The term "muscle-bound" typically refers to someone who has excessive or bulky muscles, which can limit flexibility and movement. Antonyms for this term might include words like "lithe," "nimble," or "flexible," all of which suggest a slim, agile physique. Other antonyms could include "lean," "slender," or "svelte," which suggest a more streamlined and toned appearance without excessive muscle. Additionally, words like "weak," "frail," or "flabby" might serve as antonyms, suggesting a lack of muscle development altogether. Ultimately, the choice of antonym will depend on the context and intended meaning of the original term.