Kinesiology of Exercise


When it comes to doing the fly, the dumbbell variant is preferred mainly because it allows for better control and stricter form. However there are some machine variants that are also very effective.

Major Muscles and Actions Involved

Horizontal adduction, which involves the anterior deltoid, coracobrachialis, and pectoralis major muscles, occurs in the shoulder joint. In this action your arms move from an out-to-the-sides position toward the midline of your chest. Your arms remain on a level with your shoulders. In addition, the serratus anterior and pectoralis minor muscles are involved in shoulder girdle abduction. In this action the scapulae move away from the spine out toward the sides of the rib cage.

Sports Uses

The combination of shoulder joint horizontal flexion and shoulder girdle abduction is important in sports that require forward reaching and grabbing actions pushing and throwing skills. This includes the shot put, discus throw, javelin, baseball pitch and throws from the outfield, tennis forehand, etc. The muscles and actions are also used in the sports of gymnastics (rings, parallel bars, free exercises), boxing (round house), martial arts (execution of various punches), football (tackling and grabbing an opponent from the side), and in power-lifting (bench press).

Exercise Analysis

  • Breathing rhythm is very important in the execution of this exercise. When you inhale, your rib cage (chest) expands and stabilizes so that the muscles have a firm base upon which to pull during their contraction. If your rib cage is not held firm, your ribs will move slightly as your muscles contract resulting in less force, and the potential for greater stress, especially on the backward movement.  
  • The total movement should be under control at all times. You should not relax your muscles in the most backward (downward) position unless you use light resistance and are looking for increased flexibility. When the resistance on the return is great, you should reverse directions once your arms are level with the shoulders. Allowing the eccentric return to bring your arms all the way to the rear with your muscles relaxed can cause tearing or stretching of the pectoralis major or anterior deltoid or some of the connective tissue. 
  • The butterfly exercise is one of the best exercises to fully develop the middle of the pectoralis major muscle. This is the only exercise in which both the mid-upper and mid-lower muscle fibers are involved; all other actions which involve the pectorals use only the upper or lower portions. To ensure that the entire middle section of the pectoralis major is worked, you must keep your upper arms perpendicular to your trunk when executing this exercise. 
  • If you wish to work the lower pectorals on an exercise machine, adjust the seat higher so that when you exercise, your elbow will be pointed down approximately 20-30 degrees. If you desire more upper development, switch to another exercise such as the front arm raise or overhead press. Having your elbows higher than your shoulders when you do this exercise can be injurious. 
  • When using butterfly machines (also known as "pec decks"), be careful not to overstretch your arms in the rear position because the stress in this range can be severe. This exercise has been identified as the main reason for bodybuilders and athletes getting stretch marks across the shoulder-chest area.

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