Muscles are expensive for the body to maintain. If you quit using them, your body will break them down because it doesn’t think you need them anymore and it can better use the resources it uses to maintain them elsewhere.

In order of your questions:

• Your body essentially eats them. Muscle fibers require a ton of resources to maintain, and your body is evolved for efficient use of scarce resources. If you don’t use them, your body gets rid of them.

• While it happens pretty quickly, a week or so after working out, provided you’ve been eating and sleeping, you won’t be noticeably weaker. A good rule of thumb is after about 2 weeks of not doing a lift, drop the weight by 10%.

• See the first bullet for the next two questions.

• Barring unusual circumstances, it’s easier to regain muscle than it was to get it in the first place.

Edit: To elaborate a bit on that first bullet, one pound of muscle takes, conservatively, about 140-150 calories per day to maintain. while that may not sound like a lot, in the environment our species evolved in, even 10 pounds of muscles is a good half of what you could expect to scavenge. For comparison, a pound of fat takes about 40-50 calories per day. To give you a sense of what that entails, I'm 6'4 at about 205lb, in the 85th-95th percentile by most strength metrics, though nowhere close to anyone who does it professionally. My base metabolic rate - that is, the amount I'd need to maintain weight in a coma, is well over 2,000 calories. To gain weight at a reasonable pace, with a caloric surplus of ~200-300 calories/day, I need to eat around 3200 calories per day. That is an insane amount of food for a species that evolved in subsistence conditions (that many people still live under), and something that takes serious effort (and money) to maintain in a healthy way.

TLDR muscle is crazy expensive, energy-wise.

58 YO male who stopped going to the gym in 1990. At the time I was 6'4" and weighed ~275 lbs. with around 10% body fat.

In October of 2021, I started a new job and stepped on a pallet scale, and weighed 316 lbs. Last Monday I was 299, and the wife and I have started moderate workouts. Swimming yoga, cardio.

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All this by way of telling you that your muscles WILL atrophy from lack of use, and it is NOT easy to regain either muscle mass OR flexibility once you do. We plan on retiring in a couple years, by which time I hope to be down to around 250, and have more flexibility and cardio, if not necessarily bulk.

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It’s still hard but it’s much easier to get them back than it was the first time. Hard to gain hard to lose.