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Anytime you can select the resistance, it will never match your strength.

“It’s like everyone’s riding horses and you build a car—but then you govern it so the car can only go as fast as a horse can run. There’s a reason cars look like cars, not metal horses with wheels.” — Jim Keen, Director of Training at FastFit®

Is the fitness industry going through a “digital weights” revolution? Machines like Tonal let you tap a screen to select a resistance. They’re sleek, popular, and give the impression that they’re more advanced that traditional free weights. But understanding why we refuse to use them reveals what actually makes an exercise stimulus effective.

First, let’s get clear on our terms. Isotonic means “same tension”—a weight is 225 pounds the whole way up and down. That’s what a digital weight mimics. Isokinetic means “same motion”—constant speed, with the force varying to match whatever you produce. Companies like Tonal spend millions of dollars, build motors and gearboxes, and patent special firmware that takes hundreds of force readings per second, all to replicate what a simple barbell already does. Jim Keen has an analogy: It’s like building a car and then governing it so it can only go as fast as a horse. The FastFit® machine, by contrast, is almost “dumb” in comparison. It’s a two-horsepower motor that holds a constant speed like an inexorable wall. And that simplicity is exactly what lets it deliver perfectly matched resistance automatically.

The core problem with any selectable resistance is that the moment the user (or a pre-programmed setting) picks the weight, it can no longer match the user. Jim explains why. With a barbell, you have to sacrifice mechanical loading—lifting 65 pounds when your max is 100—just so the set lasts long enough. You’re only truly matched at the final failure rep. The Nautilus cam improved on this by making the weight heavier where you’re strong and lighter where you’re weak, which is why those machines outperform free weights and are still respected decades later. But even the cam can’t account for everyone’s different leverages. And it can’t address two other realities: you’re stronger lowering a weight than lifting it, and your concentric strength fatigues faster than your eccentric strength—at a different rate every single rep, for every muscle group, in every person. No firmware can calculate how to adjust for this. But a motor holding constant velocity does it perfectly, every rep, for everybody (and every body).

Jim then introduces a concept he coined: the “orderly drop-out of fibers.” With weights, you recruit muscle fibers gradually—slow twitch first, fast twitch only at the end when you’re already systemically wiped, breathing hard, circling the drain. On an isokinetic machine, every recruitable fiber is engaged from the very first rep, when you’re fresh. The fastest-twitch fibers fire first and drop out as they fatigue, then the next fastest, and so on. Every rep is a “stimulating rep”—high mechanical tension at maximum motor unit recruitment.

This is one of the reasons why seasoned lifters who plateau for years often start experience muscle gains once they get on an isokinetic exercise machine. Because they’re finally getting more mechanical loading than weights could ever provide. All roads may lead to Rome, but isokinetic resistance takes you to the nicer neighborhood—faster, with a lower learning curve, and to a higher cruising altitude.

In this episode, you’ll learn:

• The difference between isotonic “digital weights” like Tonal and isokinetic exercise technology
• Why whenever resistance can be selected—by the user or a program—it can never truly match your strength
• How the Nautilus cam improved on free weights, and where it still falls short
• The three ways your strength changes during a set—and why only isokinetic resistance accounts for all of them
• Why every rep on a FastFit® machine is a stimulating rep
• Why digital weight machines are capped (e.g., ~200–250 lbs) while isokinetic resistance and Nautilus stacks are not
• Why plateaued lifters grow again on isokinetic exercise machines

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