The significant relationship demonstrates that the less flexible distance runners tended to be more economical, possibly as a result of the energy-efficient function of the elastic components in the muscles and tendons during the stretch-shortening cycle. -ncbi
MY COMMENT: They used a sit and reach test on 20 year old subjects. I suspect that the result might be different if hip range of motion or even hamstring flexiblilty were measured in masters runners. There are much better tests than sit and reach for assessing flexibility.