What happens to skeletal muscles if you don't exercise?

They age. Like human beings, muscles without exercise get shorter, fatter, and weaker. Using muscles stimulates growth of new muscle & replacement of injured muscle (as long as you rest to allow the process to complete itself) it also signals other healthy processes in your body. No exercise=reverse of that: no repair, no signals & eventually replacement of muscle cell w/ fat.
Get weak. They'll get weaker and smaller with calories from foods going to make bigger fat cells.