Sometimes called butterfly arms, the butterfly apparatus is a strength training station for targeting your chest muscles. A butterfly is usually found on home gyms, multi-gyms and as a stand-alone machine in fitness centers. There are two types of butterfly machines, one is called a butterfly or pec deck, and the other is called a fly.
Sit on the upright bench with your feet flat on the floor. Place your forearms against the arm pads. Your forearms should be perpendicular to the floor and your upper arms parallel to the floor. Push against the arms enough to lift the weights off the stack. This is the starting point. Squeeze against the weight until the butterfly arms almost touch in front of your chest. Keep your chest elevated; don't let it sink in. Pause for a count then slowly lower back to the starting position.
The butterfly station is also called the pec deck station. As described above, your forearms are placed perpendicularly to the ground. A second type of butterfly station is simply called a fly. However, the difference is in your arm position. During a fly, your arms are almost fully extended and parallel to the ground as you push the weight together. During the butterfly, you have a 90-degree bend in your elbow, placing your forearm in a vertical position.
The butterfly and the fly both target your chest muscles. These are isolation exercises so the movement only occurs at one joint -- your shoulders. This keeps the emphasis on your chest and eliminates involvement from your triceps, which are the muscles on the back of your upper arm. Your biceps, the muscles on the front of your upper arm, assist during the fly movement. This is the result of your arms being fully extended instead of bent.
Although "butterfly" and "fly" are often used interchangeably, they are two separate exercises. The butterfly movement can only be done on a butterfly apparatus. You need the arm pads to keep your forearms in a vertical position. However, the regular fly can be done with other types of equipment, such as a cable machine, dumbbells or a resistance band. Because the arm lever is shorter for a butterfly -- your arm is bent -- you should be able to push more weight on the butterfly than the fly exercise.