The Short Answer: Despite the shift from a toned to a softer appearance, nope, muscle does not turn into fat with decreased use.
It’s actually impossible. Muscle tissue and adipose tissue – aka fat – are different types of tissue, from function all the way down to the molecular structure. Muscle tissue turning directly into fat would be the everyday equivalent of turning water into motor oil. But that doesn’t change the fact that muscle tissue will change appearance through decreased use.
So why do our muscles go from lean and hard to small and soft?
The shift of larger, harder muscles to smaller, softer muscles is due to a loss of muscle size and muscle tone. Appearance can also be affected by overall levels of body fat. Of those contributing factors, muscle tone has the greatest effect on how much a muscle looks like, well, a muscle.
A muscle’s “tone” is it’s level of contraction at rest. That may seem contradictory, but all of your muscles maintain a very low level of contraction at all times. This helps keep you upright and in control of your body. Muscle tone is also developed when your muscles do regular, intense work (like strength training). Done frequently, heavy lifting will stimulate your nervous system to maintain increased muscle tone even at rest, as that makes it easier to generate more force and strength when you need it. (Heavy lifting will also stimulate an increase in the size of your muscle cells, which leads to an overall increase in muscle size.) Your large skeletal muscles account for anywhere between 25-45% of your body weight, and they are heavy energy users, even with low resting tone. Increases in both size and tone increase resting metabolic rate – exactly why it’s said that muscle burns more calories than far.
When regular physical demands decrease, your body recognizes an opportunity to save energy by decreasing the size and resting tone of the muscle. The loss of tone in particular leaves the muscles looking softer and looser – giving rise to the myth that your hard-earned muscle turns into fat!