I used the analogy of the spare tire on your car – it gets you there, but it’s not as good as a full tire (unless your spare is a full tire, but you know I’m referring to cars with the mini spare tire). Same deal with muscles in the body – when a muscle is doing it’s secondary job, it tends to not be as good at it. If you continue driving on the spare tire, it’s going to either seriously limit your speed, or it’s going to blow. Same goes for when a muscle is consistently asked to do it’s secondary function in addition to it’s main function.