Forget slogging on the elliptical.
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There’s no skirting it: CAROL makes big, bold claims about its bike. Head to the website and the first line you’ll see says that the CAROL Bike is the bike that gets you the fittest, the fastest. It’s backed by science and CAROL claims that a five-minute session on this bike delivers “the same health and fitness results as a 45-minute run.”
At The Edge, we’re always up for a scientific investigation, especially when it involves fitness. To evaluate the CAROL Bike, we had a team member test it at home and consulted two physiology experts to assess its scientific claims.
Keep reading for our honest CAROL Bike review.
About the Experts
Ben Reuter, Ph.D., C.S.C.S.,*D, A.T.C., is a certified strength and conditioning specialist and certified athletic trainer. He currently serves as an associate professor of exercise science at PennWest University and the co-founder of FitLab Deep Creek.
Susie Reiner, Ph.D., C.S.C.S., E.P.-C., is a certified strength and conditioning specialist and exercise physiologist. She is the head of science at Aletha Health and the founder of TheoryEx Performance.
The CAROL Bike is a stationary exercise bike resembling a Peloton or typical gym bike, but it’s powered by AI. Its core premise is that ultra-high-intensity workouts as short as five minutes, based on REHIT (reduced-exertion high-intensity interval training), can deliver better fitness results than traditional 30- to 60-minute sessions.
The bike’s AI-driven programming adjusts resistance during REHIT intervals to match your fitness level, maximizing results in minimal time. Additionally, CAROL supports third-party apps like Peloton Digital, Zwift, YouTube, and Netflix for diverse workout options or entertainment. (You do need a separate subscription for those, though.)
REHIT is based on the idea that short bursts of maximal effort, followed by rest intervals, can deliver significant fitness gains in less time than traditional low-intensity aerobic workouts, like cycling for 60 minutes.
Here’s a look at a few CAROL Bike workouts:
As you can see, the volume of work is not high. In fact, the total working volume amounts to just 30 to 45 seconds, with the rest of the session involving warm-up, cool-down, or recovery efforts.
So, is it really possible that just 30 to 45 seconds of intense exercise can produce the same benefits as a 45-minute jog, like CAROL says? It’s not a straightforward yes-or-no answer. While research does show that short, high-intensity workouts elicit a number of physiological benefits, they do not provide the same kind of fitness as long, aerobic workouts such as a steady-state run or zone 2 cardio (1).
Steady-state workouts build endurance and ramp up fat burn for energy without putting a lot of stress on your body. High-intensity workouts improve lactate threshold and VO2 max, and burn calories much faster because of the higher demand on your heart and muscles. All of these workout adaptations are important for longevity, health, and performance.
HIIT workouts are a good option for the busy average Joe, since they make a big impact in a shorter amount of time. But if you’re training for something specific, like a 5k or marathon, REHIT won’t build endurance like steady-state workouts. In other words: Both high- and low-intensity cardio are worth doing, if you have the time.
As stated by our product tester, “CAROL is designed to capture maximum adherence, and I think it does. There is rightful consternation about the outlandish claims the brand makes in its marketing, but I think getting people into very high-intensity workouts that only last a few minutes is a pure mission that the bike succeeds with.”
While few studies have shown REHIT outperforms HIIT in terms of health and performance, the concept still holds promise, according to Susie Reiner, Ph.D., C.S.C.S., E.P.-C., a certified strength and conditioning specialist and certified exercise physiologist.
HIIT stresses the cardiovascular system more efficiently than steady-state cardio by requiring the heart and lungs to work at near-maximal capacity during short, intense bursts of exercise, explains Reiner. This repeated maximal engagement stimulates greater adaptations throughout the cardiovascular system, enhancing oxygen delivery to working muscles. This, plus changes in the way that muscle cells produce energy, boosts aerobic capacity over time (2, 3).
HIIT also recruits a large proportion of both slow-twitch and fast-twitch muscle fibers, unlike endurance exercise, which predominantly engages slow-twitch fibers (leaving those fast-twitch fibers typically underutilized). According to Reiner, this increases overall muscular endurance.
“The high-intensity nature of HIIT intervals pushes the body to its limits, resulting in greater cardiovascular strain per unit of time than longer, steady-state efforts,” she says. “Given the similar targeted protocol [of REHIT], I would expect the outcomes to be similar as long as the intensity provides a high enough stimulus to stress the cardiovascular system appropriately.”
That said, Reiner points out that HIIT workouts typically last longer and have more intervals than REHIT, so they may be more advantageous for performance, particularly for athletes.
The key to REHIT is actually reaching the required intensity level during the work intervals, says Reiner. It takes a lot of mental determination to truly exercise at maximal or supramaximal efforts. HIIT can often induce negative feelings during an exercise session that impact overall exercise enjoyment and corresponding adherence, she explains.
“I wouldn’t recommend this protocol for someone who’s been sedentary and just starting their exercise journey, but instead for those ready to supplement their moderate-intensity workouts with something more challenging,” Reiner says. Typically with shorter intervals or workouts, the intensity of the intervals has to be very high to provide enough stimulus to the cardiorespiratory system, which can be tough for beginners to pull off, she explains.
Additionally, certified athletic trainer Ben Reuter, Ph.D., C.S.C.S.,*D, A.T.C., points out that AI isn’t smarter than us (yet). “If the AI is poorly designed, it could result in incorrect programming,” he says. For example, if CAROL overestimates your fitness level, you might not be able to maintain that intensity or good form for the duration of the interval—upping your risk of injury.
The critical point to remember: exercise equipment is a tool. Ultimately, you need to understand its (and your) limitations and learn to recognize how your performance is stacking up.
The CAROL Bike costs $2,595 upfront along with a $19.95 per month subscription. That’s a lot of dough, no matter how you slice it.
As Reuter puts it, this high cost of entry severely limits the potential audience, since most people do not have the disposable income to spend thousands of dollars on their exercise. And, bluntly, you don’t need the CAROL Bike to do the workouts it prescribes: You can perform them on any exercise bike, albeit without the automatic resistance applied by CAROL AI.
Our tester says the bike is well-built and provides a nice exercise experience, but so do many other exercise bikes.
All that said, other top-tier stationary bikes have a four-figure price tag, too:
If you’re someone who is short on time, has enough disposable income, and wants a well-built bike with built-in guidance from AI, the CAROL Bike might be worth the jump. Between all the integrated programming, third-party app compatibility, and high-end build quality, it’s a bike that can grow with you as both your fitness and appetite for working out improve.
Our product tester, The Edge editor Will Price, spent a few weeks with the CAROL Bike to get a feel for how it handles, and whether it actually improves fitness in the way it claims.
Will ran into a delivery issue due to a storm passing through his area but says it was quickly resolved via a phone call with the contracted local delivery service. The bike came the next day without issue, he says, and the white-glove delivery was seamless. “The bike itself is something like 120 pounds, so if you didn’t have a delivery team, it would probably require a partner or neighbor to haul it into your home,” he says.
The third-party service also took care of assembly. “My bike was assembled upon arrival with the exception of the touchpad screen, which was in a separate box and slotted into its spot easily,” Will says. If you choose to forgo the assembly service (it costs an additional $299), you’ll have to set it up yourself, which could take a couple of hours or longer.
With a square footage requirement of about 7 feet, the CAROL Bike might not work in small homes or apartments without a dedicated workout space. “Ours is set up in the living room, which isn’t exactly sightly but is the only spot I have for it,” Will says. “It’s about as compact as it could be, but I wonder if the bike could’ve been designed to break down—or at least shrink—when not in use.”
Even though it takes up a good bit of space, Will says it’s well worth it for the ease of use it offers. The bike frame does feature transport wheels, so you can theoretically move it out of the way when you’re not using it. But at 120 pounds, that may be difficult for some people, particularly on carpeted surfaces.
The bike suits riders from 4′ 7” to 6′ 7”, and safely supports up to 330 pounds in the saddle. “I think that’s a typical and expected range for a bike from a brand like this (AKA a real company, not a knock-off on Amazon) and I don’t think there’s good reason to ask for more,” Will says. “The weight limit is a hair above Peloton, which is just 300 pounds.”
On the ergonomics front, Will says the CAROL Bike feels run-of-the-mill, in a good way. The seat moves up and down and backward and forward; the handlebars, which allow for a number of grip styles, are also on an up-down lever. The pedals are fully compatible with cycling cleats should you have them, but work just fine with sneakers, too.
“I expected ease of use to be a sticky subject because AI tech and exercise can be a bit fraught,” Will says. “I give it 3 out of 5 points here because I had some issues getting my baseline V02 max recorded (as have some other reviewers), which was frustrating and affected a few of my early workouts.”
“Basically, the V02 max test kept resetting, claiming my heart rate wasn’t rising high enough to record it accurately,” Will continues. “I wound up doing several V02 max assessments before it finally relented.”
Day-to-day use is simple, though, he says. “The AI driving the resistance levels on the bike works exceptionally well—setting itself just hard enough for long enough to where I want to quit, but won’t—and the app is relatively easy to navigate.”
Although the CAROL Bike can be used in your home, it’s built with commercial use in mind. Will has no big notes about durability, saying it feels solidly built and there are few, if any, places where an unfixable break could occur. The frame, flywheel, handlebars, seat, pedals, clips, and other components feel sturdy, he says.
The integrated AI programming is the major selling point of the CAROL Bike. While you can employ REHIT protocols on any cardio exercise equipment, the difference with CAROL is that the bike automatically locks in the resistance for high-intensity intervals. Once you trigger the work interval (by pedaling harder), the resistance increases to the AI-determined set point and stays there. On the contrary, you’d need to manage that set point yourself on a different exercise bike.
The practical draw is the fact that the workout sessions themselves are very, very short.
“I didn’t expect to like the very brief, very intense workouts CAROL advertises, but I did,” Will says. “The fact that I can waddle out of my desk chair after a long meeting, slip on my New Balances, and be in the heat of a genuinely challenging workout that I’ll wrap up before a pot of water boils is excellent. I use it as a complementary workout to my usual weightlifting, and I have very few negative things to say about it.”
It’s a bonus that you can also do “free rides” on the bike or connect it to an external app, like Zwift or Peloton Digital, which makes the equipment much more versatile.
There are no other exercise bikes that apply the same exact training protocol in the same AI-powered way as the CAROL Bike. That doesn’t mean you can’t do CAROL workouts on other bikes—it just means it’ll require a bit more brainpower and manual action to toggle the resistance up and down than doing those same workouts on the CAROL Bike.
A third-party app can help you achieve this stimulus on a different bike. For instance, a Zwift-enabled exercise bike may have “erg mode,” which, like CAROL, creates unchangeable intervals that force you to pedal at the desired intensity level. Such apps may not have the same exact workouts as the CAROL Bike does, though.
You could recreate a CAROL workout on any stationary bike, but it’d be entirely up to you to ensure you’re actually pedaling at maximum intensity and progressing over time. And that can be tough to pull off on your own, whether you’re new to interval workouts or simply struggle to push yourself.
The CAROL bike and similar protocols can boost health and performance in beginner to advanced exercisers. However, the physical and mental health benefits of low- and moderate-intensity cardio can’t be overlooked and can’t be achieved with a REHIT protocol. HIIT workouts (including REHIT) are a vital part of a workout routine, but shouldn’t be the only part.
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