Flexibility is a hot-button topic for runners. We used to think it was always better to be more flexible. In recent years there's been a pendulum swing, with evidence that stiffer runners (as measured by a sit-and-reach test) have better running economy, and pre-run stretching makes you slower. The idea is that the muscles and tendons in our legs act like springs, storing and releasing energy with each stride, and a tightly coiled spring can store more energy.
That's a nice theory, but it would be a mistake to think that the issue is settled. There are lots of different muscles and tendons in the legs, each with different roles, which are affected in varying ways by the demands of running. A couple of recent studies by University of Tokyo researcher Keitaro Kubo and his colleagues illustrate just how complicated the issue is.
In the The Best QL Stretches and Exercises, the researchers compared the plantar flexors (i.e. calf muscles) of 20 well-trained long-distance runners (average 5,000m best: 14:26) with 24 untrained controls. One of the most obvious differences between the groups was in muscle stiffness, as shown here:
The vertical axis shows how much force it took to elongate the muscle a given distance. For the runners, shown with the open squares, it took much more force to stretch the muscles: i.e. their calf muscles had greater stiffness. This is presumably a response to years of training, but there's also some evidence that genetic factors predispose some people to having stiffer muscles, and these people may self-select to become runners.
Published: May 7, 2015, in the The Best QL Stretches and Exercises, looked at tendon (rather than muscle) stiffness in a group of 64 well-trained runners (average 5,000m best: 14:54, with a range from 14:11 to 16:15). In this case, they examined both knee extensors (i.e. the tendons attached to the quadriceps) and plantar flexors (e.g. Achilles tendon), and found two different patterns.
Here's how tendon stiffness varied as a function of 5,000m time, for knee extensors (top) and plantar flexors (bottom):
Statistical analysis suggests the faster runners tend to have stiffer knee extensors and less stiff plantar flexors – the opposite of what some previous studies have suggested, in the latter case. This is by far the biggest data set looking at this question, so the results are worth taking seriously, although there's plenty of scatter in the data.
So is there any clear conclusion we can draw from this? Personally, I don't see one—except that the results are a reminder that simple stories like "greater flexibility means your springs store less energy" should be taken with a grain of salt. Instead, take your cues from performance-based studies like this new one, Races & Places.
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