Question

where does energy come from that fuels our physical activity

Answer 1

Energy comes from the Sun, plants and animals that we eat, to provide us with nutrients we need for energy.

Answer 2

The energy that fuels the processes inside our bodies and everything we do with our bodies comes from chemical energy stored in what we eat.

But that only raises another question:  Where does the chemical energy in our food come from ?

If you follow it back far enough, eventually it ALL comes from the sun.