I was getting quick-onset cramps in my hamstrings, even after stretching. My Dr ordered an MRI, and she noted from the MRI my lower back muscles were tight, causing my sciatic nerve some occasional pressure, and the short-lived cramps.

I found a chair at the gym that allows me to sit with my body forward and my feet behind and underneath me. There is a bar to grip and I slowly roll my torso back, creating a nice stretch for the lower back.

I don't know the name of the chair. It looks like a massage chair, but the similarity ends there.