Into the Fold
So you have some nagging, persistent low-back pain. You've heard that your tight legs are probably part of the problem and that stretching them is a good idea. Since forward bends mightily stretch the backs of your legs, you decide to add some to your home practice a few times a week. Good idea?
Actually, it depends. While forward bends can be wonderfully relaxing and introspective, they can also strain or injure your low back—especially if the backs of your legs are tight.
In fact, there's a right and a wrong way to do forward bends. Learning to do them correctly will keep your back safe from harm but requires that you pay close attention to the mechanics of your body. The crucial muscles to understand are the hamstrings, particularly in their interactions with your pelvis.
The hamstrings, of course, occupy the backs of your thighs. They originate on the ischial tuberosities (sitting bones), which project from the bottom of the pelvis. From the sitting bones, the hams extend about two-thirds of the way down the backs of the thighs, at which point they connect to the long tendons that cross the back of the knee to attach on the lower leg bones, the tibia and fibula.
The lateral (or outer) hamstring is known as the biceps femoris (biceps means "two heads"; the second head originates on the back of the femur, or thighbone). The two medial (inner) hamstrings are called the semitendinosus and semimembranosus.
When the hams contract they flex, or bend, the knee. You can feel this if you sit on the floor with one knee bent and your fingers placed on the back of the lower thigh near the knee. Dig your heel into the floor, pulling your heel toward you against the friction of the floor, and you should feel the hamstring tendons pop out into your fingers. At the hip (with the help of the gluteus maximus), the hamstrings pull the thigh into line with or behind the torso. This is called "hip extension." The hamstrings also have leverage to rotate the femur bones. This action is called "hip rotation": The biceps femoris externally rotates, and the two inner hamstrings internally rotate the hip.
Here's the basic anatomy of a forward bend: To stretch the hamstring muscles, you extend (straighten) the knee and flex the hip, bringing the torso and the front of the thigh closer together. Doing straight-leg forward bends like Paschimottanasana (Seated Forward Bend) and Janu Sirsasana (Head-of-the-Knee Pose) is an excellent way to stretch your hamstrings.