What is a Machine Fly?

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  • Written By: Bobby R. Goldsmith
  • Edited By: Susan Barwick
  • Last Modified Date: 14 February 2020
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A machine fly is a machine-assisted weight training exercise that isolates the pectoral and anterior deltoid muscles. Also known as the chest fly, the machine fly is performed in the seated position, with the back firmly pressed upright against the seat at a 90 degree angle to the floor. The machine fly is a simple exercise that requires a fluid motion of both arms simultaneously. To perform the exercise in the vertical seated position, requires a specific machine, but a chest fly performed while seated on a training bench will yield similar results with the use of simple dumb bells.

The motion of a machine fly is designed to focus the effort of the exercise directly to the pectoral and front shoulder muscles. Unlike a free weight exercise, the machine-assisted aspect of the machine fly allows the user to perform the exercise without training or straining the smaller stabilizing muscles in the biceps and triceps. The advantage of this is that the user will see quick gains in pectoral strength and endurance. A disadvantage is that the stabilizing musculature does not also progress, limiting the overall performance gains for the user.


The machine fly is performed on a pectoral fly machine that limits the range of motion of the user. The pectoral fly machine consists of a tall vertical beam, with twin arms at a 90 degree angle to each other, one to the left and one to the right. Attached to the front of the beam is a flat seat with a rigid, upright backrest. At the top of the beam is a pulley with cables that attach to the arms and to the weight plates located behind the seat.

To perform a machine fly, the user sits on the seat with his back pressed firmly against the backrest. Each arm operates one arm of the machine. In one motion, the user brings both arms of the machine from the rest position at his sides to a point directly in front of him. The pulley system connected to each arm lifts the selected number of weight plates at the rear of the machine.

There are several variations of the machine fly that can be performed, both with the machine and without. With the pectoral fly machine, the user can vary the weight, number of repetitions and range of motion for one or both arms of the machine. The user can also swing one of the arms at a time to increase the isolation to one side of the pectorals or the other. The user may also use free weights to perform a resting chest fly while seated on an incline or decline bench. A free weight fly consists of the exact same motion as a machine-assisted fly, but the free weight variation incorporates the use of the smaller stabilizing muscles as well.


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