If someone is musclebound, heavy training has given them very large muscles, but those muscles have gone stiff, so the body does not move freely. Shoulders look huge and a bit locked; arms cannot swing loosely. It is usually not a kind remark. Muscular and strong praise the body; musclebound quietly says too much gym, not enough easy movement. You hear it in sport, gyms, and everyday talk more than in doctors' notes. It is informal to neutral. You will also see it written muscle-bound, with a hyphen; the meaning is the same. It is muscle plus bound, and bound here means held tight — as if the muscles have tied the body up.