While one pound of weight always weighs the same, sometimes the labels on the weights we use don’t tell us the whole story.
Free weights typically vary a bit in weight simply because they’re not manufactured very precisely, but this is rarely actually an issue.
Machines, on the other hand, can feel very different with the same weight, due to differences in the pulley systems they use, the pattern of movement the weight follows, and the exact angles of the arms in the machine.
In general, it’s best to stick with the same equipment in order to adhere to the principle of specificity. If you can’t do that because you travel a lot and change gyms regularly, using free weights for more of your training is a better idea because you’re less likely to have issues.
When does one pound of weight not weigh the same as another pound of weight?
A common question I get has to do with the machines that we use every day in the gym - when you move to a machine in a different gym, even if you’re using the same weight, it may often feel much lighter or heavier than the one you’re used to at your home gym.
Before we’d get started, I’d like to quickly add that yes, machines are a useful training tool and are not inherently inferior to free weights. I’ve written on this previously.
Why do some machines feel different than others, even with the same amount of weight? Ultimately, it’s mostly related to the construction of the machines in question.
When it comes to free weights, we can be generally sure that they always weigh about the same. A 45lb plate in your gym probably weighs about the same amount as any other 45lb plate.
Now that doesn’t mean that all of those plates weigh 45lbs - because in fact, manufacturers are not that precise when it comes to making equipment. Calibrated plates (which are checked to ensure that they weigh almost exactly the amount on the label) weigh almost precisely the given weight, but these are more expensive and harder to make. Your more common plates may vary by a few pounds here and there. Aside from that, all plates lose a bit of mass over time as they get worn out from regular use - especially prevalent in gyms, where plates will see constant usage.
Now, that doesn’t mean that they’re useless, or that you’re always going to be lifting more or less than you think - the reality is that these small variations average themselves out, and over time you’ll still be getting roughly accurate numbers. This means that while the plates may be a bit lighter or heavier than their label suggests, these variations tend to cancel each other out. In training, especially if you’re using the same set of plates over and over, it’s rarely an issue.
However, calibrated plates become useful for things like competitions, or setting world records, where we want to be very sure about exactly the weight being lifted.
Unlike free weights, where the weight is generally accurate, the same cannot be said of machines.
A pulley system is a simple mechanical device used to make it easier to lift a weight. A cable is fed through one or more wheels to make it easier to lift, and to redirect that cable to a place that makes it easier to pull. Pulley systems are the basis for all modern weight machines, which typically affix one end of the cable to a stack of selectorized weights, and the other to some kind of handle which the exerciser pulls or pushes on. This provides the advantage of making these machines both relatively compact and also easy to change weights on.
However, different machines are designed differently. The angles of the mechanical device, the movement pattern of its handles, the number of pulleys used, all of these can affect the way the weight “feels” on the other end. 15lbs on one machine may “weigh” slightly less or more on another machine due to the complexity of the pulley system involved.
Using an unknown machine also means violating the principle of specificity, meaning that you’ll struggle until you adapt to the exact movement patterns of the new machine. Your brain thinks of this as a different movement pattern, so it will take time to adjust.
In general, it makes far more sense to stick with machines and equipment that you’re used to. If you travel a lot, this may be hard to do, simply because of the number of different gyms you have to use. In these cases, this problem can be alleviated simply by making greater use of free weights, which as examined above are a bit more resistant to this kind of problem. You could still step into a new gym and be thrown off a bit by the feel of their barbells, or the precise size of their plates, or by pound/kilo conversions, but this will generally be less of an issue.
Machines, like anything, are subject to the principle of specificity, and should be treated as such. Don’t use new machines if you don’t need to.
About Adam Fisher
Adam is an experienced fitness coach and blogger who's been blogging for 5+ years, coaching for 6+ years, and lifting for 12+ years. He's written for numerous major health publications, including Personal Trainer Development Center, T-Nation, Bodybuilding.com, Fitocracy, and Juggernaut Training Systems.
During that time he has coached hundreds of individuals of all levels of fitness, including competitive powerlifters and older exercisers regaining the strength to walk up a flight of stairs. His own training revolves around powerlifting and bodybuilding.
Adam writes about fitness, health, science, philosophy, personal finance, self-improvement, productivity, the good life, and everything else that interests him. When he's not writing or lifting, he's usually hanging out with his cat or feeding his video game addiction.
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