A chest fly is a strength training exercise that focuses on the pectoralis major — two broad, fan-shaped muscles that are often referred to collectively as simple 'the pectorals.' The exercise is usually performed on a weight bench using dumbbells, or can alternatively be carried out in an upright position using a specialized machine. The chest fly is considered to be an excellent exercise to work the inner pectoral muscles, and to add definition to the chest in general.
The movement begins with the athlete lying supine on a narrow bench, with a dumbbell in each hand and the arms slightly bent to minimize stress on the elbow joint. The athlete inhales and extends the arms outward, parallel to the floor. Exhaling forcefully, the athlete then contracts the pectoral muscles to move the weights in an upward arc, bringing the upper arms perpendicular to the floor. At the top of the movement, a brief isometric contraction can be held, to focus the stress on the inner pectorals. The movement is repeated for the desired number of repetitions, usually in the range of eight to 12.
A chest fly machine simulates the same motion, but in an upright position. This normally involves the use of cables or padded bars. Chest fly machines are useful for pregnant women or anyone who is restricted from supine exercises. It should be noted, however, that free weight movements are typically associated with more complete muscle fiber recruitment. As another variation, an incline bench can be substituted for a flat bench, increasing the proportion of stress directed at the upper region of the pectoral muscles.
During any fly exercise, the arms act as long levers to increase the load placed on the working muscles. Chest flyes, in particular, have a heightened potential for injury, because the muscle fibers are stretched at the bottom portion of the movement and the weight has a mechanical advantage over the muscles. To prevent muscle tears and other injuries, this and any exercise should always be performed with caution. The chest fly should normally only be performed using light weights, and with slow, controlled movements.
In addition to promoting gains in muscular strength and size, the chest fly can increase thoracic expansion and promote flexibility in the pectoral muscles. By increasing thoracic expansion, chest flyes are able to actually increase pulmonary capacity, which in turn contributes to the cardiovascular fitness of the body as a whole. Chest flyes are also known to promote flexibility in the pectoral muscles. These two factors make the chest fly a popular choice among swimmers, gymnasts, or other athletes requiring a combination of strength, endurance, and flexibility in the chest region.