If I start off in a crouched position, with my feet on the floor, and I extend my hips and knees so that I am now standing, my centre of mass has accelerated upwards, and work has clearly been done on me.
My understanding is that the centre of mass of a system cannot be accelerated unless there is a net external force acting upon it. When I extend my hips and knees, there are forces generated by my muscles, and there is a ground reaction force.
The ground reaction force is the only external force acting upon me, so it must be what is doing the work of accelerating my centre of mass upwards. Yet the ground reaction force can do no work.
Can someone help me resolve this apparent paradox?