At its natural resting length, a muscle is close to its optimal length for producing force. As the muscle contracts, the maximum force it can deliver decreases. When a muscle is at approximately 80% of its natural length, it cannot contract much more and the force it can produce drops drastically. For a muscle stretched beyond its natural length, the same is true. At about 120% of its natural length, the force that a muscle can exert against drops drastically. This muscle length to force relationship can be demonstrated by doing a chin-up. As you hang from the bar, your biceps muscles are stretched and can produce only a relatively small force. As you get close to the bar, your biceps muscles contract substantially, and you again experience difficulty. The easiest part of the chin-up occurs somewhere in between, when your muscles are close to their natural length.