The hamstrings are the muscles that run up the back of the thigh. They cross both the knee and the hips and so are affected by both joints.
To stretch the hamstrings the knee has to be straight and the pelvis tilted forwards.
It may help to think of straightening the knee as opening the back of the knee joint. Meanwhile tilting the pelvis forwards opens the back of the hip joint.
If the knee joint is bent then tilting the pelvis forwards only stretches the hip extensors and not the hamstrings.
If the knee is straight then tilting the pelvis forwards can stretch the deeper "single joint" hip extensors as well as the hamstrings.
If you have tight hamstrings then it can be helpful to focus on keeping the back straight when doing forward bending yoga poses so that the focus is on opening the back of the hips and stretching the hamstrings. (Read more about doing a hunchback free seated forward bend.)
And you may find it helpful to do a hip extensor stretch like low lunge prior to stretching the hamstrings.
While it is a bit difficult if you are close to the floor, you could try activating the front of the hip in this yoga pose by trying to pull upwards on your front knee even as you let your chest sink downwards.
Yoga Hamstring stretches can be divided into two main groups, standing and seated. You can also stretch the hamstrings while reclining in plow pose.
If you have tight hamstrings and have difficulty stretching them you might find it easier to start with the standing gravity assisted stretches and then move to seated stretches
A slightly more advanced option is muscle assisted forward bends where you use either the arms or the hip flexors to assist the forward bend.
To make it easier to feel your hamstrings and control them you can also try hamstring strengthening exercises.
In standing yoga hamstring stretches the hamstrings may tighten up to help control the forward tilt of the body.
In the pose picture above, called Triangle Forward Fold or Parsvottanasana, if my arms are lifted as in the first picture my hamstrings have to activate to keep my torso horizontal.
And if I want to tilt my pelvis forwards so that my torso moves closer to my front leg then the hamstrings have to lengthen while staying active.
(This is known as eccentric contraction. But you can think of it as the hamstrings "lengthening while activated".)
With lack of experience, your hamstrings may lock and prevent you from deepening the forward bend.
They won't be able to lengthen while active. You could think of this as a fear response.
And so one goal of hamstring stretching is negate the fear response.
One way to overcome the fear response is to use the arms to help support the body.
In the second picture above I'm resting my hands on my legs. If I press my hands down against my leg then my arms support my upper body meaning that my hamstrings can ideally relax.
If I can keep my hamstrings relaxed I can gradually tilt my pelvis forwards. So that my ribcage can lower I can either bend my elbows as I tilt my pelvis forwards or slide my hands down my leg towards the floor.
Another option for overcoming the fear response is to practice activating and relaxing the hamstrings.
Using the same pose, we can start with the hands on the leg. Then, slowly lift the arms and reach them back.
The hamstrings will have to activate to support the body.
Then put the arms back down on the leg. With the arms supporting the body the hamstrings can relax again.
Repeat this a few times and then switch sides.
One you have the basic feel for this exercise a variation uses the following repeated steps:
If you practice a few times you'll learn to feel what you need to do to make your legs strong in just the right way to support the weight of your body.
With the legs strong, your spine then has a stable base (the pelvis) from which to lengthen. Your then have a firm foundation for lifting the arms.
Each time you lift the arms you practice engaging the hamstrings. You then give yourself, and your hamstrings, the confidence and experience so that you can begin deepening your hamstring stretch.
When do you stretch the hamstrings in this exercise?
When you put your arms back down on your leg or the floor.
With your hands on your leg (or on the floor) you can gradually bend your elbows as your hamstrings relax.
As your elbows bend your ribcage can move closer to your leg.
However, rather than bending the spine so that this happens, tilt your pelvis forwards so that your ribcage and pelvis tilt forwards as one unit.
You'll then be stretching your hamstrings.
Another way to support the body while doing a standing forward bend is to use the butt muscles.
Standing with feet parallel and hip width (or shoulder width) apart, you can slowly bend forwards while keeping your butt engaged. If you like rest your hands on your legs, or on the floor if you can reach it.
You could also focus on squeezing your butt cheeks and then relaxing them.
Each time you relax your butt cheeks see if you can also relax your hamstrings so that you can stretch them.
If you find that your lower back is really stiff when doing standing yoga hamstrings stretches, you might find that activating your gluteus maximus muscle is really helpful.
Another way to work around the fear response while doing hamstring stretches is to focus on what you are trying to do. In the case of the standing wide leg hamstring stretch (prasaritta padottanasana) you can focus on lengthening your hamstrings by reaching your sitting bones upwards, away from the backs of your knees.
The sitting bones are the bones that you can feel when you sit on a hard chair.
If you aren't sure about your sitting bones you could also focus on feeling the bottom of your butt cheeks. Reach the bottom of your butt cheeks upwards while bent forwards.
If you can get your hands onto the floor then use your arms to pull your ribcage towards your legs (Prasaritta Padottanasana A).
You can also grab onto your big toes (Prasaritta Padottanasana D). In this case pull your arms forwards, so that your ribcage moves back, towards your legs.
In both of these versions keep your spine feeling long. Make your legs feel long also.
If you have your hands on your waist (Prasaritta Padottanasana B) or you have your hands clasped behind your back (Prasaritta Padottanasana C Shoulder Stretch) then you can use your hip flexors, as well as gravity, to drive the stretch.
To help activate your hip flexors you can imagine drawing your legs forwards, towards your chest.
In seated yoga hamstring stretches I often find that pressing down through the backs of the knees helps improve hamstring flexibility.
You may find that this action (pressing the knees or heels forwards) adds tension to the thighs and that tension is desirable since it gives more control over the hip joints, which is what is desired in a forward bend. With control of the hip joints you can use that control to help increase your forward bend.
Some instructors may shy away from this instruction because of the tendency for those with hypermobile (or hyper extensible) knee joints to hyper extend the knee. However in a seated stretch for the hamstrings this is less of a concern.
And more importantly, for those who have difficulty stretching the hamstrings in the first place, this may be one action that helps.
If you're hamstrings are so tight that you can't sit upright with the legs straight then this action may not work. (You could try sitting on an inclined sit up bench so that your butt is higher than your feet so that you can then lean your upper body forwards.)
In the following seated hamstring stretches you can stretch the hamstrings either by tilting the pelvis forwards relative to the pelvis (such as in "Wide Leg Seated Forward Fold", "Janu Sirsasana A", or "Paschimottanasana").
You could also pull the legs (one leg at a time) towards the front of the body as shown below (Compass Pose preparation).
In both types of hamstring stretches, the knees are straight (or as straight as possible.)
These stretches can be made more pleasant if you focus on feeling your breath.
Generally, the seated wide leg seated forward bend is the easiest of seated yoga hamstrings stretches.
Janu Sirsasana A is also a seated yoga 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.)
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 to 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 yoga hamstring stretches 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.
A slightly different way to stretch the hamstrings is to grab on to the foot or big toe (or ankle) with one or both arms and while pressing the knee down, pull up with the arm to help pull the ribcage down, towards the thigh.
Rather than relaxing the leg, keep it active and actively resist the arm. The action of the leg pressing down and the arm pulling up may help bring your ribcage closer to your thigh.
Keep the chest open and neck long when doing this action also.
This option can be used in most (if not all) of the asymmetric seated hamstring stretches above.
In the previous seated hamstring stretches you tilted you pelvis forwards relative to your thigh (or attempted to.)
In this compass pose preparation, you can move the leg relative to the pelvis to stretch the hamstrings, one leg at a time.
Here too you can lengthen 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.
If you can straighten your leg in the above pose then you might want to give compass pose a try.
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).
For more on the anatomy related to the hamstrings (and the glutes) you can read hamstring anatomy.
For more on hip anatomy in general you can read hip joint anatomy.
For more on strengthening the hamstrings (and learning to activate them) read Hamstring Strengthening Exercises.
So how do you go about using the hip flexors to stretch the hamstrings?
How do you learn to activate them?
I've included the exercises that I've found most helpful in teaching Active hip flexion in the video Muscle Control. This covers the basics which I expand upon in the ebook Active Stretching. They include exercises for activating the hip flexors as well as the muscles that oppose them, the hamstrings and glutes. It also shows you how to recognize when these muscles are active (so that you know that you are using them).