The hamstrings and lower back are connected by the pelvis. The hamstring is made up of tendons and three muscles that run from the back of the knee to the back of the pelvis; this network is responsible for bending the knee behind the midline of the body when extending the hip (such as when running or walking). Through the hip connection, tight hamstrings can cause lower back pain.
When a muscle is tense, its length shortens. This can occur in several ways. Overusing the hamstring without stretching it can cause it to become chronically tight. This is often accompanied by weakness of the quadriceps and hip flexors, such as the psoas, as the hamstrings do not relax and do not allow them to work. Lack of stretching and exercising in a balanced way can lead to this muscle imbalance. On the other hand, the opposite imbalance can cause the hamstring to tighten. If the hip flexors and quadriceps are too tight, the hamstrings will stretch and become weak. Weak muscles can quickly fatigue due to their lack of strength. Tired muscles have a hard time relaxing. If a weak hamstring doesn’t relax when the quadriceps contracts, it can tear or pull. This is a very common type of injury, especially in athletes.
Muscles work like a kind of pulley system; if they are not balanced in length, strength, and flexibility, then any number of structures within the body can become misaligned. The hamstrings and hip extensors are closely related; if one is tight, the other is bound to be too. If these are tight, the muscles that perform the opposite functions (hip flexors and knee extensors (quads)) will be weak and elongated. The muscles of the upper legs and hips are a crucial part of pelvic stabilization. Since the pelvis is the base of the spine, pelvic stabilization is the foundation of spinal stabilization.
The shortened and tight hamstrings exert a downward pull on the pelvis. Since the quadriceps and hip flexors are long and weak, they cannot balance this force with an equal pull in the opposite direction. As the pelvis is pulled down into the back, the lower back loses its natural arch (called lordosis). As the spine is forced to flatten, the discs between the vertebrae experience increased pressure at the front, possibly leading to early disc degeneration.
The lower back muscles are also tight from tight hamstrings. The correct way to lean forward is at the hip joints. However, when the pelvis is pulled back, it must bend from the lower back. This type of work is not natural for the muscles of the lower back and can lead to chronic strain.
Due to the compensatory tendencies of tight muscles, it is necessary to loosen them before attempting to exercise. While stretching is a good way to restore length and flexibility to tight muscles, a technique called automyofascial release (SMR) is more effective and the best first step of treatment for tight hamstrings. In this technique, you use a dense foam roller to compress and loosen the hamstring, effectively shutting it down so other muscles can start working. See this hamstring SMR video for a better idea: http://www.youtube.com/watch?v=TNm79lJLyz0. As with any form of exercise for back pain, it’s best to have a physical therapist train you in proper technique.
Once the hamstrings have been neutralized, you can move on to developing the quadriceps and hip flexor muscles, which will help stabilize the pelvis. See a physical therapist to make sure your exercise plan doesn’t create the same or another painful imbalance. Educating yourself about how muscles work together will help protect you from back pain.