You see them everywhere, people yanking an ankle behind their back before setting off on a jog. It looks like sensible preparation for exercise, but it may do more harm than good. Not only does it fail to reduce the risk of injury, it may also hinder performance.

Research on 23 studies of athletes who performed stretch exercises before performance tests of sporting performance showed nearly all had a bad effect. One study showed that static stretching before a jump test reduced the maximum height by three-quarters of an inch. A review of six studies of stretching before exercise found that not one demonstrated it prevented injury. Ian Shrier, a Canadian epidemiologist who conducted both reviews, in the Clinical Journal of Sports Medicine, said: "At first people thought I was crazy." But other researchers had since reached similar conclusions, he said.

The best way to prevent injury and prepare for exercise was to do a proper warm-up routine to get blood flowing to the muscles, he said. Doing calf stretches before a run does not benefit the leg muscles because they are never stretched in the extreme position while running. Almost all over-use injuries are strains that occur when the body is in the normal range of motion and are the result of improper training.

But stretching should not be ruled out. Dr Shrier, of the Jewish General Hospital in Montreal, found seven other studies which showed how regular stretching exercises included as part of an exercise routine improved performance. "If one stretches, one should stretch after exercise or at a time not related to exercise," Dr Shrier said.

James Brown, a specialist in sports medicine in Wakefield, West Yorkshire, and spokesman for the UK Association of Doctors in Sport, said: "Stretching before exercise probably increases the risk of injury. Your muscles are never going to get warm. Unfortunately you do still see people doing it everywhere. You won't see elite athletes doing it. They will do a warm-up. If you go jogging or to the gym at lunchtime there is no need to stretch first."

WARMING UP

* Start with a gentle warm-up, of the muscles you plan to use

* Increasing blood flow to muscles gets them contracting the way they will need to for the exercise

* Stretching after exercise is recommended

* Static stretching can stiffen muscles

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