I’d like to address a common injury to the hamstrings, those powerful muscles in the back of your thighs. It is not unusual for students who don’t warm up the hamstrings slowly, who push themselves in hamstring-stretching poses, or who do a lot of jumps into and out of forward bends and Chatarunga, to injure this area in the form of strain from overstretching, or, in more serious cases, tearing of the muscle fibers.
The hamstrings all start off from the same starting point, your sitting bones or ischial tuberosity, and head down toward the knees. They comprise three muscles—semitendinosus, semimembranosus and biceps femoris—and their corresponding tendons. The semitendinosus and semimembranosus split off to the inside back of the lower leg bone at the knee, and the biceps femoris heads to the outside back of the lower leg at the knee. So the hamstrings cross two joints, your hip joint and your knee joint. When the they contract, they can either pull you upper leg, the femur, back behind you into extension, or they can help your knee to bend or “flex,” or they do both things at once.
If you have your femur pulled back and your knee flexed, like in Dhanurasana (Bow Pose), your hamstrings are at some of their most contracted and shortened. When you are bending forward at the hips in poses like Paschimottanasana (Seated Forward Bend), they are required to go to their maximum length or stretch. And when you are springing back for Uttanasana into low Plank, or springing forward from Down Dog to Uttanasana, your are putting a sudden intense demand on the hamstring muscles.
The vulnerable and most commonly injured area is where the muscles originate at the sitting bones. The short tendons that anchor the muscles to the bone heals most slowly, due to a poor blood supply. Once present, this injury can take a long time to heal. In addition, once the injury has occurred, it gets aggravated by any stretching of that muscle, which can delay healing even more. Since stretching the hamstring is detrimental to healing, at the very least, you will need to modify (by bending the knees deeply) or skip all forward-bending poses until the inflammation in the tendon (and the accompanying pain) has disappeared.
As you can imagine, this would likely preclude you doing vigorous vinyasa practices without the very likely chance of re-injuring the tendon again and again. In addition, more static styles of practice like Iyengar or Anusara yoga would need to be modified if forward bends are part of the sequence. I learned a trick from one yoga teacher for modifying the front leg in Triangle to eliminate painful pulling of the hamstrings. Instead of turning the front foot out 90 degrees, you would turn it out more like 100 degrees or so. This would shift the stress more laterally on the tendon, to an area that might still be healthy and intact.
Your yoga strap can be of some help as well. Make a loop and place it snuggly up the thigh as high as you can and so it won’t slip down (but not too tight). This creates a kind of brace that will shift the stretch of forward bends to the strapped spot on your hamstrings and away from the sitting bone.
Roger Cole recommends that post injury, you rest for at least 72 hours to let the inflammation cool down, then focus on strengthening the hamstrings before returning to forward bends. This can be accomplished beautifully by Locust Pose (Salabhasana), and I like to have students do a one-legged version, where the affected leg is lifted just few inches off the floor, keeping a sense of the leg lengthening backward. Warm up with a dynamic version, inhaling up and exhaling down for 4-6 breaths, before holding the pose for a few breaths. If this causes any pain, you probably need to rest the area for a while longer.
Hamstring injury requires patience with your body. It can take months of slow, methodical work to allow the area to heal to the point of returning to a regular class. And even then, you'll need to spend some time at the start of each practice warming up the hamstring muscles before diving into a vigorous and strong asana sequence. Once injured, this area will be vulnerable to re-injury, so being mindful about your actions and your body will help you keep it healthy and your practice strong.