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I have noticed in my nearby gym that they have machine equipment like: lat press/lat pull down/machine fly/etc. all of which have their successive weights as: 5kg, 12kg, 19kg, and so on.

On the other hand, the dumbbells are in steps of 2.5kg i.e. 2.5kg, 5kg, 7.5kg, etc.

Considering that the same human body exercises on both types of the equipment, why does one set of equipment increase in 7kg and the other in 2.5kg? What factors would make one number a better fit for one type of exercise, and the other for the other type?

I personally find the number 7 an odd step to choose as it is a prime number and has a non-terminating decimal expansion (1/7=0.(142857))

  • As well as the answers below just to add - Dumbells tend to be harder to increase the weight as theres a lot of a control element. Not to mention the machine going up my 7 it usually means 3.5kg each side so its not really 7 – Twyxz Jul 2 '18 at 6:30
  • @Twyxz "the machine going up by 7 it usually means 3.5kg each side so its not really 7" Good point, I didn't think of that! "a control element" What do you mean by that? – Gaurang Tandon Jul 2 '18 at 13:18
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    So when you use dumbells your muscles have to use stabilisation muscles to control the weight wheres on machines you just push so the increment is bigger because free weights tend to be more difficult – Twyxz Jul 4 '18 at 8:54
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Weight machines that use a pin-selected weight stack have larger increments than free-weights because it is would be too difficult or expensive to manufacture a weight stack with 2.5kg increments. You couldn't make the plates lighter by using thinner plates, as they need to be thicker than the pin and still have enough strength to hold the weight of all the plates above them, and you can't make the plates lighter by using narrower plates, or otherwise the stack would need to be three times taller. You could have a system for adding a single, additional smaller weight on top of the stack to fine-tune it (and some do), but that adds complexity and hence cost.

The reason for the seemingly unusual value of 7kg is that they're actually manufactured as 15lb increments, and are then labelled as 7kg for countries that use the metric system.

Requiring users to go up in increments as large as 7kg is highly suboptimal from a training perspective, but this isn't seen as a problem, as machines aren't designed for serious training, they're designed to allow gyms to push through large numbers of paying customers without requiring any significant coaching or instruction.

The obvious solution to this problem is to just not use machines!

  • Thanks for your answer! Though, I don't understand how having as large an increment as 7kg allows "gyms to push through large numbers of paying customers without requiring any significant coaching or instruction". The machines have a rod-insert mechanism, that makes changing the weight load a breeze. Moving the rod and changing the weight load does not at all require any "significant coaching or instruction", at least in my experience. Then why do you say so? – Gaurang Tandon Jul 2 '18 at 13:16
  • It is not the mechanism of changing the weight that makes it easy for gyms to allow customers to exercise with no instruction, it is the use of machines instead of free-weights. Gym managers like machines because anyone can sit down on a machine and perform a leg extension without having received any instruction, whereas an inexperienced person, given a barbell and squat stands, could not be expected to properly perform a back squat without any coaching. This allows them to push more people through the gym and make more money. – David Scarlett Jul 2 '18 at 15:10
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I doubt it has anything to do with numbers and more to do with the limitations of machine plates vs. dumbbells. With machine plates, they often go from 5 kg to maybe 100 kg or more. If they went by 2.5 kg, then the stack would have to double in size. The machine would have to be enlarged to accomadate the size or the manufacturer would have to make the plates out of a denser material to keep the size down. Both would increase the cost and complexity of the machine.

Dumbbells on the other hand are limited by the size of the gym itself and theoretically could go on forever. Gyms don't technically need to buy dumbbells in 2.5 increments, but they often do.

I will say that some machines that use a double pulley system may also have wider increments because the double pulley actually makes lifting the weight easier. So you would barely feel a difference in a 2.5kg increment for a machine that is built like this. It may also be that machine manufacturers use wider increments because the machine itself makes it unnecessary. Dumbbells are just free weight so there's nothing aiding the lift and doesn't require a wide increment.

As to why they chose 7 kg? I have no idea. In the States, machines go by either 10 lbs. and some go by 15 lbs. I've seen some that go by 20 lbs. Dumbbells go by increments of 5 lbs. 2.5 kgs is a little over 5 lbs. and 7 kgs. are a little over 15 lbs. Manufacturers most likely just use the same exact equipment and change the labeling.

  • Sorry but I am not sure where you're going with the first paragraph. I think you misunderstood the machines in my question. My machines are using a pin-selected weight stack, so I don't understand how the "machine plates" come into play. Could you please explain? – Gaurang Tandon Jul 2 '18 at 16:23
  • That's what I meant. I was referring to the stack of "plates" that you select with a pin. Maybe that's not as common a terminology as I thought. – DeeV Jul 2 '18 at 16:26
  • Oh! I accidentally confused your "plates" with those circular plates that are put on by weight lifters. – Gaurang Tandon Jul 2 '18 at 16:33
  • The plate stack wouldn't double in size... stepping up by less weight means each plate is thinner. The mass takes up the same amount of room bro. – WakeDemons3 Jul 9 '18 at 20:39

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