Do whatever exercise you re capable of, and the Doctor/Therapist has OK'd. Other muscles will still be sympathetic to exercise elsewhere, and will try to keep their tone. Amazing, but true. (A tip I got from my Physical Therapist - thanks Jeri!)
p.s. I don't think she weighed 90# dripping wet, in full gear, but, boy, could she inflict pain. Her name, muttered under my breath, replaced J.C. in my vocabulary, when I was in pain...... And, she approved! She also made my knee work again.....