How to equate weighted crunches with an abdominal crunch machine

Question

What's the equivalent reps of weighted crunches to a given number of reps on an abdominal crunch machine?

I keep track of my workout routine on each machine I use at the gym, and practice linear progression to slowly increase the weight over time. However, sometimes I travel or can't get to the gym for scheduling reasons, and so have to workout without access to the fancy stack weight machines and do just regular crunches or situps while holding some weights.

When I use the abdominal crunch machine, I've been setting the weight to 145 lbs. I'm now trying to find what the equivalent weight would be when doing regular crunches while holding a weight at chest level.

At first, I naively thought that, with the average male torso being about ~55 lbs, I could simulate doing crunches with 145 lbs by simply holding two 45 lbs weights at chest level so 45+45+55 = 145 lbs. Granted, it's not exact, but I thought it would be close enough to approximate the experience.

Boy was I way off.

That amount of weight was way too much for me to handle. I couldn't even do one rep. So clearly, the abdominal crunch machine is employing some sort of leverage so that 145 lbs becomes a much less actual resistance.

Is there any way for me to calculate the weight I should use when doing regular weighted situps or crunches, so I can work the same muscle group at the same level of resistance regardless of which method I use?

I've tried trying a bunch of lesser weights to find what "feels" about the same, but that seems too subjective. I'd like to find a formula that tells me 145 lbs on the machine equals X lbs without the machine.

Answer 1

There is not one. Use RPE in your desired rep range.

There’s no way to convert these measurements objectively. A crunch machine and a weighted crunch aren’t even the same movement. While the rectus abdominis as the prime mover, they will use different supporting muscles in different ways. They just aren’t the same movement.

Instead use RPE: rate of perceived exertion. Keep your perceived exertion and rep range the same for each exercise, and you can expect a similar training stimulus. This article from Dr. Eric Helms gives a good introduction to using RPE in training.