Why do machine weights increase in steps of 7kg, while dumbbells in steps of 2.5kg?
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))
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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.
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!
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