Seated hamstring stretches stretch the back of the legs as well as opening the backs of the knees and hip joint.
They can also be thought of as forward bending yoga poses.
Note that not all forward bends are hamstring stretches.
You may find that doing low lunge first makes the following seated hamstring stretches a little bit easier.
Both of the poses shown below are seated hamstring stretches. However in the first one, Janu sirsasana A you stretch the hamstrings by tilting the pelvis and upper body forwards relative to the straight leg.
In the second pose, Modified Compass Pose you stretch the hamstrings by moving the leg close to the torso.
In both types of hamstring stretches, the knees are straight (or as straight as possible.)
The pictures below show the beginning and end step of this sequence while doing seated wide leg forward bend.
Generally, the seated wide legged forward bend is the easiest of seated hamstrings stretches. It's also a good yoga pose to practice the above actions in.
Janu Sirsasana A is also a seated hamstring stretch. However only one leg is straight in this yoga pose.
As with the wide legged seated forward fold, you can first practice lengthening your spine while seated upright.
In the pictures below I'm turned forty-five degrees in from my straight leg while I practice straightening and relaxing my spine.
Bending forwards you can first bend forwards while facing between both legs.
If you start of with this variation, shown below, you can increase the difficulty of the pose by gradually turning your body so that your torso is reaching over your straight leg.
In the photos below I'm turned towards my straight leg.
If your bent knee doesn't touch the floor you can either place a block beneath it so that you can then press that leg down against the block or focus on pressing the outer edge of the foot into the floor as you lengthen your spine. (The foot of the bent knee leg that is.)
Janu Sirsasana B Seated Hamstring Stretch
In Janu Sirsasana B position your pelvis so that your anus is on top of your heel. then bend forwards. You can point the bent knee foot too the side or forwards. Try both variations and notice how they feel.
Here are some suggestions for getting comfortable in the Janu Sirsasana C foot position as well as some modifications.
After doing the above seated hamstring stretch on both sides you can then do a seated hamstring stretch with both legs together. This is easier with the feet about hip-width apart (my favored position.) It can become a lot more uncomfortable with feet together.
As with the wide leg seated front fold, you can also do this seated forward bending yoga pose with knees bent.
As you tilt further and further forwards in this yoga pose, try to widen the top of your inner thighs (close to the groin) while keeping your feet in the same position. With inner thighs widened, press your inner thighs downwards so that you can use your psoas and iliacus to help tilt your pelvis forwards. Or focus on using your outer glutes (gluteus minimus and medius) to help tilt your pelvis forwards relative to your thighs.
The pictures below show lengthening the spine while sitting upright with knees bent.
You can practice the same thing, lengthening the spine and reaching the arms forwards, with your pelvis tilted forwards a little bit. You can do this with knees straight (shown) or with knees bent (not shown.)
In either case, press your legs down as you lengthen your spine.
Finally, as your hamstrings lengthen, or to deepen the stretch of your hamstrings, try to reach forwards even further. But rather than bending your spine focus on lengthening it as you inhale.
If you find that your back rounds excessively in this pose or you have difficulty lengthening your spine the tips in seated forward bend may help.
In the previous seated hamstring stretches you tilted you pelvis forwards relative to your thigh (or attempted to.) In this Heron Pose modification you can move the leg relative to the pelvis to stretch the hamstrings, one leg at a time.
Here too you can lenghten your spine. You can also press the foot of your bent knee leg into the floor. But so that you can straighten your leg in this yoga pose, relax your shoulders.
I've included a step-by-step in seated hamstring stretch.
You may find that it feels good to do ardha matsyendrasana after doing modified heron pose or just before it.
Since seated hamstring stretches tend to stretch the hamstrings and open the back of the body, you can counter pose them by opening up the front of the body with back bending yoga poses like table top yoga pose and or reverse plank pose (purvottanasana).***
To begin with start with your legs open about 90 degrees. Turn towards one leg. Then bend forwards with your hands on the floor.
Repeat the following actions on one side:
To assist with the arms, grab the foot or ankle. While grabbing the foot repeat these steps:
For wide leg seated forward bend you can do much the same series of steps.
For the muscle assisted version (using the arms) grab the toes or sides of the feet and then pull up with the arms after first pressing the legs down and opening the chest.
For seated hamstring stretches janu sirsasana could be thought of as the equivalent to the "standing on one leg" hamstring stretch above.
Sit upright with both legs straight and together, then bend one knee and place the foot of that leg against the inner thigh of the other leg.
For this hamstring stretch you may find it easier if you turn slightly towards your bent knee side and then bend forwards. Then as you hold the stretch your can gradually pull your torso towards your straight leg.
You can start with your hands on the floor. Do the same as above: open the chest, (lengthen the neck also.) Press the straight leg down. Press the foot of the bent knee leg down. Lift the arms and then reach them forwards. Then relax and repeat. Each time move gradually closer to the straight leg.
You could also grab onto the foot with both hands. Hold on to the edges of the foot. After opening the chest and pressing the straight leg down pull up with both arms.
After doing the above two seated hamstring stretches you may find that seated forward bend is a little bit easier.
Prior to doing the above exercise bend forwards and notice your legs, then focus on feeling your hips, then your lower back and your ribcage.
Slowly open your ribcage and lengthen your neck. Then press down with your legs. Lift your arms and reach them forwards.
Focus on making your spine long. (That means you can reach it up and forwards, out of the pelvis.) Then focus on making your arms long. Reach them forwards.
Reach forwards with your eyes also.
Then relax and repeat.
When holding on to your big toes again make your spine feel long, and press your legs down.
You can try and make your legs feel long also. Reach them forwards out of your pelvis.
And then pull upwards on your big toes with your arms.
Relax and repeat.
You could also try not using your arms and using your hip flexors (and quads) instead.
Press your legs down, but then imagine pulling up on the front of your thighs and shins. However instead of pulling your thighs up, this action tilts your pelvis forwards and brings your ribcage down.
In modified heron pose (or compass pose preparation) you stretch the hamstrings by moving the leg towards the ribcage instead of the other way around. Fold the bottom leg to the inside so that your foot is near the inner thigh of the other leg.
Relax your shoulders. Imagine using your leg to stretch your arms. Open your chest.
Slowly press your thigh away from you but resist with the arms at the same time. Make your arms stronger than your leg and bend your elbows so that you slowly draw your leg towards you.
You can use a towel or strap to hold on to the foot. (Not shown.) Wrap the towel around the lifted foot and hold on to the ends with either hand. As you draw your leg towards you, you can grab the foot directly.
Another option is to keep the spine long and make the leg long also. (Make the leg "feel" long.) Slowly draw the leg back towards the chest.
When I first started doing this stretch I found it easier to use a towel or strap to hold the foot. Then as my hamstrings lengthened, then I grabbed the foot with both hands.