The hamstring muscles down the back of the thigh are shortened in many people I see in clinic. I see hamstrings as crucial to our mechanical well-being and regularly evaluate their parameters on most of my clients.
Hamstring efficiency is paramount to lumbopelvic rhythm and sacroiliac joint function and healing / prevention of low back pain. (see: Principles of Manual Sports Medicine, Karageanes, Lippincott Williams & Wilkins)
Stretching the hamstrings to increase their functional range requires a slow and steady approach and maintenance of these muscles is essential to ideal function of the low back and pelvis. There a many ways to stretch the hamstrings and I generally like the ones that isolate this large, strong and often tight muscle group. Check out this two and a half minute how-to video here.
As with all things physical, listen to your body, take it slow and steady and seek professional advice if you are not confident with home exercises. Pain is a message from the body to the head that something is not well.
Take care, Pete.