the loose. This is stretching, what it does and doesn't do for runners. We follow five questions, one per chapter. Does it prevent injuries, and does it ease soreness? What does it do in the minutes before you run? What do weeks of stretching change? And last, does a more flexible runner use less oxygen? First, the three kinds of stretching that studies test. Static means taking a muscle to where you feel the stretch, and holding it there. Dynamic means moving a joint through its range again and again, like leg swings. And P N F mixes a static stretch with tensing the same muscle. For general health, the American College of Sports Medicine describes holding a stretch for ten to thirty seconds. That adds up to about sixty seconds per exercise. Keep that sixty in mind, because it comes back. So, the first question: does stretching prevent injuries?

1:23 · Does it prevent injuries?

The recruits' trial is the largest that tested stretching only before exercise. Both groups did the same warm up. One group also held a twenty second stretch for each of six leg muscle groups. There were one hundred and fifty eight injuries among the seven hundred and thirty five who stretched. Among the eight hundred and three who did not, one hundred and seventy five. By our arithmetic, that is about twenty two in every hundred, in both groups. The authors worked out that preventing one injury would take some two hundred and sixty hours of stretching, with a very wide margin. A second trial asked two thousand three hundred and seventy seven active adults to stretch before and after every hard session. About a third of them were runners. That meant at least thirty seconds per muscle, for twelve weeks. The risk of any injury came out the same as without stretching. Three reviews that pooled these and other trials landed in the same place, with no clear difference. There is one hint the other way. In some studies, muscle injuries went down, mostly in sports with sprinting, and one review saw no such drop in endurance running. A panel of twenty stretching researchers, judging partly their own reviews, points out that bone and joint injuries went up in the same studies. It does not recommend stretching to prevent injuries in general. And in the main review on running injuries, five of the six stretching trials were in soldiers. The one trial in recreational runners mixed stretching with a warm up and a cool down, and fewer than half of them stuck to it. In one recent trial of novice runners, a hip and core strengthening programme did better. It had about a third fewer overuse injuries than the group that stretched. So, no protection shown, in thousands of people. Next, the morning after a hard session.

3:06 · Does it ease soreness?

A Cochrane review, a large independent summary of trials, pooled small studies of soreness, mostly in the lab. Stretching beforehand cut soreness the next day by half a point, on a scale of a hundred. Stretching afterwards cut it by one point. Before, after or both, none of it was clinically important. The big trial of two thousand three hundred and seventy seven people saw a little more. Bothersome soreness came up in about one week in four with stretching, against one in three without. But people rated that themselves, and they knew whether they were stretching. The effect was larger in those who believed in it. Whether stretching afterwards speeds the return of strength, we don't know yet. So soreness does change, but only a little.

3:52 · Just before you run

Now the minutes before you run, and here the stopwatch decides. Two large reviews pulled together over a hundred small lab studies of strength, power and sprinting. With holds under thirty seconds per muscle, the average change was minus one point one per cent. Thirty to forty five seconds gave minus one point nine. Neither of those was a clear effect. Sixty seconds or more per muscle, and those tests dropped four point six per cent. There is that sixty again, now as the point where stretching starts to cost you something. Those tests mostly came three to five minutes after the stretching. In studies that waited more than ten minutes, the loss was usually trivial. And when some dynamic exercise followed the stretching, no clear loss was seen. The panel advises against holding a muscle for more than sixty seconds before a maximal strength or sprint effort. It sees no such loss from short stretches inside a dynamic warm up. And what about stretching right before a run? Four trials stretched runners for eight to twenty minutes first. In one, ten trained men ran three point four per cent less far in half an hour, five point eight kilometres against six. In another, by our subtraction, ten college runners were thirteen seconds slower over an uphill treadmill mile. But the first trial's lab repeated it with twelve trained women, and they did not change at all. And eleven club runners were seven seconds slower over three kilometres, again our subtraction, which could easily be chance. None of those four trials tested a short static stretch inside a normal warm up, before a long run. A review that pooled fifteen such studies found no clear effect of one session on running economy. The studies were small, so we don't know yet. On the bike, one small study saw oxygen use rise only in the first five minutes. In another, nine active men cycling to exhaustion lasted twenty six per cent less long. Dynamic stretching gave opposite answers in a single lab. Against sitting still, seven runners lasted eighteen per cent longer at a fast pace. Added on top of a normal warm up, eight other runners lasted sixteen per cent less long. Both of these trials were very small. Dynamic stretching seems to work mostly as a warm up. On top of a warm up, it did these runners no good. So in strength and sprint tests, a short stretch costs next to nothing and a long hold costs a little. Before a run, the answer is still open.

6:14 · Weeks of stretching

So far, we looked at a single session. Now what weeks of stretching change, and after that, the result that surprised us most. Weeks of stretching do work, for flexibility. Seventy seven studies found a clear gain in range of motion, how far a joint can go. The gain was larger with static stretching and P N F than with dynamic. But what actually changes in the leg? In a classic study, seven women stretched the back of one thigh twice a day for twenty days. The other leg was the control. Taken to the same angle as before, the muscle resisted just as much. What changed was how far they let it go before it became too uncomfortable. Pooled studies of three to eight weeks found no clear change in how stiff the muscle or the tendon was. The panel does see the muscle itself loosen, but only with far more stretching. That means four minutes or more per muscle, five days a week, for at least three weeks. After a single session, range of motion was still up at least twenty minutes later. Two to six weeks after stopping a programme, part of the gain remained. In eleven trials, some in older adults or patients, strength training raised range of motion about as much as stretching. So at the doses most people use, the extra reach comes mostly from accepting more stretch.

7:31 · Flexible, or economical?

So does a more flexible runner use less oxygen at the same pace? In the studies that looked, the stiffest people were often the most economical. In a hundred adults walking and jogging, the stiffest third used nine per cent less oxygen than the loosest third. In nineteen sub elite runners, a stiffer ankle went with better economy. Sit and reach is how far you can reach towards your toes. In thirty four international runners, the worse their sit and reach, the better their economy. That shows a link, not a cause. The ankle study's authors call their own guess speculative: a stiffer leg may store and return more energy with each step, like a spring. They add a warning: clinicians assume stiffness raises the risk of running injuries, so a stiff body is not simply better. Two small trials then stretched people for weeks, one for three weeks and one for ten. In the longer one, sixteen students stretched for forty minutes, three times a week, while sixteen others did not. In both trials, economy did not change.

8:32 · What this leaves you with

So stretching has not prevented injuries in the trials, and it barely touches soreness. In strength and sprint tests, a short stretch cost next to nothing and a long hold cost a little. If you like how it feels, the trials give little reason to stop. If you do it to avoid injury, they give little reason to start. Every source and figure is on the page at enduranceproof dot com. Still to come on this channel, running economy, the oxygen cost of your pace and what can move it.