The phrase "breaks one back" typically means an action that is difficult, tiring, or burdensome. Some antonyms for this phrase could include "effortless," "easy," or "simple." Alternatively, you could describe an action as "refreshing," "invigorating," or "energizing," indicating that instead of draining a person's energy, it is actually revitalizing them. Other antonyms might depend on the specific context - for example, if the phrase refers to long hours of manual labor, an antonym could be "sitting down," while if it refers to a challenging mental task, an antonym might be "mindless." Ultimately, the antonym for "breaks one back" will depend on the opposite qualities that best describe the action in question.