He told me the only lifters he has seen with a shake or wobble in the legs at the beginning and ending of the squat had a weakness in the low back.He then went on to say a weak back leads to hamstring issues. Strengthen the low back and hamstring pulls are a non-factor. He had seen dozens of football players over the years with hamstring pull after hamstring pull. Every one of them had a weak low back and flat erectors.
via Hamstring Pull.