I have previously posted about some studies addressing the relationship between flexibility and running economy. The studies find that less flexibility correlates with greater running economy. The reason is probably that elastic recoil of muscle and tendon is an important contributor to running power. Just as a golf ball will bounce higher and longer than a squash ball, a runner will bounce higher and further off stiff hamstrings and calves than floppy ones.
I’ve recently come across two more studies on the same issue, discussed at the excellent Sweat Science blog, which I highly recommend.
According to the first study, about 60-70 percent of flexibility is heritable, and one gene that helps determine flexibility level is called COL5A1. The researchers were curious whether variations in this gene would correlate with endurance running performance. It turned out that runners with the “stiffer” version of COL5A1 were faster. Interestingly, the stiffer people were not faster at biking or swimming, where elastic return of energy is less important. The results were essentially repeated in a second study looking at a different set of runners.