In a previous post I argued that flexibility is often massively overrated as a desirable physical quality for sports performance. Nowhere is this point more clear than in the case of running economy, as shown by a few recent studies.
Running economy basically means efficiency - a runner with better economy uses less energy to go the same speed and distance as a runner with poor economy. Running economy can vary between runners as much as 30%. Not surprisingly, studies have shown that running economy is an excellent predictor of successful performance, even among elite runners.
Many runners believe they can increase their running economy by becoming more flexible. At first glance, this theory appears plausible. As runners age, they typically lose efficiency and flexibility. Perhaps there is a connection there. Further, one might imagine that better flexibility would allow greater stride length or perhaps reduce muscular resistance to movement at the end ranges of motion. However, research on this issue does not support this theory. In fact, many studies have found that running economy actually correlates with less flexibility, not more.
In one representative study on distance runners, researchers asked subjects to perform the sit and reach test, which is basically a hamstring stretch. The runners who reached further were found to have poorer running economy. Several other studies have reached similar conclusions, including a recent study measuring flexibility at the ankle.
Why would less flexible runners be more efficient? One reason is that elastic recoil of muscle and tendon is an important contributor to running power, accounting for as much as 25-40% of the necessary energy. Stiff calves and hamstrings may enhance the storage and return of elastic energy, just as a tightly inflated ball will bounce higher and longer than a deflated one.
Based on these facts, you might guess that stretching before running would be detrimental to efficiency. One recent study found exactly that. Researchers from the University of Florida had ten extremely fit runners either stretch for sixteen minutes or do nothing immediately prior to a one hour run. The runners who did not stretch on average covered more ground and burned less calories. It should be noted that several previous studies have reached different conclusions, finding that either acute or chronic stretching, including stretching that increases flexibility as measured by the sit and reach test, did not affect running economy one way or the other.
Based on this evidence, I think the best that could be said for stretching is that it might not make you slower. (This assumes you are not sprinting, in which case it will definitely make you slower.) By the way, another thing we can definitely say about stretching is that it won’t help prevent injuries.
For some ideas on how to be more efficient in your running or anything else, see here.
For an excellent way to avoid "pulling a hammy" without stretching, see here.