Please assist my mental pull up

August 23, 2012 9:38 AM

My gym has an assisted pull up machine that provides varying degrees of assistance depending on how much weight is selected. I'm pretty sure it's lying.

The way it works: You select an assistance amount by putting a pin into a stack of weights. A cord attached to the weights wraps around a pulley overhead, and is connected to a padded platform on the other side. You kneel on the platform and hoist yourself up. In theory, if you weigh 150 and 50 pounds of assistance is selected, each pull up would require as much effort as if you weighed 100. Makes sense, right? BUT! If I set the machine to 50 (it's minimum assistance amount), and I put a 20 pound weight on the platform, the platform goes down, lifting the "50" pounds. Putting 17.5 pounds on the platform seems to bring the two sides into equilibrium. So, aren't I really only getting 17.5 pounds of assistance? Is the minimum weight mislabelled, or are some crazy pulley physics messing with my head? Side note: selecting 50 + x pounds of assistance causes the equilibrium amount to change to 17.5 + x, so it's apparently proportionate.

posted by Gee Your Hair Smells Terrific to health & fitness (9 answers total)

Not sure about the meaning of 50/17.5 here, because as I'm reading it you should be getting 37.5 lb of assistance (50-17.5).

Regardless, there may be other factors: the weight of the platform itself, leverage (if the platform has a fulcrum point), and so forth.
posted by IAmBroom at 9:50 AM on August 23, 2012

There's one like that at the gym I go to - it was telling me I weighed upward of like 220 which was a DAMN LIE. From what I could tell, it looked like the part of the mechanism you stand on is longer than the part of the mechanism that is connected to the weight stack, so whatever weight is on that part has a mechanical advantage. Look up levers on wikipedia, for instance.
posted by logicpunk at 9:52 AM on August 23, 2012

Can you clarify how exactly the pulleys on the machine are set up? You're right in thinking this has to do with the pulleys but there are also other factors such as accuracy of the labeled weights, weight of the machine components, etc.
posted by Loto at 9:52 AM on August 23, 2012

Just to clarify, are you talking about a machine like this and this, which has a single line going from the back of the platform, over a single pulley to the top of a stack of weights? Or is there a more complex block and tackle-like pulley system?
posted by metaBugs at 9:58 AM on August 23, 2012

Thanks all. The machine is this one. It appears to be a simple single pulley, not a block & tackle as far as I can tell, although part of the cording is obscured by protective covering.
posted by Gee Your Hair Smells Terrific at 10:54 AM on August 23, 2012

Perhaps I'm just forgetting the weight of the platform itself...I suppose that could account for the "missing" 37.5 pounds?
posted by Gee Your Hair Smells Terrific at 10:55 AM on August 23, 2012

Yes, it's the weight of the platform. The labels are directly on the weights on the back, right? It's marking a 50 pound weight, possibly interchangeable with other weights on other machines, which helps lift both you and the platform--and the platform itself weighs 32.5 pounds.
posted by anaelith at 10:59 AM on August 23, 2012

You got it anaelith. You get best answer for agreeing with me! You would think they would account for the platform on the weight labels? Thanks all!
posted by Gee Your Hair Smells Terrific at 11:02 AM on August 23, 2012

On a lot of machines (I frequent hotel gyms, and this is one of my favorite machines of late) they don't even mark it in pounds, they just have integer numbers on each weight in the stack. I.e. 1 is lowest assistance, up to 13 or 14 for highest assistance. So I think to the manufacturers of the machine, they only intend for you to use the values to mark your improvement and so you can have consistency from one workout to the next, not as any absolute indicator.

That's how I'd recommend thinking of them (on virtually all machines, since sometimes the amount of mechanical advantage or disadvantage isn't obvious) anyway.
posted by Kadin2048 at 1:02 PM on August 23, 2012