Lose upper body muscle mass

In college I was 6 ft. and ran at 145 pounds. In my mid- to late 30s I really got into weight training and got up to 218. I got tired of weighing so much and wanted to get back into running. I really cut back on lifting and just began running - and I was never to follow the 10% rule. I was running 70 miles a week fairly quickly and quit eating 5 times a day. I dropped 35 pounds in 6 weeks. I even got down to 145 again in my 50s running 85 miles a week.

It depends on your body type. I’m a mesomorph and it took a really long time (multiple years) to noticeably decrease my upper body muscle mass. If you’re an ectomorph, it’ll be much easier for you to do so.

I recall watching a YouTube short video where someone was talking about this exact scenario. He was a dedicated lifter that needed to lose muscle. He somewhat stated that the fastest way to lose muscle mass is to remain bed-ridden like a hospital patient. The more you move, the more growth you encourage. You’d have to stay still to allow your muscles to atrophy quickly.

Your control over that is limited. You need a high caloric deficit, low protein intake, and to not use your upper body much, but beyond that, your body will pick the ratio of fat and muscle to burn, and you also can’t avoid losing some lower body muscle in the process even if you continue training it.

Don’t do it.

Don't stop benching brah. Keep it Heavy.

Dude it's 10lbs of muscle mass - that's like 6-8 months of noob gains. It probably makes you look better and is overall a net positive for your health. Just lift 1x/week full-body for maintenance and general health and start running.

The quickest way is simply to fast. In my experience, a 10-day water fast, or a 5-day fasted hike, will wipe off 5kg at least. If you’re not excessively fat, most of that weight loss will be muscle. (It’ll take you about the duration of the fast to recover full energy and zip for running.)

You're a lifter's dream, ie an anabolic machine.. which just so happens to be bad for a long distance runner. To progress as a runner, you actually want to be catabolic so your body uses fat as fuel (rather than carbs), you just have to eat plenty of calories, otherwise you will breakdown protein/muscle.. but you seem to want that specifically. In my opinion, you don't actually want less muscle, you want more efficiency, which doesn't actually require less lean mass. Take beta alanine after anytime you eat carbs or after any run faster than marathon pace. The other option is do a lot of your workouts near 10k pace, this essentially does the same thing, but you can't do it everyday like you can take beta alanine.

This, technically, should work too, but is more dangerous than beneficial because of the electrolyte imbalance that could occur if you're still training seriously.