I am talking about the static stretch here. Make sure you stretch the target muscles when your body is warm. Otherwise, it is not only that effective, but also you may injure yourself by stretching.
Why is an inflexible hamstring bad?
During training
Hamstring injury occurs most often during the squat when the torso is too far forward. The hamstring muscle group is in an extremely stretched position and contracts forcefully straighten the pelvis. This can lead to tearing, most often at the high or middle portion of the muscle group.
For daily life
Remember that one muscle that is not normal, affects the other part of the body. Hamstrings neighbour is glutes, then low back muscles. If hamstrings are too tight, its neighbour muscles have to compensate for the movement to complete. This leads to the round back during the squat movements In daily life, how often do you have to squat to pick up something from the floor? Or simply to tie your shoe lace would be hard with tight hamstrings.
Standing hamstring stretch
Standing in front of a small step, one leg is forward and its knee is extended, the other leg knee is flexed, then slowly bending forward. To increase the intensity of this stretch, you should flex the knee and hip of the limb not being stretched. Try to not round the back but just maintain the neutral.
Pros
You can do it everywhere or even during breaks from office work.
Cons
Since you are in a standing position, the target muscle is not fully relaxed but still effective, though.
Seated hamstring stretch
You may think of this stretch for the hamstring first. Sitting down on the floor and the target leg knee should be fully extended and the other leg is flexed just like the image. Then slowly bending forward toward the toe. As you stretch, your knee may want to flex but in order to make this stretch effectively, make sure the knee is extended throughout the stretch.
Pros
The muscle is more relaxed than standing position. If you want to stretch the muscle, the muscle should be fully relaxed.
Cons
It may not be suitable for someone with low back pain since this includes the spine movement.
Hamstring stretch in a lying spine position
By far, I think it is the most effective and safe for hamstring stretch. Lying down on the back, then bring one leg up to the ceiling, the other leg is on the floor, then use a towel to pull the leg towards yourself. Keep the knee extended throughout the movement. If the other leg starts to go up (specifically the hip) , that’s where your hamstring’s range of motion. Ease out a bit.
Pro
Low back is safe and the hamstrings can be relaxed. You will feel a great stretch.
Cons
Need a space and a towel for most of the people.
Partner stretching at the end of personal training sessions
I can help you to improve your flexibility, too. At the end of sessions, we often finish with a partner stretching. When someone helps you stretch, it is absolutely more effective since your muscles can be fully relaxed while I am helping you stretch.