As @lioness said, muscle atrophy occurs when you are unable or don't use a muscle for a long period of time. Typically, people experience muscle atrophy (loss) post surgery because you are unable to move a body part until you are healed. From my own experience, not following a PT's guidance on recovery plans can accelerate atrophy and make recovery harder than it needs to be. Also, for me, dealing with the loss of muscle from surgeries is one the hardest things I struggle with, both physically and mentally.