Or it could be a combination of short hamstrings heredity and cycling. There is a book out, called, appropriately, Stretching, by a fellow named Bob Anderson. An entire book dedicated to specific stretches for every sport and activity. There are pages for both cycling and mountain biking. Many are modified yoga asanas, it appears, and they work well for me. I have both short hamstrings from heredity and cycling, and I usually stretch after any ride of over 10 miles.