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I've always thought that the weight listed on a weight stack (putting the pin at 100lbs) meant that you were lifting 100lbs after all factors were taken into account. After reading this answer, I am uncertain about the meaning of the weight on the weight stack.

I have also recently switched gyms and went from 90 to 40 on rope pulldown, which I attributed to the conversion between weight units (It matches which could be a coincidence). Could it have simply been the different brand of machine that meant the weight is actually different due to the setup of the machine?

  • Most likely one of the machine uses imperial pounds (90lb), the other metric Kilograms (40Kg). Just google "90lb in kg". – zeFrenchy Aug 15 '16 at 13:40
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    That was my train of thought until I read the answer I linked to, I figured that machine was in kg, but now I have doubts. – Yousend Aug 15 '16 at 14:09
  • The numbers on the stack usually represent the weight of it .. however, because of pulleys and/or mechanical lever inherent of their respective designs, very few machine will require you to produce the same amount of work you would do with free weights. Most use non-circular pulley design to 'optimise' the torque required during typical motion (they try to make it easier around full-stretch posture) so that will have an impact of perceived effort. Absolute numbers don't matter much anyway, so long as they are consistent and you can evaluate your progress. – zeFrenchy Aug 15 '16 at 14:33
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There is no direct relationship between the numbers listed on the stack, and the weight you're lifting.

It could well be that the marker with "100" indicated that the stack now weighs 100lb or 100kg, but since the machine uses pulleys, it leverages your force by a lot, so you're not actually lifting that much weight.

Also, since different manufacturers uses a different number of pulleys, and different sizes, and different lubrication, 100 is going to be different from machine to machine.

So the answer is yes; if you switched machine brand, 100 is going to feel different.

EDIT: To answer your question in the comments; no, they don't use pulleys to boost your ego. They use pulleys so that when you push forward on the machine, you're lifting the weight straight up. I mean, that's what pulleys do. They allow you to shift the direction of force.

  • Distance between pulleys has no influence, the number of them does (and their diameter) ... explainthatstuff.com/pulleys.html – zeFrenchy Aug 15 '16 at 13:43
  • Good catch. Edited. :) – Alec Aug 15 '16 at 14:04
  • From what I understand some brands put more bells and whistles in order to boost your ego? – Yousend Aug 15 '16 at 14:15
  • Plus having the plates locked into the columns they are stacked on means all energy/output directed in the same very general direction all gets applied to raising the weights. If you tried that with free weights they'd fly out of control, so many of the muscles used there are used to control the movement of the free weights, in space. That makes a big difference on how much weight you can move in those two scenarios. – PoloHoleSet Aug 15 '16 at 14:55
  • @akadian - No, it's not to boost your ego. It's to change the direction of force. See edited answer. – Alec Aug 15 '16 at 15:41
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The weight in which you are lifting does not perfectly correlate to the numbers on the stack.

The weighted plates that you put the pin into are showing the weight of the stack up to where the pin is. If you see the stack increasing in 10 lb increments that means that each plate is 10 pounds. Due to the mechanics of the machine, you are not necessarily doing 10 more pounds of work.

The best example of weights that actually show how much weight you're lifting would be with free weights.

  • So if I put the stack at 100lbs, some magic happens and I'm now magically at an unidentified weight? – Yousend Aug 15 '16 at 14:11
  • Essentially yeah. I like to think of physics as magic, since that doesn't require me to know anything further about it, haha. – Mat Aug 15 '16 at 15:30
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Here is a real-world example that you can verify for yourself in gyms with those large multi-station cable machines with a pull-up bar in the middle. Typically, they have a pair of lat pulldown stations, a pair of cable row stations, a pair of triceps pushdown stations, and the interior pair is a multi-purpose cable cross.

If you observe the pulleys for the triceps pushdown station and compare that against the pulleys for the cable cross station, you'll see that the cable goes through an additional pulley on the latter. This halves the force needed to move the same amount of weight (it also doubles the distance needed to pull the cable to lift the weight the same height off the stack but this is not relevant to the amount of exertion needed). So if you were to set up the stacks with the same amount of weight for these stations, and you configure one of cable cross station pulls to do the same triceps pushdown, you will find it easier on the cable cross station because you're only using half the effort.

So the takeaway is that the weight of the plates on a machine only tell part of the story in terms of the force needed to lift the weight, and it is the amount of force that relates to the exertion or effort. Consequently, for a machine of a given, fixed mechanical design, the relationship is also fixed, but it may vary from machine to machine. The only way to maintain comparability of the weight of the plate to the effort needed to lift it, is to look at the same machine or machines of equivalent design.

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While the weights might not be consistent between different brands, the weight increases on a particular machine are consistent meaning the increase of approximately 10 pounds is the same whether you're going from 40 to 50 pounds or 80 to 90 pounds. The number doesn't exactly correlate to the weight you are lifting but it's still a useful number for tracking progression. If you went from 40 pounds to 50 pounds on the weight stack then you've gotten stronger, regardless of if you actually went from 37 pounds to 47 pounds once the leverages of the pulleys etc is taken into account..

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