When I jump in the air I have a sudden increase in kinetic energy, this energy must have come from the work done due normal force on the system (me).
My question is since the work done is (force)$\times$(displacement of point of contact) Why does the normal force do work since the point of contact (At my foot) has no displacement during the jump.
This question is similar to my question, but I do not agree with the answer. The answer there uses an internal forces argument. I mentioned the system to be me, so internal forces (muscles) should not have any role in my increase in kinetic energy.