How can I get a basic equivalent between free weight and machine "weight", i.e. stack pin?

Question

I know that machines are different from free weights. That's a given: the machine limits the trajectory, eliminating the need for stabilizing forces, and reducing effectiveness in building core strength and coordination, etc. Machines also provide mechanical advantage. So the pin position in the stack can mean anything - or nothing.

My question is what is the conversion factor? A machine with a 200 pound stack used in seated chest press mode requires a pressing force equal to some amount of barbell lifting force (which equals the weight of the barbell). Depending on the pulley arrangement, lever arms, and cams the weight on the stack could be anything. It does not really matter if one is simply trying to build up strength but what if you want to know when you will outgrow the machine? I could previously bench 300 pounds (Olympic barbell and flat bench) 5 times. I suspect I will soon, after returning to training, be able to do 5 reps on the machine at full stack (200 "pounds") but I doubt I will have returned to my free weight bench of 300.

Manufacturers are not forthcoming with their mechanical data. I can compute it after careful measurements, if the mechanism is visible, but I wonder if there is a convenient reference. Does anybody know of one? After you outgrow the max machine weight, the machine becomes a calisthenics machine more than a strength training machine; still useful but limited.

Answer 1

This answer is based on thinking (almost) purely of the metal mechanics of the machine; others can address the biomechanics far better than me.

In at least some manufacturers there can easily be a factor of 2 difference between notionally similar machines if one (often more versatile though I'm thinking of an older model) routes the cable round a pulley to give a mechanical advantage and another doesn't. Labels on the plates don't take this into account. Lat pulldowns are an example exercise 2 adjacent machines can differ like this.

So any conversion is per machine/exercise, even for a very rough approximation, and even if the movement is the same. Then the leverage often changes through the range of motion, meaning a single conversion factor only holds for one point.

Taking another simple example, a linear leg press is illustrative. Compared to a squat it of course gives much more support, but I'll neglect that (important) aspect. The effective mass of the plates can be calculated by trignometry (the machine axis is at about 40° on that model, and sin(40) = 0.64, so each plate presses down on you by about 64% of what it says on the label). But you have to add on the mass of the carriage that supports the plates, so load = 0.64*(plate_mass + carriage_ mass). But when you're accelerating those plates, the inertia comes from the whole mass, not the mass reduced by trigonometry. The load is then different in the static and dynamic cases, and therefore varies depending on how fast you move, in a different way to a vertical lift.

Answer 2

I don't think you'll find a universal conversion factor. And even if you find the conversion factor for a specific machine, it still won't be super useful because it may not necessarily transfer for you specifically to free weights.

How much much you can lift with free weights depends a lot on technique and whole body tension to produce the necessary force, whereas machines isolate muscle groups a lot more and they are a lot more forgiving with technique, only the major muscle group needs to contract to move the weight, without much concern for direction.

This means that the conversion factor will vary a lot from person to person and even for the same person for different exercises or angles.

Instead of that, I instead propose you measure your 1RMs on both machine and free weight. Then you'll know your conversion factor for that exercise.

This doesn't mean that it will go up linearly though if you only train one of the two. You could train on just machines for a while and increase your absolute force production 1.5x, but your free weights 1RM will be perhaps only slightly higher than it was before because you haven't trained technique and the stabilizing and tension muscle structures. Conversely you could spend a lot of time on free weights and improve your technique loads, allowing you to lift a lot more weight without necessarily improving your maximum force production by much.