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I understand the practical differences between these two exercises And that then you work the just in different ways, but my question is why... from a more technical standpoint? I ask this because when I do dumbbell press I go from the very bottom(touching my chest) to the very top(arms virtually straight), using a full range of motion. Also, with pec deck for instance, the arms are in a neutral position (not pronated or supinated), meaning the elbows point down slightly and the lats can be engaged. Given these similarities, and assuming that the pressing and the flys use weights that result in similar intensity, what causes the differences in how these exercises work the chest?

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I hope I'm getting the main point of your question. The chest press and fly exercises primarily target the pectoralis major muscle of the chest, but they do so in different ways due to their different mechanics, resulting in different secondary muscles being engaged, different muscle fibers being targeted within the chest, and different aspects of strength being emphasized. Here's a more detailed explanation:

Chest Press:

  1. Compound Exercise: The chest press is a compound exercise, which means it involves multiple joints and muscles. While the pectoralis major is the primary muscle targeted, the triceps and deltoids are also heavily involved as secondary muscles.

  2. Movement Pattern: In a chest press, the arms move straight out in front of the chest in a pressing motion. The pectoralis major is involved in pushing the weights away from the body (shoulder horizontal adduction).

  3. Muscle Fiber Activation: The press primarily activates the sternal head (lower part) of the pectoralis major, which is responsible for the adduction and extension of the humerus (upper arm bone).

  4. Strength and Power Focus: Because multiple muscles are involved, you can typically use more weight in a press, making it an effective exercise for developing overall chest strength and power.

Chest Flys:

  1. Isolation Exercise: The chest fly is an isolation exercise, which means it primarily involves a single joint and targets one main muscle group. In this case, that's the pectoralis major. While other muscles do assist, they are less involved than in a press.

  2. Movement Pattern: In a chest fly, the arms move out to the sides and then come together in front of the chest. The motion primarily involves shoulder adduction, which is one of the main functions of the pectoralis major.

  3. Muscle Fiber Activation: The fly motion activates both the sternal and clavicular (upper) heads of the pectoralis major. The clavicular head is more responsible for shoulder flexion, which is included in the motion of a fly.

  4. Muscle Squeeze and Stretch Focus: Because of the movement pattern and the ability to focus more directly on the chest, the fly is often used to emphasize the 'squeeze' and 'stretch' of the pectoral muscles, which can promote muscle hypertrophy (growth).

So while both exercises target the same primary muscle, they do so in different ways and have different profiles regarding strength, hypertrophy, part of the muscles, secondary muscles.

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