You know you hear it all the time, not just in yoga, but in every exercise cool down. Instructors want you to straighten your leg, sometimes locking the knee joint in order to get a “good” hamstring stretch. We say, NOT TRUE! In fact, that instruction can sometimes lead to injuries. For most of us, straightening the leg and trying to keep an elongated spine is not an easy relationship to maintain. Usually, this means, we sacrifice the long spine to straighten the leg. To demonstrate, try a standing forward bend. Straighten the legs completely and notice what happens in your back. You lower back will most likely round. Now try the forward bend with the knees bent enough for the lower spine to be neutral and the belly to rest on the thighs and then start to straighten the legs without losing this feeling in your spine or abdomen. You may notice now how the feeling of the stretch has moved and is no longer in the low back (if it was there before), but is in the middle of the hamstring. You may able to access a deeper stretch here.
Why? The upper attachments of 2 of your hamstrings are on the ischial tuberosities or “sitting” bones. If you have straight knees, but a rounded back, you are only stretching the lower attachments by your knee. Typically this means you are feeling a stretch in your back. If you keep doing “hamstring” stretches or forward bends this way you won’t develop the articulation to know the difference between spinal movement and hip movement.
To switch up that pattern, try bending your knees, laying your chest on your thighs. Build up a connection between your chest and legs and keep that relationship active while doing a forward bend. In other words try keeping your chest glued to your legs (without forcing, as this could also create a bad scenario), with a long spine (bend your leg as much as you have to in order to get there) as you start straightening the legs. Stop when the chest begins to lose connection with the thighs. Respect that you won’t be able to go as far as maybe you are used to. Do you feel your hamstring stretch here? Note that this is also the difference between stretching your tendons/ligaments versus your muscles. When you do this stretch in particular you want to keep the sensation in the belly of your hamstring (meaning closer to the middle of your thigh bone). If you feel it behind the knee or near the sitting bones you are most likely stretching tendons, which are not supposed to stretch, as opposed to just muscle tissue. This you do NOT want and can lead to destabilization and damage in a joint.