Muscle Failure: Each set is designed to work the muscle to failure. Muscular "failure" is defined as when the muscle can no longer move or contract concentrically. During a session, each set allows you to progress through all 4 motor unit types. Your body will start by recruiting your slow twitch fibers first, as the set progresses and those fiber weakens, your body will recruit your fast intermediate muscles fibers (type 2a and 2b); finally for the last 2 to 20 seconds of the set, your body will recruit your fast twitch fibers. That progressive overloading seems to stimulate the biggest amount of muscle fibers and creates the greatest metabolic response.