The chest or pectoral fly primarily works the pectoralis major muscles to move the arms horizantally forward. If medially (internally) rotated, it is assisted in this by the anterior (front) head of the deltoideus in transverse flexion. If laterally (externally) rotated, the contribution of the deltoid is lessened and the pec major is strongly emphasized as the transverse adductor. The hands are usually brought out further than the elbows, in which case the flexors contract isometrically to inhibit undesired excess flexion of the elbow. Muscles which do this are the biceps brachii, the brachialis and the brachioradialis. The biceps may also play a limited role in shoulder flexion. The straighter the elbow is, the more stretch in these muscles. For safety, many avoid locking out the joint. -Wiki-
