Recruiting motor units to produce tension in a muscle is like playing tug-of-war. If two people begin pulling on a rope from either end and gradually additional people begin to grab hold and tug on...

Recruiting motor units to produce tension in a muscle is like playing tug-of-war. If two people begin pulling on a rope from either end and gradually additional people begin to grab hold and tug on that rope at each end, the rope gets tighter and tighter. As more and more motor units are recruited, the tension in a muscle will continue to increase.