Muscle doesn't turn into fat when you stop training

And fat doesn't turn into muscle no matter how hard you train

'Stop training and your muscle turns into fat' is physically impossible, even though it sounds plausible because of how the process looks. Muscle and fat tissue are fundamentally different cell types with different functions, and one can't directly convert into the other — you can't turn a brick into a tree just by reshaping it.

What's actually happening when training stops: muscle fibers gradually shrink (atrophy) from lack of stimulus, and if eating habits stay the same (the appetite built up during active training often sticks around), the resulting calorie surplus gets stored as fat. Two independent processes happening at the same time create the illusion that one turned into the other.

It works the other way too: building muscle doesn't directly 'burn' existing fat — those are also two separate processes. Strength training just creates conditions where both can happen in parallel, given the right diet.