Pec Deck Fly

Target Muscle: Chest
Equipment: Chest Press
Level: Intermediate
Pec Deck Fly

The Pec Deck Fly (also called the Butterfly Machine) is a chest isolation exercise that targets the pectorals through horizontal adduction — bringing your arms together against resistance. Unlike pressing movements that recruit triceps and front delts, the pec deck isolates the chest by keeping the elbows fixed in a slightly bent position throughout the arc. This makes it the best machine for building the inner chest line and overall pec sweep. The pre-stretched starting position places the pecs under deep load at the most lengthened point, which research shows is a powerful hypertrophy stimulus. Use it as a finisher after heavy pressing or as a pre-exhaust movement to maximize chest pump.

Video Demonstration

Step-by-Step Instructions

  1. Step 1

    1. Adjust the seat height so the handles are at chest level — when you grip them, your upper arms should be parallel to the floor

  2. Step 2

    2. Set the arm pads or handles to the widest comfortable starting position that gives a deep chest stretch

  3. Step 3

    3. Sit with your back flat against the pad and grip the handles — elbows should be slightly bent (about 15-20°) and fixed throughout

  4. Step 4

    4. Squeeze your chest to bring the handles together in a wide arc — imagine hugging a large tree

  5. Step 5

    5. Hold the contraction at center for 1 second, actively squeezing your pecs

  6. Step 6

    6. Slowly open your arms back to the start position, feeling the stretch across your chest — do not let the weight stack drop

Pro Tips

  • Keep your elbows slightly bent and FIXED — the angle should not change throughout the movement. If your arms are straightening and bending, you are turning it into a pressing movement that recruits triceps. Do not use momentum to swing the weight; the movement should be slow and controlled in both directions. Focus on pulling with your elbows, not your hands — this improves mind-muscle connection with the chest. For rear delt work, simply face the opposite direction on the same machine, which is a common dual-function design.