The phrase "muscle bound" refers to a person who has overly developed muscles that restrict their movement and flexibility. Some of the synonyms that can be used for this term include "rigid," "stiff," "inflexible," and "unyielding." However, these words may have negative connotations and imply a lack of athleticism or grace. Alternatively, one could use the phrase "overly muscular" or "heavily muscled" to describe someone with a lot of muscle mass but who still maintains flexibility and agility. Another option is to use the term "bodybuilder" or "weightlifter" to describe someone who is intentionally cultivating their muscles for competition or aesthetic purposes.