Always be stretching your hamstrings. Suggest you look up: "Bret Contreras 5 Things You Should Do Everyday"The book I refer to is called The Stark Reality of Stretching. The basics are: find the 1st hint of tension, then the second, then gear down from the 2nd point and hold for the traditionally prescribed 30-45 seconds.Try all 5 variations to find the best that works. By work I mean it has noticable difference in flexibility longer than 4 hours. Also, try to determine if your hamstrings are tighter at the hip or the knee/calf connection.