My 17 year old son is a fairly talented runner, but for the past 5-6 months has had a series of muscle pulls and strains. (hip flexor, calf, hamstring). The athletic trainer says he has some of the tightest and least flexible muscles he has seen. He eats well, plenty of fluids, consistent stretching before and after workouts. Any ideas on what an underlying cause might be

Simple, scar tissue and or trigger points. Find a qualified MT or PT to help with this. Also another effective technique is A.I.S.