If you have tight hamstrings, then other areas of your body, like your low back, will be forced to compensate.
Take bending down to pick something up for example. Relying on your spine to bend is not necessarily a bad thing. There is nothing inherently wrong or dangerous about rounding your back.
Healthy movement is all about having options. The more options your body has, the more balanced, natural and effortless movement will feel for you.
The Mechanics of Tightness
If your hamstrings can’t lengthen enough to perform certain basic movements, your movement options will be limited. The nervous system will take what it has access to. Over and over.
This will put unnecessary stress and load on other parts of the body, which can lead to pain, discomfort, fatigue, and a feeling that your movement is stiff or forced.
Supine Hamstring Stretch
It’s always wise to start your progression with a supine (lying down) version of a stretch. This will usually be the easiest version because other parts of your body don’t have to do much and can just relax.
Seated Hamstring Stretch
Moving to a seated position, the lower body can still relax quite a bit. But now we ask the torso to remain upright and remain in a good position while we stretch the hamstrings.
This is why the goal in this stretch is not depth but how straight we can keep our spine. Instead, we want to find the depth where we can keep our back straight but still feel a stretch in the hamstrings.