As with a lot of the stretches we have looked at on this page, there are a few different ways you can stretch your Hamstrings and you can stretch them either by yourself or with the help of a friend. I will cover a seated self-stretch here because as always I feel it is the safest method. As you become more familiar with stretching your body you can investigate other stretches by yourself.
Sit on the floor with one leg outstretched and the other leg flexed (bent) at the knee, so that the plantar surface (or bottom) of the foot of that leg is up against the medial (inside) aspect of the outstretched knee. Put your hands together and reach them out towards the foot of the outstretched leg, and let your head fall forward between your arms so that you don't put unnecessary strain on your neck and/or shoulders.
You may be feeling a light stretch through your hamstrings and/or the back of your knee already. If you are, then be careful not to push yourself too far with this stretch the first few times. Always remember that stretching is an individual thing, and is unique person to person. Any stretch should only ever be pushed to the point of tension, and if you are feeling that in your Hamstrings without flexing at the waist to reach for your foot then that is enough of astretch for you at this time. As you stretch more and more, your flexibility will increase and you will be able to reach further towards your foot ... until you can grab your foot, then you might need to look at a more challenging stretch.
If you are not feeling tension in your Hamstrings to the point of stretch already, then you are ready to reach for your foot. Take a deep breath in, then as you exhale gently bend forwards to reach for your outstretched foot. The stretch will be more effective if you can initiate the flexing from the hips, but this can be very difficult. Flexing at the waist is good, just remember to try and initiate your flex from as close to your hips/waist as possible. If you try to reach forwards from the shoulders it is likely to make the stretch not only more difficult but also less effective.
Hold the stretch for at least 15-20 seconds, but there is no danger in holding this position for longer if you feel comfortable and want to hold it for a minute or two. If you do hold the stretch for an extended period of time, be sure to breathe normally and stay relaxed to get the most out of your effort.
Add on to stretch the lower legs as well:
When you find you can reach your outstretched foot reasonably comfortably, you can always grip your foot and pull your toes towards your body. This will cause Dorsi Flexion at the ankle and put your calf muscles on stretch as well.
Always be carefull to not let your outstretched foot drop to one side or the other when you stretch, your toes should always be inline with the rest of your body/trunk. This means toes directly up towards the ceiling if you are not gripping your foot, and pulling your toes evenly towards your body centre if you are gripping your foot. The reason for this is that if your toes fall outside your allignment it can put unbalanced tension on the different muscles of the Hamstrings. Over time, this can lead to an imbalance between the different muscles of your Hamstrings and possible increase in the likelihood of injury.
Always remember to relax into your stretches - don't fight against the stretch (it may cause more harm than good), and always do each stretch 3 times, at least twice each day.
Taken from "The Anatomy of Stretching" by B. Walker - pp 118