Tight hamstrings limit forward bending. When one wishes to bend further forward than the hamstrings allow, many compensate by overstretching their lumbar spine and straining their backs. By lengthening the hamstrings, one reduces this risk.
Hamstrings anatomy and action
The hamstrings are three muscles that originate together on the hip at a the ischial tuberosity. The muscles continue along the femur before the different muscle bellies branch off, cross the knee joint and insert onto the tibia and fibula bones of the lower leg.
As the hamstrings cross two joints (the hip and knee), they have an effect on both. When the hamstrings contract, their primary action is to flex (or bend) the knee. A secondary action is to assist in extending the hip. So to lengthen the hamstrings, we do the opposite: we extend (or straighten) the knee and flex the hip.
In the leg sequence of our lower back yoga class, we start with practices to warm up the hamstrings before stretching them fully and in a supine position to limit risk of compression to the lumbar discs.
In both practices above, the knees are straight, not bent, and the hips are flexed, not extended, thereby lengthening the hamstring muscles all while the lower back remains in a safe supine position with floor support. These simple stretches will enable freer movement of the pelvis, thereby reducing stress on the lumbar vertebrae in forward bending.
A note for those with knee problems
During the hamstring stretches, if you can straighten the knee, please do so, but the knee should not be locked nor hyperextended. In the first practice above, you can keep the knee “soft” while the femur and tibia form a straight line.
Some people have difficulty straightening the knee. Straightening should not be forced as one must honor the limitations within the joint capsule. For the first practice above, a cloth belt or elastic band is used to allow the leg to remain further away from the body which makes it easier to straighten the knee. However, if straightening is still not possible, continue with the knee slightly bent.
Another option is to lie within a doorway with one leg up against the door frame and the other down on the floor through the doorway. As the hamstrings of the leg on the door frame relaxes into the length, you can gradually inch closer to the door frame.
Important: Do not stretch so far that the same-side hip lifts from the floor and rounds the lower back. Stay long on the floor through that side with the hip down.
For the second practice above, one lies against a wall with a pleasant passive stretch on the hamstrings. However, if the knees cannot straighten, simply move slightly away from the wall to make it easier for the knees to straighten. The idea is to remain comfortable while gently lengthening the hamstrings over an extended period of time.
Tight hamstrings limit our forward bending which may strain our lumbar spine if we bend forward too far. Read "Bending with Back Pain".