The following set of yoga poses can be used as hamstring strengthening exercises. They work the hamstrings in a variety of different positions for developing all round hamstring strength. This may help improve hamstring flexibility.
The first exercise is extended cat pose (shown at the right).
While on all fours, reach one leg back with the knee straight and pointing downwards.
Pull the sacrum forwards (to arch the lower back) and then slowly lift the back leg higher.
This actually strengthens all the posterior chain if you focus on using the spinal erectors to bend the spine backwards (focus on drawing the sacrum forwards) and in addition use both the butt muscle and the hamstrings to lift the leg higher.
Work at lifting slowly and hold for a breath or two then lower and do the other side, repeating both sides 5 or more times.
An option to add a balance component is to shift weight to the down leg side and lift the arm on the same side as the lifted leg.
A possible counterpose is to tuck the toes under and lift the knees less than an inch off of the floor.
For a fun challenge see how little you can lift the knees as an exercise in body awareness. Try for a 1mm lift.
Sitting with your hands on the floor behind you and your legs straight forwards, the next hamstring strengthening exercise is to press the heels down so that the butt and legs lift up.
To work into this gradually try pressing the heels down slowly and stop just your butt is about to leave the floor and then lower.
Repeat this a few times.
While bending forwards, start with your hands on the floor, close to your hips (as opposed to closer to your feet):
Repeat a few times. Then after opening the chest:
Once you get used to the exercise you can start with your hands closer to your feet.
To turn table top into a hamstring strengthening exercise, sit with your hands on the floor behind you and with knees bent and feet flat on the floor.
Repeat a few times, noticing the feeling of increased pressure in your feet as you lift and also the increase in tension in your thighs.
Each repetition lift a little higher and as you do so retract your shoulder blades and reach your chest up (out of the shoulders) and back, away from the knees.
Hold for the final rep, looking for space to push your hips and ribcage higher.
Sitting with legs open wide to 90 degrees, bend forwards.
Position your hands in front of you but close to your pelvis so that they can easily support the weight of your ribcage.
You can gradually tilt your pelvis forwards during the relaxation phase so that you stretch your hamstrings as well as strengthen them.
This hamstring strengthening exercise can be used in any forward bend where at least one leg is straight. (Press the knee down and then reach the arms forwards.)
You can use bridge pose as a glute or butt strengthening exercise. Because the knees are bent in this pose, it's a bit harder for the hamstrings to activate effectively.
While laying on your back with knees bent and feet flat on the floor:
To increase the workload in this butt and hamstring strengthening exercise:
A variation of the above hamstring strengthening exercise is to lay on the floor with legs straight.
Push the heels down to lift the hips.
You may have to walk the feet back slightly so that you have room to lift the hips off of the floor.
For both bridge options you can press the shoulders down into the floor and work at opening the front of your ribcage.
This hamstring strengthening exercise works the hamstring of the standing leg both on the way down and on the way up.
To begin with, balance on one foot with your standing knee straight.
Lift the other leg and pull the knee forwards slightly, allowing it to bend as you do so.
If you reach the lifted leg back, your hips will move forwards relative to the standing leg meaning that the hamstring of that leg does less work when you stand back up.
If you find strong pulling sensations or extremely strong lines of tension in your legs or hips, you need to adjust the way that you do the the above standing hamstring strengthening exercise.
It is a sign that something isn't working properly within your body.
And so the sensible thing to do is slow down.
Rather than trying to force yourself, notice what you do on the good side (hopefully one side is "good") and try to find the same feeling on the "not so good" side.