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Machine chest fly

Also called pec deck, machine fly, pec fly, butterfly.

What is a machine chest fly?

The machine chest fly, or pec deck, is a seated exercise that brings the upper arms together against resistance. It mainly trains the pectoralis major, with the front deltoids assisting.

Anatomical illustration of a seated pec deck fly before and after the arm pads move together, with the chest highlighted.

How do you perform a machine chest fly?

Set up

  1. Adjust the seat so the arm pads sit around chest height, then place your back against the pad and feet flat.
  2. Rest your forearms on the pads with your elbows bent and open the arms only as far as your shoulders comfortably allow.

Execution

  1. Bring the pads together by moving your upper arms forward in an arc.
  2. Pause when the pads meet or nearly meet without letting your shoulders roll forward.
  3. Open the arms under control until you return to a comfortable chest stretch.

Common mistakes

  • Setting the seat too low or high

    A poor seat height puts the pads above or below the intended chest-height arm path.

  • Letting the shoulders roll forward

    The torso leaving the back pad shortens the fly and turns the finish into a reach.

  • Dropping the stack on the return

    The opening phase should remain controlled instead of letting the machine pull the arms apart.

Where it fits in a session

Usually after heavier chest presses or as the main fly variation in a chest session.

Choose a load that lets the torso stay against the pad and the return remain smooth.

Muscles worked

Front
Front — Schematic surface view. Some muscles are partly visible or not shown.
Back
Back — Schematic surface view. Some muscles are partly visible or not shown.

Primary

Secondary

Select a muscle name to locate it.

Schematic surface view. Some muscles are partly visible or not shown.

The pectoralis major brings the upper arms together, with the front deltoids assisting.

Variations

  • Handle machine fly — Uses straight or slightly bent arms on handles instead of forearm pads.
  • Two-frame anatomical illustration of a flat-bench dumbbell fly on a white background. In the left frame, a man lies on a flat bench with his feet on the floor and holds a dumbbell in each hand. His arms are open wide at chest level with a small bend in both elbows and his palms facing inward. In the right frame, the same man has brought the dumbbells together above his chest while keeping the slight elbow bend. The pectoralis major is shaded bright red in both frames. Lighter shading also appears over the front of the shoulders, biceps and parts of the forearms.
    Dumbbell flyUses free weights while lying on a bench.
  • Two frames of a standing cable crossover, side by side. A muscular figure stands in a split stance, one foot forward and flat and the rear heel lifted, torso upright, holding a handle in each hand. The machine itself is not drawn: each cable runs up and away out of the frame, so the two high pulleys are implied rather than shown, and there is no upright or weight stack in the picture. In the left frame the arms are open wide at shoulder height with the cables taut and the elbows slightly bent. In the right frame the arms have swept down and forward along a wide arc so the two handles meet in front of the hips, the elbow bend unchanged. The chest is shaded red in both frames, across the whole of the pectoral muscle rather than an upper or lower portion; the rest of the body is drawn in grey.
    Cable crossoverUses independent cables from a standing position.

Frequently asked questions

What muscles does the machine chest fly work?

It mainly trains the pectoralis major, with the front deltoids assisting the arm movement.

Is a machine chest fly the same as a pec deck?

Pec deck commonly means the forearm-pad machine shown here. Some machines use handles instead, but both guide the arms through a chest-fly path.

How far back should the arms go?

Open until you feel a comfortable chest stretch while your back and shoulders remain settled against the pad. Do not force the deepest machine setting.

Should the elbows stay bent?

Yes on the forearm-pad pec deck. Keep the elbows supported and move the upper arms rather than changing the bend to press the pads.

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