I walk without pain, and run with basically no pain, but if I try to sprint, my right hamstring screams at me.  Therefore, I don't do speedwork.  This has been going on for a year or so.  This morning I got on a hamstring machine at LA Fitness.  It isolates the hamstrings - you do one leg at a time.  My right hammy is now much weaker than my left, and it hurts like heck to lift weights with it.  Any suggestion as to what my issue is (strain? muscle tear?)?  Any thoughts as to what I can do to improve it?