There are many synonyms for the phrase "using every muscle," which refers to the act of exerting oneself physically to the fullest extent possible. Some possible alternatives include "exerting maximal effort," "pushing oneself to the limit," "working out all muscle groups," and "utilizing every ounce of strength." Other similar phrases might include "straining every sinew," "putting in one's all," "going all out," or "throwing oneself into the task." Regardless of the specific wording used, the underlying idea is the same: that the person in question is putting forth their greatest possible effort in order to accomplish a given task, challenge, or athletic endeavor.