Like most home gyms, the discontinued Weider Pro 4100 is designed to offer at least one or two exercise options for every major muscle group. In this case, the two-station gym offers two options for working your chest muscles. You could have more exercise variety in less space with a dumbbell set, but the machine guides you through an appropriate range of motion and offers conveniently adjustable resistance.
Chest Press
Both chest exercises are done on the left-hand station as you face the gym. In fact, they both use the same attachment. To work your chest, arms and shoulders with chest presses, sit down on the bench seat facing out. Grasp the horizontal press bars and press them straight out in front of you. Don't hunch forward; your back should stay in contact with the seat back through the motion. Bend your arms, letting the handles come back toward you, then press them away again.
Chest Flyes
Chest flyes work your pectoral muscles almost exclusively, and are a good way to tire these large muscles if your triceps and shoulders are fatigued but your pecs are not. Sit down in the same bench as for chest presses. Place your forearms on the vertical, padded flye arms. Your arms should contact the padding from elbow to wrist, and your elbows should be at or slightly below shoulder level. Squeeze the padded flye arms together in front of you, then spread your arms apart to prepare for the next repetition. Make sure your elbows do not, at any point, go farther back than your shoulders; that places your shoulder joints in a very unstable position.
Adjustments
Because you do both chest presses and pec flyes using the same attachment, you must make a minor adjustment between exercises, switching which plane of motion the handles move in. To do chest presses, place the two locking pins that came with the unit in the two "press holes," located on the top of the press arm itself, to either side of the main axis where it hangs down from the Pro 4100 frame. To prep the attachment for doing pec flyes, place the same two locking pins in the "butterfly holes," which are located in the short crossbar just behind the main axis where the attachment hangs down from the frame.
Resistance
You adjust the resistance for each exercise by sliding a selector pin into the Pro 4100's weight stack. Your options range from 6 lbs. of resistance to 106 lbs., but the mechanics of the pulley system mean you'll feel slightly difference resistance levels from each attachment. The press arm attachment offers an actual minimum resistance of 20 lbs., and a maximum resistance of 180 lbs.; the butterfly arms offer a minimum resistance of 16 lbs. per arm, and a maximum of 108 lbs. per arm.
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