I'm afraid you've got the wrong idea. Break a leg refers to acting in Shakespeare's time. Poor people who couldn't afford seats would stand up directly in front of the stage in a pit, so they could even lean on the edge of the front of the stage. As performances were very rowdy and much more casual than today, people would often get drunk with low hygiene levels and as a result a thick layer or grease, slime, alcohol and sweat formed around the front edge of the stage.
If actors performed badly then they would get shouted off, probably with vegetables thrown at them; but if they did well then they were safe to come right up to the audience to be congratulated without being attacked. One such actor, who did very well came right up to the edge and bowed, but because of the thick layer of slime he slipped and broke his leg.
People say 'break a leg' before a performance because they want your performance to be so good that you could do what that actor did and, metaphorically, come right up to the edge safe knowing that the audience enjoyed your performance so much that they wouldn't harm you.