The hamstrings are a group of 3 muscles that run from the pelvis, along with the back of the leg, to insert into the two bones of the lower leg - the tibia and fibula.
These muscles are considered muscles of the knee - involved with (running, kicking, etc.), but because the hamstrings are muscles of both the knee and the hip. Because the hamstrings originate in the pelvis (and not the femur) the hamstrings can facilitate (or inhibit) movement of the hip or the knee.
When the hamstrings shorten, they become rigid, and difficult to stretch. You can see this in action if you try to kick a football with tight hamstrings - your foot won't travel as high as it would if you'd kicked the ball with relaxed hamstrings. That's an example of how short hamstrings inhibit the knee, but a similar problem occurs in the hip.
If you bend over to tie up your shoes with tight hamstrings, your pelvis won't tilt so far forwards. You probably won't notice, however, because when your pelvis "locks", your lower back makes up the difference. In practical terms, it's a tiny change, but that little extra movement through the low back can cause back pain, especially if you have an existing back injury.
The small joints between the bones of the lower back and the long, thin muscles of the lower back become easily overloaded, and that further movement can be enough to do it.
Luckily, preventing hamstring-related low back pain is easy. Just stretch your hamstrings regularly. If you're not sure how to do a good hamstring stretch (or even if you think you do know how), watch this video to see the best way to target your hamstrings and prevent low back pain.
For more information on hamstrings, email me directly via the contact page. For specific information on an injury you have, or for treatment, contact one of my clinics directly. You can find that information here.
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Dr Mitch Clark