Tonal vs IsokineticWhy We Refuse to Use Digital Weights

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. Key Moments: 0:00 - "A car governed to go as fast as a horse" 1:21 - Welcome back: part two 2:04 - Recap: why the exercise is adaptive 2:25 - The digital weights problem (Tonal & co.) 2:49 - Isokinetic vs. isotonic, explained 3:58 - The fork in the road: recreating weights with millions 4:38 - The three mismatch problems with weights 5:11 - The horse-and-car analogy 5:40 - The paradigm shift: damage → fatigue → stimulus 6:31 - Why digital weights are so mechanically complex 7:11 - The 250-lb ceiling problem 7:30 - Eccentric/concentric: cool tech, still limited 8:09 - Every solution reveals a new problem 8:35 - The ratio that changes every rep 10:21 - Why we didn't put digital weights in our studios 11:33 - Mechanical tension: the chain analogy 12:21 - The Nautilus cam explained 12:53 - The bicep curl breakdown 14:02 - Why Nautilus outperforms a barbell 15:35 - So why go all-in on isokinetic? 16:35 - The three strength changes, solved instantly 17:19 - Orderly recruitment of muscle fibers 18:23 - Jim coins "the orderly drop-out of fibers" 18:48 - Every rep is a stimulating rep 20:19 - The "all roads lead to Rome" pushback 21:29 - Kyle's 10 lbs of muscle in 6 months 23:06 - Why seasoned lifters start growing again 24:10 - Rome vs. the Hollywood Hills 25:08 - The defining rule of adaptive resistance 25:49 - Wrap-up & teaser: why muscle matters as you age