Just out of curiousity, is there a safe (i.e. not involving locking yourself in a cell for a week with no food.) way to lose muscle mass, while still going about normal activities that just happen to require muscular movement of some sort?
Get married.
Don’t eat much protein and keep on excercising. I guarantee that you’ll lose muscle mass. You’ll also lose fat, if you care.
Eat nothing but carbs and do aerobic exercise (not weight bearing) for long periods of time at easy-moderate intensity for a few days (i.e. all day hikes).
Why???
Stop exercising (AKA stop USING) the muscles you desire to lose mass in. This is critical step number one.
Reduce ALL caloric intake, not just protein.
Proteins are essential to building muscle, BUT… unless your are running near minimum body fat, if you continue to use your muscles strenuously or even moderately - even if you are well below your recommended protein intake - you will maintain your muscle mass.
Muscle atrophy comes with inactivity, no matter how much protein you consume.
If you are gifted with large leg muscles that you wish to reduce, you have almost no shot. Genetics are the hardest thing to defeat. Some have been known to boost the size of proportionatley smaller muscles, but I would imagine that losing mass is more difficult because effective atrophy exists through inactivity (the kind that exists when your leg is in a cast for several months. Atrophy is so strong, despite a good diet, that rehab is required to return your muscle to size and strength just to BEND it, let alone walk on it.)