Physics application?

Question

why is it more difficult to do sit-ups when you knees are bent that when your legs are streched?

and, which of the ff situations produces less stress on the lower spinal column:

a. touching your toes while seated on the floor with outstreched legs

b. touching your toes from a standing position

Answer 1

When u do sit-ups, ur pelvis acts as a fulcrum. When ur legs are stretched, they act like a lever and provide more balance to ur upper body than when keeping ur legs bent.

Touching the toes while seated is more difficult and requires more flexibility. I think the reason is that, when ur sitting, ur whole body weight is taken by the leg/thigh muscles and there is very little room for contraction or expansion. So that shud produce more stress on the lower spinal column.