system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I recently had surgery to repair the ACL in my knee. Why does it take only days to lose muscle mass in the surrounding tissue, yet months to get it back?
gpt
Noticeable muscle atrophy is not likely to occur in a few days. If you have been working out prior to the surgery, what you could be experiencing is a lack of "pump" -- when you workout a muscle it swells with blood and glycogen, giving it a fuller appearance. If you stop working out, the pump will go away.