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I don't use the peck deck machine. If someone say can do 100 kg on the peck deck how much can he do with dumbbells flyes?

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In addition to the point @Alec made, that the pulleys reduce the amount of force the user needs to apply (I think usually by about half), there's two other considerations that come to mind:

1) The lever-arm: when using dumbbells the force is applied where the weight is held, at the hand. Thus, the torque that you need to apply is (Weight x Arm Length), that's why it's so much easier to do flys the more you bend your arms (and makes it harder for people with long arms, given the same amount of weight). In a pec-deck, the weight is applied at the point where the cable attaches to the handles of the machine. This is typically much closer to body than the hands, maybe at about elbow length, give or take. Since you are applying force at your hand length, but the resistance is applied at roughly half that distance, you need to apply roughly half the force. (And, since the resistance point is fixed, the pec-deck doesn't "penalize" people with long arms).

So, rough estimate, about a quarter of the force of the weight stack gets applied at the hands, depending on the pulley system and arm length.

2) The pec-deck applies force much more consistently throughout the range of motion. Regardless of whether your arms are fully open or nearly closed, the handles are pushing back with nearly equal force. With DB flys, the force is always straight down. Thus, the force that is needed to close your arms is proportional to the cosine of the angle between your arm and the floor. That means that in the fully open position, the DBs are completely resisting the movement. At about 2/3rds of the way through the motion, they provide half the resistance, and by the end, all the effort is just holding the weight up, not pushing them together.

#2 is the primary reason why it doesn't really make sense to compare the weight done using DB flys with the pec-deck (and, of course, the fact you don't have to balance machines like you do free weights).

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Even if the weight-stack is 100kg, all the pulleys make it so the actual weight you need to push, is far less than 100kg. And the number of pulleys, as well as their size, lubrication and distance between each other, is different depending on manufacturer. So there is no one-to-one relationship between how much you can do with free weights and how much you can do on the machine equivalent.

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    Also the dumbbells put a lot more stress on the joints to stabilize the weight. I was looking for some numbers even if not 100% exact. Anw thank you. – llllllllllllllllllllllllllllll Aug 13 '16 at 17:38
  • @Alec You should clarify that there is still a benefit from using machines. – rrirower Aug 13 '16 at 17:45
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    @rrirower - Why? I mean, I certainly agree, but I don't think the asker has any doubt about that. I saw it only as a question of "if I can do x on machine, how much with free weight?" But of course, both methods are beneficial. – Alec Aug 13 '16 at 17:51

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