The Science bit…
Hamstring muscles are attached to the bottom of the pelvis, in a similar area to the bony points you sit on. If this muscle is tight, it will pull on the bottom of the pelvis and encourage it to rotate backwards. If there is also tightness around the low back itself, this backwards rotation movement will be resisted by the back and therefore the increased stress will be taken up in and around the bottom of the Low Back or the Sacro-Iliac Joint. This increased stress is likely to lead to low back pain.
The simpler explanation….
In short, tight hamstrings pull the Pelvis down, straightening the curve in the Lower back causing strain and weakness leading to pain!
Ways to combat this is to make sure you have good flexibility in the hamstrings, by making sure you warm up properly, stretching before AND after exercise. For best results, have the muscles warm BEFORE stretching. You will get a greater stretch if you do.
If you are prone to tight hamstrings as many are… regular deep tissue massage (weekly/monthly) throughout the sporting season or throughout a fitness program will encourage flexibility and help prevent injury.
So people, go forth and prosper! Play well, stay fit and prevent injury!