Six Exercises You Should NEVER Do-#3

The "Pec Deck"

The myth: It's a super safe and very effective way to work your chest muscles.

The truth: This apparatus, also called the chest fly machine, can overstretch the front of your shoulder and cause the muscles around the rear of your shoulder to stiffen.

The result: Doing this movement frequently can lead to shoulder impingement syndrome.

The alternatives: Forget this machine, and stick with exercises such as the push up, dumbbell bench press and dumbbell incline press; they're easier on your shoulders and the best way to build your chest overall. In fact, Truman State University researchers found that pectoral muscles are activated for 23 percent less time during the chest fly, compared with the bench press.