A number of studies have found that runners with variations in their COL5A1 gene that make them less flexible can also make them faster. (http://www.ncbi.nlm.nih.gov/pubmed/20798666 and http://www.ncbi.nlm.nih.gov/pubmed/24755981). This is because tighter muscle fibers can help your muscles store more energy. You may find that after you have been running for a while that it’s harder for you to touch your toes. This was definitely true for me. This inflexibility may help improve your run times. This is also why static stretching before running may slow you down and dynamic stretching may be preferable. Check out this overview of dynamic warm-ups. Want to find out how your genes are influencing your body’s response to exercise and nutrition? Sign up here to get early access to the test that we are developing.