Ask any jogger or step-class instructor, and they will insist on a series of stretches before launching into your exercise regime. Indeed, most exercise classes do warm-up and cool-down stretches before and after the aerobic workout. The idea is that the increased flexibility from stretching will prevent muscle injury.
But is there any evidence for this?
As with so much of sports, preexercise stretching has entered the lore of good practice without much proof.
In fact, there appears to be no evidence that stretching will prevent injury. When the US Centers for Disease Control and Prevention did a review of all randomized trials and cohort studies in sports medicine, they found only six studies that compared stretching with other ways to prevent injury. The CDC found no significant association between stretching and a reduction in total injuries (Med Sci Sports Exerc, 2004; 36: 4). Indeed, those who stretched were no more or less likely to suffer injury, although a warm-up did increase blood to the muscles.
Stretching does not appear to benefit male runners. In one study, 47 per cent of all male runners who stretched regularly were injured during a one-year period compared with only 33 per cent of those who didn't stretch(www.pponline.co.uk/encyc/0396.htm). How-ever, this increased risk did not apply to female marathon runners, suggest-ing that stretching may be overextend-ing men's muscles.
Stretching also has no effect on oxygen use. A 10-week study compar-ing the treadmill runs of volunteers who'd spent 40 minutes doing calf and thigh stretches beforehand with those jumping on the treadmill cold found that regular stretching did not affect oxygen levels while running (Scand J Med Sci Sports, 2001; 11: 260-5).
Does stretching prevent injury or loss of muscle strength in novice exercisers? Researchers at Umea Uni-versity in Sweden asked 10 women to perform 10 deep, full knee flexes with both legs after a five-minute warm-up on a stationary cycle. However, before the flexing, they also did four 20-second static hamstring stretches on only one leg. The scientists measured their soreness and loss of contractile force after one, two and four days, and found no differences between the non-stretched and stretched legs (Scand J Med Sci Sports, 1999; 9: 219-25).
The same researchers carried out a more extensive experiment with seven women working on their quadriceps muscles until exhaustion. Seven days later, they repeated the experiment, but with passive stretching before and after the exercise. Again, they found no difference in muscle response between the two experiments-and once again, passive stretching didn't prevent mus-cle soreness (Scand J Med Sci Sports, 1998; 8: 216-21). Stretching before strength training also did not prevent increased muscle stiffness (Scand J Med Sci Sports, 1998; 8: 65-77).
In fact, the true benefit of stretch-ing may have more to do with elongat-ing the muscles and tendons for doing activities that require it (such as dance or yoga) (Trans Orthop Res Soc, 1989; 14: 294). This theory is supported by a Danish study of skeletal muscle during stretch manoeuvres that showed that three weeks of stretching increased a joint's range of motion. Even just five repeated stretches caused the muscles and tendons to elongate (Scand J Med Sci Sports, 1995; 5: 342-7).
The type of stretch can also make a difference. The most common form is static stretching-stretching a muscle group to a length or angle just short of pain, and holding it for between six seconds to a minute. This is thought
to increase the length of a muscle's stretch reflex.
With ballistic stretching, an athlete uses quick repetitive bounces, alter-nately stretching and resting the muscles, but this has been largely abandoned because of a possible risk of muscle strain.
Proprioceptive neuromuscular facili-tation, or PNF, is a systematic resist-and-relax technique designed to stimulate muscle receptors to prevent the body from straining or tearing muscle fibres. When a muscle is stretched to its maximum, the pro-prioceptors signal the central nervous system to contract and then relax the muscle.
When doing PNF, the athlete holds a stretch, but contracts the muscles against the resistance of a partner for 10 seconds, then relaxes the muscle for 10 seconds, after which the partner slowly moves the muscle into another stretch.
The most effective PNF technique is said to involve stretching the muscles and their antagonists (the muscles that move a limb in the opposing direction) because this will rapidly cause the muscles to lengthen (Trans Orthop Res Soc, 1989; 14: 294).
How long to stretch?
If you decide to incorporate preexercise stretches into your regime, some studies suggest working at it. On comparing four-week with eight-week bouts of hamstring stretching, passive resistance of the joint was still seen in the four-week group. However, as reduced resistance can lower the chances of injury, the researchers recommended training with stretches for at least eight weeks (Scand J Med Sci Sports, 2001; 11: 81-6).