Even if you hate cardio, you might love the Carol Bike. Developed by scientists, it’s an AI-powered smart stationary bike that promises to deliver the benefits of a 45-minute run in less than a minute. Its primary workout protocol consists of three weekly rides, each of which lasts less than nine minutes and requires just 40 seconds of hard effort. These reduced exertion high-intensity training (REHIT) workouts aim to trigger your body’s “fight or flight response,” causing it to burn the sugar stored in your muscles as fuel. At $2,395, the Carol Bike costs about as much as a Peloton, but it’s not trying to replicate a group fitness experience or entertain you. Instead, it’s all about getting your workout over and done with as quickly as possible. It’s best for people who don’t have the time or desire to work out, or those who focus on a different activity such as weight lifting or yoga, and want to quickly get in their cardio.
HIIT vs. REHIT
You’ve likely heard of high-intensity interval training (HIIT), but there’s a good chance you’re unfamiliar with REHIT. As its name suggests, this training technique is a modified form of HIIT that requires less exertion.
Like HIIT, REHIT workouts alternate short bursts of intense exercise with recovery periods. The difference is that REHIT workouts involve fewer, shorter bursts, performed at maximum intensity.
Your first six workouts on the Carol Bike are introductory rides designed to help familiarize you with the system and teach the bike’s AI how hard to push you. Based on your performance, its algorithms calculate the amount of resistance required for you to reach your maximum intensity. Carol’s AI then continues to monitor your metrics, and tweak your resistance as needed to challenge you as your fitness level improves.
At the heart of Carol’s REHIT program are the company’s Intense rides. They start with a gentle warmup, followed by a 20-second sprint, a recovery period, another 20-second sprint, a cooldown, and you’re done. The entire workout lasts just 8 minutes, 40 seconds.
Carol says these short workouts deliver the same benefits as a 45-minute run, and that completing just three Intense rides a week for eight weeks has “been scientifically proven to deliver double the health and fitness gains of regular exercise.” Those are some pretty big claims, and here’s the kicker: The company encourages you not to sweat during Intense workouts.
Sound too good to be true? A peer-reviewed study by the American Council on Exercise (ACE) found that Carol’s short REHIT workouts were “more potent” than 30-minute moderate-intensity continuous training (MICT) sessions.
Premium and Pricey
The Carol Bike features a sleek black steel frame with red and silver accents, a 10.1-inch touch screen tablet (a bit on the small size for this category) that doesn’t tilt or rotate, and a comfortable “ergonomically sculptured” saddle.
Measuring 42.5 by 22 inches (LW) and weighing 130 pounds, the bike is a good size if you’re short on space, and is fairly easy to move around. On the bottom, it has four leveling feet and two transport wheels. It can accommodate riders ranging from 4 feet, 11 inches to 6 feet, 7 inches and up to 286.6 pounds. The bike’s minimal design doesn’t offer any built-in storage areas.
It features a rear aluminum flywheel with a cast-iron center, a silent belt drive transmission system, and a frictionless magnetic Eddy current brake for resistance. For safety, it features a freewheel clutch so you can stop pedaling at any time and safely get off the bike, even if the flywheel is still turning. Its software also offers safety algorithms that will warn you if your heart rate is unusually high.
The bike has heart rate monitors on the handlebars, and sensors to measure your cadence (how fast you’re pedaling, measured in rotations per minute, or RPMs) and resistance level. The tablet features dual front-facing speakers, a 3.5mm jack so you can plug in wired headphones, and Bluetooth connectivity for wireless headphones and heart rate monitors (but no ANT+ support).
Like the Bowflex VeloCore, the Carol Bike features dual-sided pedals that can be used with or without clip-in cycling shoes. On one side, it has an adjustable toe cage for secure footing while wearing regular workout sneakers. On the other side, it features SPD brackets. The bike comes with a pair of compatible cleats you can screw to the bottom of your cycling shoes. I easily attached them to the Pearl Izumis I used to test the SoulCycle At-Home Bike.
Like Peloton, Carol offers several different packages starting with the Essentials bundle ($2,395), which includes just the bike and a one-year warranty. For $200 more, the Standard bundle ($2,595) also includes a chest strap heart rate monitor, a tablet holder, and upgrades your warranty to two years. The Premium bundle ($2,795) is the most expensive and comes with all of the previously mentioned accessories, plus a floor mat, a water bottle holder you can attach to the bike frame, and a three-year warranty. The accessories are also available to purchase on their own.
These prices make the Carol one of the most expensive smart exercise bikes on the market, surpassed only by the $2,495 Peloton Bike+ and the $2,500 SoulCycle At-Home Bike.
US-based customers have the option to finance the Carol over 12, 18, or 36 months via the third-party lender Affirm. Carol offers a 30-day return policy, so if you’re not satisfied with the bike, you can send it back free of charge, and the company will issue you a full refund. The company offers 24/7 customer support via phone, chat, or email.
For this review, Carol sent me the bike plus a chest strap heart rate monitor ($59 when purchased separately). I recommend getting a heart rate monitor, as without one, the bike will stop recording your heart rate every time you take your hands off the handlebars. The company’s Bluetooth chest belt is an excellent option; it’s easy to use and automatically connects to the bike. It’s also compatible with many smartwatches and fitness apps, including the Garmin Venu 2 that I’ve been wearing lately.
As is customary for smart home gym equipment, you have to pay extra for a subscription, but Carol’s is less expensive than most at $144 a year (which works out to $12 a month) after a three-month free trial. Most competitors charge anywhere from $29 to $39 per month for a class membership, which typically includes workouts on and off the machine, performance tracking features, and the ability to set up multiple user profiles per household.
Carol doesn’t offer trainer-led classes or workouts off the machine. Its subscription gives you access to the company’s personalized, AI-guided workouts and lets you create up to eight individual profiles so everyone’s stats are kept separate. The bike’s AI automatically adjusts the resistance during every ride based on your ability, so it’s important that everyone who wants to use the Carol sets up their own account. Without a subscription, you can still use the Carol as a regular stationary bike, but you lose the AI features.
For more variety, you can install the Peloton Digital app ($12.99 a month after a 30-day free trial) directly on the Carol’s tablet. When streaming Peloton classes on the Carol, you’ll see metrics including your RPM, output, and heart rate on the left side of the screen, and be able to manually adjust your resistance from 0 to 100 with a slider on the right.
You can’t use Apple Fitness+ ($9.99 per month) on the Carol tablet, but you can put the bike in Free Mode and follow along with Apple’s cycling workouts on a separate device. If you want to go that route, you’ll need a phone or tablet holder for your bike; Carol sells one for $99.
Carol charges $150 extra for professional assembly. If you choose this option at checkout, a technician will set up the bike, connect it to your Wi-Fi, and haul away all the packaging. If you decide to assemble it yourself, you’ll want to enlist someone to help, as some of the pieces are heavy. Carol says that self-assembly should take two people about 45 minutes. All the required tools come with the bike. This article details the assembly process so you can see what you’re in for.
My test unit arrived fully assembled (the perks of being a professional product reviewer), so I simply had to connect it to Wi-Fi, create an account, and I was ready to go.
The Carol Bike is comfortable and sturdy. I have it in a room with wall-to-wall carpet, and have never felt any rocking, even when I’m sprinting on it as fast as possible. My test unit has never experienced any hardware or software issues.
Working Out With the Carol Bike
At the bottom of the Carol interface, there are tabs for Select a Ride, Dashboard, Rides, Trends, and Weekly Leaderboard.
In the Select a Ride tab, Carol offers several different workout options, including Intense (20-second sprints), Energizer (10-second sprints), Fat Burn (10 minutes, 30 sprints), Fat Burn (20 minutes, 60 sprints), Free Ride (continuous power), and Endurance (continuous ramp up).
As a baseline, the company recommends doing two to three Intense rides per week. Then, depending on your goals, you can optionally mix in the other workouts. If you’re looking to lose weight, for instance, you might do two to three Fat Burn rides in addition to your two to three Intense rides each week.
Carol has a section on its site detailing its goal-based programs, but could do a better job of incorporating this guidance into the bike app interface. Peloton, for instance, just overhauled its Programs section to offer more regimented training plans with recommended schedules, a progress report at the end of each week, and badges for your achievements.
After choosing your workout on the Carol, you have the option to select one of six music options for your session: Tiger, Triumph (my personal favorite), Mellow, Escape, Electro, or Lounge. When you’re ready, press Start to begin.
On the left side of the Intense workout screen is a Power meter that goes from blue to red. During the warmup, recovery, and cooldown, you’re supposed to keep your power within the blue zone (20 watts or less). In the middle of the workout screen, it shows your RPMs. On the right, it shows your heart rate. Below that, it shows scores for your best and last Peak Power and Octane Score, your target and actual number of rides completed per week, your target heart rate range, and your Max Heart Rate during the previous session. A timeline at the bottom of the screen lets you see where you are in the workout.
A timer at the top of the screen counts down to the next segment of the workout. When it’s time to sprint, the screen will turn red, which is your cue to pick up the pace and start pedaling as fast as possible. Three seconds after the screen turns red, you’ll feel the resistance increase.
The whole point of the workout is to give it your all during that 20-second sprint. If you selected one of Carol’s music options, the beat will pick up just as a sprint is about to begin (the music is only synced with the sprints during Intense workouts), encouraging you to pedal faster.
One of the nice things about the Carol is that it gives you real-time feedback about your performance. After your first sprint, a graph appears with a blue line representing your power throughout the 20-second effort. The graph highlights your Peak Power score, giving you a goal to beat on the second sprint. After your second sprint, it will add a red line to the graph showing your performance for that effort, so you can easily see if you did better or worse. I typically get a higher Peak Power score on my second sprint than my first.
To give you something to focus on during the warmup, recovery, and cooldown periods, Carol offers a breath pacer at the top of the screen, guiding you to inhale for four seconds, then exhale for six seconds to balance your nervous system.
Tracking Your Progress
Carol recently revamped the bike interface, adding Dashboard, Rides, and Trends tabs, which are full of information and graphs to help you track your progress and compare your fitness with your peers. Previously, this information was only available in Carol’s online dashboard and mobile apps, so adding it right to the bike interface makes it a lot more accessible.
As part of that release, the company also added several new user-requested metrics, including your Functional Threshold Power (the average Watt power you can sustain for one hour), Maximum Aerobic Power (the maximum Watt power you can sustain for a short period), and Maximum Anaerobic Power (your average power during the sprints in an Intense workout), so you can track them over time.
In the Dashboard tab, it shows your Octane Score (Carol’s key metric for your cardiovascular fitness), Calories Including EPOC (Excess Post-Exercise Oxygen Consumption, or “after burn”), Peak Power (measured in Watts), and Energy Output (measured in Watt Seconds) for your last Intense ride, as well as a monthly calendar showing when you worked out. Here, the app also keeps track of your total number of rides for each type of workout.
You can tap a small information icon next to each metric for more information about it. Your Octane Score, for instance, “reflects how many heartbeats are required to generate the Energy Output in an Intense ride,” with the higher the number the better. My baseline is 19.3, but during my last Intense ride, I achieved an Octane Score of 22.2. “That is 15% above your baseline…and 2% below your personal best of 22.6,” the bike says. “Your performance put you into the top 53% of your peer group of 35 to 45 year old females.”
Calories Including EPOC represents the amount of energy your body burned during and after your workout. Carol calculates it based on your power measurements on the bike. “These are ‘active calories’ only…and do not include your baseline caloric expenditure,” the bike says, warning that its calorie burn estimates may be different than the ones from your smartwatch. “Smartwatches estimate your caloric expenditure via your heart rate alone, and do not include EPOC,” so those totals will likely be lower.
The Rides tab has a list of all the workouts you have done in each category. A chart at the top lets you quickly compare your metrics for each session (Octane Score, Calories, Peak Power, Energy, and Max Heart Rate) and shows whether the software consequently increased, decreased, or kept your resistance the same. You can also click on each entry to view all of your metric graphs for that session.
In Trends, you can view graphs of your Octane Score, Peak Heart Rate, Burn (Calories including EPOC), Peak Power, and Energy Output over time.
In the Weekly Leaderboard tab, it shows how you stack up against others for the various metrics Carol tracks. The leaderboard is pretty basic: It shows that I rank as number 390 on the global Octane Score leaderboard, but it doesn’t tell me out of how many.
My Experience With the Carol Bike
At the time of this writing, I have been following Carol’s Intense protocol for about eight weeks, during which I have logged 21 rides, and can confirm that I barely sweat during these workouts.
While testing the Carol, I have become very familiar with the Peak Power metric, and strive to hit a score of at least 500, which is pretty challenging for me. When using the bike with regular athletic sneakers, my all-time best Peak Power score was 516, which put me in the top 41% of my peer group. After trying it with cycling shoes, I was stoked to get a score of 534, which put me in the top 35% of my peer group.
It’s fun trying to beat your Peak Power score. If you want an advantage, use cycling shoes, which help you ride faster and more efficiently, making it easier to pull, not just push, on the pedals.
In the Rides tab, it shows that I didn’t reach a Peak Power score above 500 until my thirteenth ride. I typically score in the high 400s on my first sprint of an Intense ride, and occasionally exceed 500 on my second effort. The graphs show that I typically peak about four seconds into a sprint before my power gradually declines.
Since Carol constantly adjusts your resistance level based on your performance, the sprints don’t get easier over time; they are always hard. During those 20 seconds, I close my eyes and pedal as hard as possible, waiting for the beat of the music to slow down indicating the sprint is done.
As a fitness enthusiast, I find it a bit difficult to drop the “more is more” mindset, but I can’t deny the benefits of sprints on the Carol. In those 20 seconds, my heart rate typically shoots up to around 165bpm, and has gone as high as 180bpm.
Carol cofounder and chief product officer Ulrich Dempfle says to monitor your progress based on your Octane Score. This metric is similar to VO2 Max, or the maximum amount of oxygen you can utilize during intense exercise, an indicator of cardiovascular fitness. He says that if you see a 10% improvement in your Octane Score, for instance, you will also see an improvement in your VO2 Max. In my Dashboard, it says my Octane Score has increased 29% since using the bike.
Overall, Carol’s AI-guided workouts feel very futuristic, and I love that it offers a different experience than most of the competition. On the other hand, doing the same workout day after day can get a bit monotonous and boring.
To spice it up, I have also tried the Fat Burn and Free Ride workouts. Fat Burn workouts alternate 8-second sprints with 12-second recovery periods, and will definitely make you sweat. During Free Rides, you manually control your resistance with buttons and a slider on the screen.
Ultimately, I prefer the Peloton Bike+ and SoulCycle At-Home Bike, which bring the experience of a group fitness class to the comfort of my home, even if their workouts are longer. These bikes offer a lot more variety, including workouts on and off the machine along with lots of instructors and class styles, so you’re less likely to get bored.
Work Out Smarter, Not Harder
The Carol Bike is all about efficiency, as its REHIT workouts promise to deliver the health benefits of other exercise modalities in a fraction of the time. Its AI assesses your abilities, then personalizes your resistance during short sprints designed to quickly deplete the glycogen stored in your thigh muscles. You work out at your maximum intensity, but only for 40 seconds, and you can monitor your progress over time and see how you stack up against your peers. The bike itself is nicely built and gives you the option to wear cycling shoes or regular athletic sneakers, and offers several built-in safety features. It’s a solid option for those who are short on time and want a quick way to squeeze in their cardio without getting sweaty.
That said, at $2,395 (plus $144 per year for a subscription), it’s one of the priciest smart stationary bikes out there, and it lacks many of the features you’ll find on competing machines, such as trainer-led classes, popular music, and workouts off the bike. The Peloton Bike+ and SoulCycle At-Home Bike offer more variety, including strength training and yoga workouts to supplement cycling sessions, though their monthly membership fees are more expensive. On the hardware side, the Carol has a much smaller screen than competitors in its price range, and no built-in storage, even for a water bottle.
If you like the idea of the Carol Bike, meanwhile, it’s also worth checking out the Bowflex VeloCore ($1,699, plus $19.99 per month for a membership). It too offers AI-powered workouts that adapt to you as your fitness level improves, along with trainer-led classes, a larger screen, Netflix integration, and a unique leaning feature that works your core and arms.
