CAROL is a stationary reduced exertion high-intensity interval training (REHIT) bike that can improve your cardiovascular and cardiorespiratory fitness using artificial intelligence. What makes CAROL unique is that its signature workouts require less than 10 minutes of your time. In fact, you can complete one of CAROL’s most effective REHIT workouts in only five minutes.
Despite that short workout duration, the CAROL bike provides the same benefits as much longer workouts. For example, one study found that riding CAROL three times per week (for a total of 26 minutes of exercise) yielded the following health benefits:
- Improved cardiorespiratory fitness by 12% as measured by VO2max.
- Increased HDL cholesterol by 6%.
- Reduced triglycerides by 10%.
- Reduced blood pressure by 5%.
- Reduced blood sugar by 2%.
I had the original CAROL bike since 2019 and upgraded to the CAROL Bike 2.0 in late 2022. In this review, I explain how effective the bike has been for improving my cardiovascular fitness and related metrics. I’ll also cover the differences between the original CAROL and the new CAROL Bike 2.0.
CAROL Bike 2.0
Summary
CAROL is a stationary reduced exertion high-intensity interval training (REHIT) bike that can improve your cardiovascular and cardiorespiratory fitness using artificial intelligence.
What makes CAROL unique is that its signature workouts require less than ten minutes of your time. In fact, you can complete one of CAROL’s most effective REHIT workouts in only five minutes.
If you want to give CAROL a try, make sure to use discount code MK250 to get $250 off your purchase.
How REHIT is Different from HIIT Training
Before we get into my review of the CAROL Bike 2.0, let’s briefly talk about the difference between high-intensity interval training (HIIT) and the reduced exertion high-intensity interval training (REHIT) concept the CAROL bike is based on.
High intensity interval workouts (HIIT) have become popular in recent years because they’re an incredibly effective way to improve your cardiovascular health and fitness with relatively little time investment.
Most HIIT workouts take less than 30 minutes, and some are much shorter than that. One of the most popular HIIT workouts is “the Seven-Minute Workout,” which, as the name implies, takes only seven minutes to complete.
Another popular type of HIIT workout is called Tabata, which only takes four minutes per round.
While you can do high-intensity workouts with all different kinds of equipment or with no equipment at all — think bodyweight exercises like mountain climbers and jumping jacks — some people prefer HIIT bike workouts to get their heart rate up and their blood flowing.
HIIT workouts on stationary bikes have the benefit that you can do them pretty much any time and from the comfort of your home, without requiring a lot of space for exercise equipment.
Intensity is what makes a HIIT session so effective, but it’s also the reason why many people don’t stick to this type of exercise routine. It’s no secret that HIIT requires a degree of willpower and mental toughness that not everyone brings to the table.
If HIIT is high intensity, REHIT is about maximum intensity. And that’s what CAROL offers — a super high intensity workout delivered in an incredibly short amount of time.
That’s why this type of exercise is called reduced exertion high-intensity interval training.
“In our labs, we see insulin sensitivity improve by 25-28% and fitness by 15% within 6 weeks. The CAROL stationary bike reproduces these results in the real world. The research programmed into CAROL is benefiting the public, and that’s ultimately what this science is about.”
Dr. Niels Vollaard
That’s one of the reasons why I like CAROL: each REHIT workout session lasts just 20 to 40 seconds, plus a few extra minutes for warmup, recovery and cool-down.
In other words, you have to exert yourself for only 20 to 40 seconds per ride, in the form of two 10-second or 20-second sprints (depending on the workout you choose). Those sprints are paired with four to eight minutes of casual bike riding.
CAROL Bike Review
Pros
- Minimal time investment.
- Scientifically-proven benefits.
- ISO certified for the most accurate commercial-grade ergometer.
- CAROL dynamically adapts to your progress, so you never plateau.
Cons
- Initial investment.
- Internet connection required.
The CAROL Bike has been around for a while, but it used to be called High Octane Ride. The idea behind CAROL is to take high-intensity interval training to the next level.
CAROL stands for Cardiovascular Optimization Logic, which is an AI-powered bike ergometer that the company says provides double the health and fitness gains of traditional moderate intensity exercise in 20% of the time.
Based on one of the studies involving CAROL, scientists concluded that the bike can make you 132% fitter with significantly less time investment.
That 132% figure is based on a METS Z-score, which is a combined score of various health markers that is often used in medical studies. In one study, CAROL users’ METS Z-score was 132% better than non-users at similar fitness levels.
The “less time” claim is based on the reduced time it takes to complete one of CAROL’s signature workout (the REHIT Intense: 2×20-second sprints workout) on the bike: just eight minutes and 40 seconds per day, three days per week, as opposed to jogging for 30-45 minutes five times per week.
The goal of CAROL’s REHIT protocol is to kick your nervous system into “fight or flight” mode and to rapidly empty the glycogen stores in your thigh muscles through two sprints at super-high intensity.
Scientists call this method reduced-exertion high-intensity interval training, or REHIT, because your perceived exertion is lower than what you experience during traditional HIIT workouts. That makes sticking with REHIT easier than other types of exercises.
How CAROL Works
CAROL’s signature workout program includes two maximum intensity 20-second sprints that shock your body into a fight-or-flight response and increase your body’s energy demands by 10,000% as compared to your resting baseline.
This response causes your muscle tissue to release glycogen — the storage mechanism of glucose (sugar) — to meet your body’s anticipated energy demands. As your body rapidly depletes 25-30% of its muscle glycogen reserves, it activates metabolic pathways, including AMP-activated protein kinase (AMPk) and Pparg coactivator 1 alpha (PGC-1a) that tell your body to become fitter and stronger.
REHIT causes a significant level of glycogen depletion very quickly compared to traditional exercise. You’d need to run for 30-45 minutes to achieve the same level of glycogen depletion by running.
On a cellular level, these pathways cause mitochondrial biogenesis. In other words, your body develops more mitochondrial mass, thus increasing its ability to make energy. That’s what makes you stronger and fitter!
CAROL also has significant advantages over traditional types of workouts as far as calorie burn is concerned. That’s because the second sprint on CAROL tricks your nervous system into thinking that there might be another sprint coming.
As a result, your sympathetic nervous system and the related metabolic pathways remain up-regulated for a few more hours, leading to what’s called an “afterburn effect.” In other words, you actively keep burning extra calories even after completing the bike workout.
It’s worth noting that CAROL uses artificial intelligence to personalize your workouts based on your physiology and ability. Think of the bike as your personal trainer, or as an exclusive indoor cycling studio that gives you a challenging cardio workout.
During the initial setup of your workout profile, the bike asks you for personal information, including your age, weight and height. Based on those biometrics, 50 years of empirical evidence around the REHIT method, and biometric data from onboard sensors, CAROL constantly assesses your performance and makes continuous adjustments using 1,000 resistance settings.
In other words, every time you mount the bike, you get a personalized workout that improves your health and fitness. The CAROL bike also dynamically adjusts the resistance level of the flywheel during the ride (based on your power input and data from previous rides) to make the sprints as effective as possible. That’s something you cannot emulate on regular exercise or spinning bikes.
The other nuance that sets CAROL apart from other ergometers is that the bike can replicate exact lab protocols in the real world, such as the ones used in a recently published study on the benefits of REHIT.
Dr. Niels Vollaard, a lecturer in health and exercise scientist at the Faculty of Health Sciences and Sport, said:
CAROL is the only system that accurately replicates my research outside of the labs. This scientific exercise cannot be done on a regular bike because of the supra-maximal resistance required that also needs to be tailored to each person’s physiology.
Dr. Niels Vollaard
As a result, exercising on this bike for less than nine minutes a day, three times per week, is more effective than going for a 30-minute run five times per week.
Workout Modes
The CAROL Bike offers the several workout modes in five distinct categories:
- Reduced Exertion HIIT (REHIT): Ultra short and highly-effective workouts that take between five and nine minutes to complete and revolve around a low number of 10 to 20-second sprints.
- Fat Burn: A series of highly-effective workouts to burn fat and lose weight (when combined with a proper weight loss diet) consisting of 30 to 60 eight-second sprints each.
- Free Ride: Great for training in your AI-personalized power and heart rate zones, if you’re using third-party cycling apps, or if you would like to participate in Peloton classes.
- Custom Ride: Custom sprint workouts of varying lengths based on research by Dr. John Babraj, Dr. Martin Gibala and Dr. Edward Coyle.
- Fitness Test: A series of standardized tests to measure Functional Threshold Power (FTP) and Maximum Aerobic Power (MAP).
Without going into the nitty-gritty of every CAROL workout, let me give you a brief overview of the various workout types you can do on the bike.
CAROL REHIT Workouts
As you can see in the image above (left), the two 20-second sprints trigger a cascade of chemical reactions in your body that result in the creation of new mitochondria (which are your cells’ power plants). During the first 10 seconds of the first sprint, your body’s energy demands increase dramatically, causing the depletion of glycogen stores. During the second half of the first sprint, AMPk and PCG-1a get released.
During the second sprint, these signaling molecules get activated in response to the stress, prompting your body to adapt and get fitter.
I’d argue that having the ability to improve your cardiorespiratory fitness in the shortest amount of time possible is the reason why most people (including myself) purchase CAROL. There is no other method I’m aware of — and certainly no other exercise bike — that offers the same benefits as CAROL without requiring more time investment.
CAROL recommends that you work out three times per week if you want to improve your performance, health and fitness. To maintain your fitness level (rather than improve it), you can reduce your workouts to twice per week.
Given how often I do CrossFit, I don’t always get to work out three times a week on CAROL. It’s not an issue of finding the time, but rather of giving my legs enough rest to recover. We’ll see how that approach impacts my octane score in the long run. For now, I think that my CrossFit training more than compensates for the missed biking sessions.
On days when I don’t feel like pushing myself to my limits, I often opt for shorter (10-second or 15-second) sprints. Those workouts are much less intense and offer slightly less pronounced health benefits. However, I’d argue that doing the 2×15-seconds sprint three times a week is more effective for improving cardiorespiratory fitness and insulin sensitivity than jogging three times a week.
But even if the benefits were equal, each ride on the CAROL Bike takes significantly less time than other workout regimens.
Note that in June 2023, CAROL added two new “experimental” REHIT workouts: the REHIT Energizer (which has three 10-second sprints) and the REHIT Intermediate (which has three 15-second sprints). CAROL says that these workouts are designed to provide more flexibility to riders who find the 2×20 sprint structure either too easy or too difficult.
CAROL Fat Burn Workouts
The Fat Burn programs consist of a two-minute warmup, followed by 30, 45 or 60 eight-second sprints with 12 seconds rest in between each, and a three-minute cooldown.
I’ve done a few of these rides and they’re absolutely brutal. They’re also effective, if burning fat and calories are your primary goals.
Based on my last full-body scan, I have approximately 8% body fat. As a result, I’m not concerned about burning fat. However, I’ve been using the Fat Burn workouts in combination with intermittent hypoxic training to improve my body’s oxygen-carrying capacity.
If you have a couple of extra pounds to lose, I highly recommend including three Fat Burn rides in your weekly exercise routine. Combined with a healthy diet that supports weight loss, you’ll dramatically increase your chances of shedding those extra pounds (while also improving your metabolic fitness).
Free Ride
Free rides are good for when you’d like to retain full control over your ride intensity. Free rides are also the best choice for when you want to use third-party fitness and entertainment apps, including Apple Fitness, Peloton, Digital, Swift, YouTube, Netflix and more.
In case you’re wondering, you are able to download these (and other) third-party fitness and entertainment apps to CAROL’s Android-powered tablet.
Custom Ride
The CAROL team designed custom rides based on the research of scientists such as Dr. John Babraj, Dr. Martin Gibala and Dr. Edward Coyle. Two of those three custom rides are significantly more challenging than CAROL’s signature ride, but also deliver more health benefits.
Fitness Test
If you want to assess your fitness based on standardized tests used in sports medicine and research, you can use one of the three fitness test protocols. I once completed a ramp up test on an assault bike wearing an oxygen mask to determine my VO2max and basal metabolic rate (BMR).
Delivery and Setup
CAROL bikes are shipped from one of two warehouses — depending on whether you live near the east coast or the west coast — via a parcel service and arrive within 7-10 business days. I got my bike wrapped in a large cardboard box.
Assembling CAROL usually takes less than 30 minutes. I put my bike together in the garage and then wheeled it into my office.
Once I had found a good spot for my bike, I made sure it was even by manipulating the four adjustment knobs on the underside of the frame.
I have hardwood flooring in my office and was afraid my sweat might damage it; then I remembered that using CAROL doesn’t make you sweat (unless you do the fat loss routines or use the bike for free riding to build up endurance).
If so, you might want to cover your floor with a towel and rubber mat. The company also sells a special floor mat that looks similar to a traditional yoga mat you can use. Ever since I started experimenting with CAROL’s Fat Burn programs, I added the floor mat to my home gym to protect the floor from sweat stains.
Hardware and Accessories
The CAROL Bike features a commercial-grade stainless steel frame and a heavy flywheel. The weight is necessary for stability reasons.
Both the seat and handlebar are fully adjustable to ensure a comfortable ride. So regardless of how tall you are, you should find a comfortable riding position that allows for maximum power transfer.
To secure your feet to the pedals, the bike comes with (removable) straps. Alternatively, the CAROL Bike also includes a pair of cleats in case you want to use professional cycling shoes. I used to ride with tennis shoes for several years but I decided to upgrade to cycling shoes to improve my power input.
CAROL also ships with an adjustable phone holder in case you want to listen to your own music or keep an eye on important notification during longer rides.
Heart Rate Monitor
With the introduction of the CAROL Bike 2.0, the company removed the heart rate monitors that used to be integrated into the handlebars. Instead, the bike now comes with a chest strap heart rate monitor.
I appreciate that upgrade because chest straps are inherently more accurate than handlebar heart rate monitors. The other advantage of a chest strap is that the bike continues to receive heart rate information, even if you take your hands off the handlebar (e.g., during warmup, recovery or cooldown periods).
The only downside of chest straps is the risk of signal interference if you have multiple bikes and multiple chest straps (e.g., in a gym setting). I only have a single bike, so signal interference isn’t a concern.
Software and App
When you first power up the bike, you’ll probably notice that CAROL’s software is just an app running on an Android-powered tablet. That makes using and interacting with the CAROL app straightforward, unless you’ve never used a smartphone or tablet before.
Before you can start your first bike ride, you have to create a CAROL rider account. You can later access this account via web or the smartphone app (available for both Android and iOS devices).
CAROL Leaderboards
Every time you finish a workout on CAROL, you can save the ride and see how you compare to every other CAROL user worldwide on a weekly leaderboard.
I enjoy having this community connection and think it makes for a fun workout. Instead of just competing against my personal best performance (which can become tedious for even the most dedicated riders), it’s fun to essentially play an ongoing fitness game against other great athletes.
Riding With CAROL
Riding with CAROL is pretty straightforward if you follow the on-screen instructions for the workout you choose.
Before starting a workout, you can pick music or spoken instructions (this varies depending on the type of workout). The speakers that are built into CAROL’s screen are powerful enough if you’re in a quiet environment, but you might want to use headphones if you’re in a gym setting or a noisy environment.
Once you’ve completed the workout, you’ll see your score and an indication of whether you’ve improved upon your previous scores. CAROL also asks you how you feel after the workout, and if you’d like to save your score or discard it.
Cost (Is CAROL Worth It?)
For consumers, CAROL costs $2,595 plus a $15 monthly subscription fee per household. Corporate users (such as gyms) also pay $2,595, but they’re required to purchase additional licenses for their end-users.
I realize that’s more than what simple air bikes cost. However, it’s important to understand you cannot replicate CAROL’s protocol and the associated benefits with other exercise bikes without spending more time on the bike. In other words, what you pay for the CAROL Bike enables you to get the maximum benefits in the shortest amount of time.
If you want to give it a try, you can use my Carol Bike discount code MICHAEL to get $100 off the purchase price (or up to $250 during promotional periods).
It’s worth noting that CAROL offers 0% APR financing for 12 months. That means you’d pay $216.25 per month for 12 months to own the CAROL bike.
Carol also offers a 100-day in-home trial period, so you can return the bike if you don’t like it. The company even pays for the return shipping if you live in the continental U.S., the UK or Canada.
Finally, CAROL offers an extended warranty that covers the bike for two years for $150. And if you’re not willing or capable of assembling CAROL yourself, you can purchase an assembly serving for $199.
What’s New in CAROL 2.0
I’ve had the original CAROL bike since 2019 and was thrilled to learn that the company was launching a new version of the bike near the end of 2022.
The most striking different between the old and new version is the frame.
The frame of the CAROL bike 2.0 looks modern and slick, and is more comfortable to ride on, thanks to some smart changes in the angles of the various frame components and the new handlebar design, which allows you to lean more or less forward.
Aside from the new frame, CAROL 2.0 features the following updates:
- Improved handlebar with multiple grip options.
- A new touchscreen tablet that’s more powerful and responsive and has a better screen resolution.
- Instead of the heart rate monitor (HRM) built into the handlebar, CAROL 2.0 ships with a separate chest strap HRM for improved reliability and more accurate data. That means you can now take your hands off the handlebar without impacting your HRM signal.
- Two bottle holders built into the handlebar.
What hasn’t changed is the premium build-quality of all materials, the ability to swap out the seat, and to choose between using straps or cycling shoes.
Overall, I’ve enjoyed riding on the new CAROL Bike 2.0 and I appreciate how slick it looks compared to the previous version.
How Does the CAROL Bike Compare to Alternatives?
One of the most frequently asked questions I hear from people who are interested in purchasing the CAROL Bike is how the bike compares to other stationary bikes and exercise programs. Below is a brief comparison between CAROL and other training modalities I’ve using or that I’ve considered.
CAROL Bike vs. Jogging
To give you an idea of how effective cycling on CAROL is compared to jogging, below are some numbers from a study that was published in February 2019, in the International Journal of Environmental Research and Public Health.
The figures shown indicate how much better CAROL users’ scores and improvements were when compared to those of the joggers in the study:
- Risk of developing a metabolic disease: 62% better.
- Cardiovascular fitness: 78% better.
- HDL cholesterol: 71% better.
- Triglycerides: 78% better
- Blood pressure: 196% better.
- Blood sugar: 48% better.
In another study, published in 2011, scientists concluded that REHIT increased insulin sensitivity in male study participants by 28%, which lowers the risk of developing Type 2 diabetes.
I used to follow a ketogenic paleo diet before adopting a predominantly animal-based diet, and I do CrossFit and kickboxing four or five times per week. As a result, my bloodwork is already in the ideal range, and I have no fear of developing Type 2 diabetes or other lifestyle-related diseases.
However, my measurable goal with CAROL is to improve my maximum power — expressed in watts — and my octane score. The octane score is CAROL’s proprietary measure of power output on the bike in relation to your heartbeat, and you can correlate that to your VO2max.
VO₂ max is the maximum rate of oxygen consumption measured during incremental exercise; that is, exercise of increasing intensity. The name is derived from three abbreviations: “V” for volume, “O₂” for oxygen, and “max” for maximum.
Wikipedia
While octane score and VO2max aren’t exactly the same, they improve in a similar fashion.
For reference, my Octane has improved from 37 to 51 (a 37% improvement) since my first CAROL ride.
CAROL vs. Other HIIT Bike Workouts
So how is CAROL different from other HIIT bike workouts, such as spin classes?
The primary difference between regular HIIT workouts or spin classes and CAROL is the latter’s ability to rapidly empty the glycogen reserves in your thigh muscles. That’s what makes the bike so effective for increasing your aerobic capacity.
CAROL can do that because of how it utilizes science, technology and artificial intelligence. In other words, CAROL can personalize each workout based on your physical abilities at the time of that particular workout.
As a result, CAROL can improve your cardiovascular conditioning with only 40 seconds of intense work. No other workout, that I know of, can do that.
Technically, CAROL outperforms other HIIT bikes because of the way the bike automatically adjusts the resistance of the flywheel without requiring any user intervention. For example, every sprint starts with no resistance whatsoever, forcing you to peddle as fast as the muscles in your legs can twitch.
After a few seconds of high-speed peddling, the bike dynamically adjusts the resistance to make the sprint harder as you fatigue.
Based on your power input over time, the bike adjusts the resistance settings for the next sprint with the goal of achieving maximum glycogen depletion. Unless you have a computer controlling the bike’s resistance, you cannot replicate this protocol.
CAROL vs. Spinning
Before becoming a CrossFitter, I tried various types of exercises, including spinning. From a purely physical perspective, I enjoy training on a stationary bike. Spinning was tough, but I burned a lot of calories doing it and the group setting motivated me to push even harder.
The downside of spinning is that it’s monotonous. After a while, I got bored and stopped doing it.
And another disadvantage of spinning is that it takes at least an hour — and even more if you add the time required to travel to the gym and back. If you plan on using a spin bike at home, you really have to be self-motivated and disciplined.
Innovations like the Peloton bike have tried to offer solutions to these problems by introducing connected bikes that allow you to work out from the comfort of your home while still interacting with and competing against a community of riders.
While I think that’s a great idea, you still have to commit a significant amount of time for the workouts to be effective.
In contrast, CAROL requires only a few minutes of your time. And while you can’t compete with other riders live (the rides you take with CAROL are simply too short for that), you can compare yourself to others via CAROL’s Global Leaderboard.
CAROL vs. Peloton Bike+ vs. NordicTrack s22i
I’ve had many readers ask about the differences between CAROL and the Peloton Bike+ and NordicTrack s22i. While you can get an excellent workout on either of these bikes, there are some important differences you should keep in mind.
The most striking difference between CAROL and any other spinning or exercise bike is the duration of the workout. Neither Peloton nor NordicTrack offer REHIT protocols that have been shown in peer-reviewed studies to provide the same benefits as much longer workouts.
In other words, if you want to achieve the same cardiovascular and metabolic improvements on a Peloton or NordicTrack bike, you have to work out more frequently and for longer.
Here’s a side-by-side comparison of the different types of workout each of these platforms offers:
| CAROL | Peloton Bike+ | NordicTrack s22i | |
|---|---|---|---|
| Live streaming fitness classes: | Yes | Yes | Yes |
| Live streaming gaming and racing content: | Coming soon | No | No |
| Moderate intensity continuous training (MICT): | Yes | Yes | Yes |
| Reduced exertion high intensity training (REHIT): | Yes | No | No |
What’s important is that in order to participate in live streaming fitness classes with CAROL, you need a separate membership, such as Apple Fitness+, Peloton Digital or similar, because it’s not included in CAROL’s monthly membership.
Hardware and Software Comparison
Besides the workouts, there are also differences in the hardware and software between these three platforms (as you can see in the comparison table below).
| CAROL | Peloton Bike+ | NordicTrack s22i | |
|---|---|---|---|
| EN ISO 20957 certification | Class SA | Class H | Class H |
| Silent belt drive | Yes | Yes | Yes |
| Frictionless magnetic brake | Yes | Yes | Yes |
| Motorized brake system | Yes | Yes | Yes |
| Freewheel safety clutch | Yes | No | No |
| Cycling shoes optional | Yes | No | No |
| Hand heart rate monitor | Yes | No | No |
| Touchscreen monitor | Yes | Yes | Yes |
As you can see, only CAROL features a commercial-grade frame that can withstand abuse and will likely last a lifetime.
The other benefit of CAROL is that you can ride it with or without cycling shoes. I’ve used tennis shoes for the past few years because I didn’t want to invest in cycling shoes just squeeze a few extra watts out of each ride (before eventually pulling the trigger). Of course, CAROL’s pedals are fully compatible with biking shoes and cleats. Specifically, CAROL supports the Shimano mountain bike SPD system.
The only downside of CAROL is the touchscreen, which is relatively small in comparison to that of the Peloton or NordicTrack bikes. However, keep in mind that CAROL’s signature workout takes less than nine minutes to complete. Realistically, there isn’t nearly enough time (between the sprints) to watch a show or interact with the touchscreen in any meaningful way.
In other words, unless you want to use CAROL for traditional spinning workouts, you won’t miss the larger screen at all.
From a software perspective, it’s worth noting that CAROL continuously personalizes and optimizes each workout based on your previous performance and your current fitness level. Additionally, CAROL has safety algorithms built in to automatically stop the ride if it detects that your heart rate goes up too high (compared to your baseline).
Neither Peloton nor NordicTrack offer those two important features.
How Much Is CAROL in Comparison to Peleton and NordicTrack?
From a pricing perspective, CAROL is right in the middle between Peloton and NordicTrack but offers a less-expensive monthly membership.
| CAROL | Peloton Bike+ | NordicTrack s22i | |
|---|---|---|---|
| Initial purchase price | $2,395 | $2,495 | $1,999 |
| Shipping | Free (UK and U.S. only) | Free | Free |
| Monthly membership | $12.00 | $39.00 | $39.00 |
The pricing above is for customers in the United States and CAROL’s free shipping offer only applies to customers in the UK and in the U.S. (with the exception of Alaska and Hawaii).
All three brands offer interest-free financing.
Who Should Use CAROL?
I’d argue that CAROL is a good fit for anyone who wants to improve their fitness and health without having to spend countless hours in the gym. In fact, it doesn’t matter what your current fitness level is, because the bike adjusts to your physical abilities.
This indoor bike is also an excellent option for those who have had a difficult time sticking to an exercise routine. Moreover, CAROL is particularly useful for those who have a demanding job or family life that leaves little time for working out.
CAROL is also great for serious athletes who seek ways to complement their primary workout routine and improve their cardiovascular strength and capacity.
I do CrossFit and Kickboxing multiple times each week, but I also enjoy using CAROL on the days when I don’t have time to go to the gym. Additionally, I use CAROL to improve my octane score and VO2max, which helps me on those WODs that focus on metabolic conditioning (metcon).
Paired with low-heart-rate workouts, I’ve noticed that my workout performance and endurance during metcon exercises has dramatically improved.
Tips to Improve Your CAROL Score
Besides improving your conditioning by working out on CAROL at least three times per week, there are a couple of tricks you can use to get the best possible score.
- Make sure the bike seat and handlebar are adjusted properly for your height.
- Use biking shoes.
- Wait until the end of the three-second countdown before going all-in.
One of the most important factors in biking efficiently is a properly adjusted seat and handlebar. If you sit too high or too low, you won’t be able to transfer power from your legs to the bike as efficiently. So, make sure to adjust the bike according to the manual and your height.
I’ve also noticed that the pedal straps tend to loosen over time. That’s just because of the force applied when you’re peddling at full speed. For best results, I recommend getting a pair of biking shoes that you can attach firmly to each pedal.
That said, I still bike with my tennis shoes because I don’t care if I lose a few watts.
Last but not least, I recommend holding off on going all-in when the screen turns red and the three-second countdown starts. Why? Because I’ve noticed that I reach peak speed and intensity even before the 20-second workout starts.
In other words, when the actual workout starts, my performance is already declining. So, I usually wait until one second before the workout starts.
Frequently Asked Questions
It’s the principle of quality over quantity. CAROL Bike workouts do essentially the same thing as longer, more traditional workouts, but they get your body to the outcomes you’re looking for more efficiently. What you have to realize is that normally, you’re not actually in your target fitness zone over the duration of your workout. If you run for 30 minutes, only a portion of that is truly “productive” from an exercise science standpoint.
More specifically, CAROL uses technology and deep learning (AI) to give you just the right levels of resistance at just the right moments based on your particular fitness level, which quickly burns through your glycogen stores (especially in your thighs). This helps yCou burn fat (spurring weight loss) and build muscle.
You can find CAROL bikes in several fitness and biohacking studios in the U.S. and across Europe. I tried the bike at the Paleo f(x) convention in Austin, Texas, and got hooked on the platform. You can use the following link to find a gym in your area that has a CAROL bike to try.
If you don’t have access to either option, CAROL offers a 100-day in-home trial. If you don’t like the bike, you can return it for a full refund. (The company will pay for return shipping if you live in the mainland U.S., Canada or the UK.)
Yes, the eight-minute ride can help you lose weight, thanks to the “afterburn” effect that will make you burn extra calories even after the workout has ended. However, if you have the time to do them, I’d recommend adding one or two Fat Burn rides per week to supercharge your weight loss efforts.
Yes, CAROL can certainly support your training efforts by improving your cardiovascular fitness (VO2max) which will likely also lead to faster times when running, biking or swimming. Using CAROL on a regular basis will also help with reducing muscle fatigue, which means you’ll likely be able to hold your pace for longer.
However, getting better in any sport requires technical skills you need to practice, and swimming, running or riding a bike for hours are such skills. So I wouldn’t use CAROL at the exclusion of running, biking or swimming, but rather to complement your regular training efforts.
I’m not the most gifted handyman, but it took me less than 30 minutes to assemble my CAROL bike. All the instructions are straightforward and easy the follow, with one exception: I didn’t know how to attach the straps to the pedals, because the toe cage assembly wasn’t properly documented and I had never used them before.
While you need an internet connection to sign up for an account, you can use the bike in offline mode. However, the bike wasn’t necessarily designed to remain offline permanently. While your offline rides are cached on the bike, they get uploaded for analysis when the bike reestablishes a connection with the internet. This is when your ride data gets added into your overall rider profile and taken into account by the AI that personalizes your workouts.
The CAROL bike uses artificial intelligence to personalize your workouts based on your unique fitness profile, and this technology is owned by CAROL. In the unlikely event that the company were to go out of business, CAROL would still operate as a manual spinning bike, but the personalization aspect would cease to function (because your ride data is processed in the cloud, not by the bike itself). Specifically, you would still be able to use the “Free Ride” mode and manually adjust the bike’s resistance, time and other parameters.
Yes, you can! The CAROL bike uses standard bike parts. As a result, you can use your own seat and pedals, if you prefer (and if doing so makes the workout more comfortable).
If you do CrossFit, you’re probably familiar with Assault bikes — the least favorite piece of equipment for many CrossFitters. That’s because riding an Assault bike is tough and exhausting.
So you might be wondering if you could simply replicate CAROL’s REHIT workouts on such a bike, considering that they cost less than CAROL.
The important thing about Assault (and similar) bikes is that their resistance goes up in a linear pattern as you increase your energy input. That’s an entirely different concept than what CAROL offers.
CAROL constantly adjusts the resistance based on its sensor data and your physical abilities. As a result, you won’t get the same benefits (based on the same time investment) from traditional air bikes as you will from CAROL.
From a health and fitness perspective, a recumbent exercise bike is basically the same as a traditional bike ergometer. Based on everything I know, I’d say CAROL outperforms recumbent exercise bikes (which focus only on resistance training, and not on intensity training) on every level.
The Q factor of a bicycle is the distance between the pedal attachment points on the crank arms when measured parallel to the bottom bracket axle. CAROL’s Q Factor is 165 mm or 6.5 inches.
Yes, you can use your own chest strap if you don’t like the one included with the bike.
Yes, you can use CAROL in combination with Peloton Digital! Near the end of 2020, CAROL announced compatibility with the Peloton Digital platform. That means you can use the bike not only for its own high-intensity workouts, but also for the more traditional classes (i.e., longer rides) offered by Peloton.
In order to leverage this compatibility, you need a paid subscription to Peloton Digital.
Yes, you can use the Free Ride option of CAROL to participate in Apple Fitness+ classes. However, you need a separate Apple Fitness+ subscription because it’s not included with your CAROL membership.
CAROL Bike 2.0 Review: Summary and Final Thoughts
I used to be a professional sprinter and leveraged HIIT workouts to stay fit after I ended my career (and before starting CrossFit).
One of the reasons why I like CrossFit so much is because it offers a variety of exercises that help me improve my physical fitness, including weight training, circuit training, aerobic exercises and more.
That’s why I wouldn’t consider CAROL — or any other piece of cardio equipment — a replacement for strength training or CrossFit. However, I think CAROL perfectly complements my current exercise regimen. More than that, CAROL helps me to be a better CrossFit athlete by improving my VO2max and aerobic capacity.
But you don’t have to be a CrossFitter to benefit from CAROL. In fact, regardless of what sport you do, you can significantly and easily improve your performance with this REHIT bike.
If you don’t exercise at all yet, then what are you waiting for? Give CAROL a try and get on your path towards better health!
In fact, the company offers a risk-free 100-day trial period. If you’re not happy with the bike for any reason within that period, you can return it for a full refund.
Have you tried CAROL or other HIIT workouts before? If so, let me know about your experience, and whether it helped you achieve your fitness goals, by leaving a comment below!
Michael Kummer is a healthy living enthusiast and CrossFit athlete whose goal is to help people achieve optimal health by bridging the gap between ancestral living and the demands of modern society.
Medical Disclaimer
The information shared on this blog is for educational purposes only, is not a substitute for the advice of medical doctors or registered dieticians (which we are not) and should not be used to prevent, diagnose, or treat any condition. Consult with a physician before starting a fitness regimen, adding supplements to your diet, or making other changes that may affect your medications, treatment plan or overall health. MichaelKummer.com and its owner MK Media Group, LLC are not liable for how you use and implement the information shared here, which is based on the opinions of the authors formed after engaging in personal use and research. We recommend products, services, or programs and are sometimes compensated for doing so as affiliates. Please read our Terms and Conditions for further information, including our privacy policy.
Hi Mike, Thanks for the very thorough review. Have you tried to connect to Zwift on it? Just wondering if it actually interacts so I can join the races.
Unfortunately, I don’t use Zwift and never tried to connect it.
Hi again Michael. love your site.
do they have Heart rate control option?
that is, setting the bike to adjust the intensity to reach a pre-subjugated great rate level I set.
thanks
Yeah, you can set your target HR in free ride mode. See https://carolbike.com/uk/workout-page-free-rides/zone-based-free-ride/
thanks a million for your detailed reply! I’ll make sure to click from here if I ever up buying ❤️
I appreciate it!
Hey Michael,
I am noticing there are some obvious differences between true original Carol and the 2.0 (along with a lower price). This has me concerned that perhaps the second iteration may be not quite the machine as the first. What are your observations, regarding quality differences?
Thanks,
Rich
As far as I can tell, the new bike is just as robust as the previous generation. Both are built like a tank!
Micheal – what are your thoughts on using the Carol REHIIT routine around strength training? Specifically around the glucose / glycogen dump. Is it beneficial to do after a strength training routine? Or on different days than strength training / heavy weight lifting? Or before lifting?
Hey Matt,
I’d do strength and REHIT sessions on different days so you can do each with full glycogen stores. Unless, your goal is to maximize glycogen depletion (e.g., to improve insulin sensitivity).
Cheers,
Michael
Got it. Thanks for the quick reply. Goal right now is building more muscle mass (and in general a lower body fat percentage).
Yeah, I’d do those two movements on different days!
Hi Micheal, thanks for the thorough review. Do you think replicating the workout on an Assault bike might elicit a similar response. In my mind more muscles are being used so a larger glycogen release would be triggered. Would this negate the decrease in resistance the CAROL bike provides when starting each sprint?
Sometimes I think CAROL are over complicating the specifics to make their bike the only product that can achieve their results. Are you sponsored or affiliated with them?
Hey Chris,
you can definitely replicate the same glucose dump (and all other benefits) on an assault bike but not in the same (short) timeframe. CAROL doesn’t do anything better than other types of workouts. I just does it much quicker. That’s the primary value of CAROL.
I do CrossFit and use assault bikes (and other air bikes) a lot but I’ve also been using CAROL for 3+ years. I use CAROL when I need to get my workout done in less than 9 minutes.
Like I mentioned in my review (affiliate disclaimer at the very top), I’m an affiliate and earn commissions if you use my discount code. But that doesn’t change my editorial integrity. I only review stuff I actually use and love :)
I hope that helps!
Cheers,
Michael
great review & blog!
I consider testing the idea by using the gym bike, and trying to immigrate this process to see how it works for me
I suppose that I wouldn’t have the orchids all out in the gym bike. but it would give me the feeling, wouldn’t it? how far would it be as a demonstration?
PS. their 100 day guarantee is amazing. and it even applies internationally (but I’ll pay shipping both ways if returning). anyway, I would rather test first in the gym as above
Hi Jazi,
I recommend trying to find a CAROL show room near you because you can’t replicate (or even get closed to the same experience) on a regular gym bike for all the reasons I’ve explained in https://youtu.be/o4Uyr4yeSSM.
Cheers,
Michael
My bad I missed the video of you doing your routine.
Reason I wanted to see it is, yes, you are benefiting from the weighted wheels momentum.
I mean these are tough for sure, but honestly you’re really cutting down the amount of work by using this kind of device.
Also (i started on a few bikes), and I also developed some leg/sciatic nerve issues from pumping that hard seated. At least that’s my best guess, all that went away when I stopped doing ‘sprints’ on a weighted wheel based bike.
For what it’s worth I first switched from a similar weighed wheel cheater bike as I call them to an all electric resistance bike, which is not bad, there is no momentum… but then I still had leg / sciatic issues from pumping that hard in a seated position…. if you think about it, on a real bike you would NEVER crank that hard seated…
So I really don’t get how people do tabata’s, rehitt, SIT, etc on these things at all, I think you’re kidding yourself compared to actual sprints on a track.
A manual treamill is just a bit safer, and it’s also a little weighted with no momentum so I think compared to sprints outside on an actual track, the manual treadmill is a bit tougher than the real thing.
Anyway that’s what us hockey players do to train, you really can’t cheat or cut corners if you want this to be as effective as the studies show them to be.
Hey Rick!
You can’t compare CAROL with traditional spinning bikes that use a weighted flywheel. CAROL uses AI and magnets to control the resistance, so the weight of the flywheel won’t help you at all. In other words, there is no momentum to leverage.
As far as pumping hard — if you cycle uphill, you’d pump just as hard.
I used to be a professional 100m sprinter (#5 in my age category in Europe at the time) and I can tell you that riding CAROL is no kidding myself. It’s brutal and one of the toughest parts of my exercise routine (and I do CrossFit).
I encourage you to find a show room near you and try it out. I have a feeling, it’d change your mind :)
Cheers,
Michael
Why would you use this instead of a manual treadmill? That’s what I use.
The problem with most / all (?) bikes is momentum. The weighted wheel builds up a lot of energy and you’re not really doing all that much once you reach full speed. There is no cheating on a manual treadmill – or running on a track.
I highly encourage you to try a manual treadmill. It’s MUCH closer to doing actual sprints on a track.
Also – if you are going to reveiw something like this you should really record yourself doing your routine on the actual device you’re reviewing.
Hey Rick,
I used to be a pro sprinter. So I know what sprints on the track feel like. I also do CrossFit and I’m intimately familiar with all kinds of traditional ergometers, including treadmills, air bikes and others. CAROL is tougher than all of them. There is no momentum to leverage.
Cheers,
Michael
I have a barely used CAROL bike if anyone is interested in purchasing one, feel free to email me at [email protected]
Hi Michael,
This sounds great, thanks for the thorough review as always. A few follow up questions:
1. I am getting back into heavy weight strength training and usually need a recovery day (or two ) if this also requires a recovery day it seems I will need more days in the week. Have you found a device to create more days :) or what have you done to schedule around this?
2. According to the research that CAROL is based on, wouldn’t (2) 20 second sprints give similar, albeit not as efficient, results if I decide the investment is too much. It seems like the uniqueness of this method is in the research that has already been done around tricking the brain into fight or flight response. I admit it would be much more convenient to walk to the bike vs to the park.
3. Would this be compatible with a VR ride software as a bonus?
Thanks in advance!
Hi Ruben,
1. I don’t feel like I need recovery days from the bike. In fact, I started using the bike on days where I perform a regular workout. The other day, I rode CAROL right before my CrossFit class without any issues.
2. Good question that I don’t know the answer to. I feel like CAROL’s ability to dynamically adjust the resistance over the course of the two sprints cannot be replicated by sprinting on a flat surface. But it would be cool to see a study comparing the two approaches.
3. If the VR software is available as an Android app, it might work but I don’t think this has been tested yet.
Cheers,
Michael
Have you tried the bike connected to any other bike apps? Their website says it connects to the Peloton app but doesn’t give any more info. Wondering how well/easy it connects and if it would be good for regular spin classes. I’m concerned (like I think many are!) about the usefulness of this expensive bike if this young/relatively unknown company folds and no longer provides a service.
Thanks!
Hi Steve,
I haven’t tried the Peloton app on CAR.O.L yet but considering that CAR.O.L uses Android, I guess the Peloton app is as good on CAR.O.L as it would be on any other Android tablet.
Hi Michael, Great Review! I purchased the CAR.O.L and I ad looking forward to my California shipment to get the Nashville. Thank you for sharing your insights,
Thanks, Jay! I just completed a 60×8 seconds sprint on CAR.O.L and I’m wiped out :)
Thanks for the review! Does Carol bike auto-adjust the resistance when using 3rd party apps like Zwift?
Hey Mitch!
I don’t think so because in third-party mode, you’re basically free riding and can adjust the resistance yourself. Does that make sense?
That said, I never tried Zwift and don’t know for sure how the app works.
Cheers,
Michael
Thanks for the thorough review, Michael!
One thing that I am interested in was the impact this has while using Whoop Strap 3.0. What type of strain does it usually give you for the single “Intense” session or any others that you have tried?
Hey Russel,
WHOOP’s strain is based on the cardiovascular load that you’ll only build when your HR stays up for longer periods. CAR.O.L doesn’t cause your HR to stay up for more than a few seconds. As a result, WHOOP won’t recognize it as (cardiovascular) strain. It’s similar to strength training in that regard. You’ll see the impact on your recovery score but not in your strain score.
Cheers,
Michael
Hi Michael,
I am an extremely unfit grandmother of 6 and need a HIIT option to get fit with minimal time.
Would this bike be suitable – for the 10 or 20 seconds of sprint am I able to just do my very best – I really am unfit.
Thanks
Denise
Hi Denise,
Yes, I’m convinced that CAR.O.L would be a great option for you. Your level of fitness doesn’t really matter as far as the effectiveness of the bike is concerned. You just do your best you can during those sprints and you’ll see results!
Cheers,
Michael
Thorough review, so thank you! You obviously put some effort into this.
I did have one question about the specification of the bike that I did not see addressed: many purchasers of spin bikes want to know the Q-Factor of the bike. This is basically the side to side spacing between the foot pedals It is considered important both for cycling performance, and for chronic knee injury risk in the case of high mileage riding.
( https://en.wikipedia.org/wiki/Q_factor_(bicycles) ).
A typical road bicycle with have a spacing of about 155mm. A mountain bike or street bike with fatter tires might be 175mm. Many stationary bikes, particularly magnetic resistance ones, tend to be even fatter between the pedals due to the extra machinery that they contain. I’ve seen some bikes with spacings as high as 230mm. The Keiser M3i, comes in at 197mm. I’ve not been able to learn what the Q-factor is for the Car.o.l bike.
For short sprint sessions (the Intense mode), I doubt that it matters. But if someone wanted to also use the bike for a traditional spin class, or longer free rides, it might be a consideration.
Hi Craig,
I’m awaiting official confirmation from CAR.O.L but based on my measurements, the bike has a Q factor of 203 mm.
Cheers,
Michael
Hi Craig,
I heard back from CAR.O.L – the bike’s Q Factor is 165mm or 6.5 inches.
Cheers,
Michael
Mike,
Finally a proper detailed review! Yours is defiantly Still the best out there. Thanks ….
Given the COVD19 lockdown situation and ARX isn’t an option at this point… What workout would you use (other than CrossFit) to complement Carol’s fat-burning protocol?
I have been wanting to do weight lifting for strength training but terrified of injuries since I don’t have access to a personal trainer. Do you know any good online programs?
Thanks
Hey Shayme,
I follow the home workout program of our CrossFit box (which includes dumbbells, kettlebells and sandbags).
Before starting CrossFit, I used to do P90X. Beyond that, I don’t know of any online programming from the top of my head.
I’m confident plenty exist, I just haven’t looked into it.
Cheers,
Michael
Hi Michael,
Great through review. I’ve actually tried the bike at a boutique gym when I lived in Nashville along with ARX and it’s fantastic! I’m considering ordering for home use now. The free shipping along with the extra $150 off are enticing. What I would like to know is how often does the bike push updates down? Also, if the company does goes bankrupt does that mean the bike won’t work in any programmed mode? Now that we are living in Covid-19 world that is a major concern for me. Plus, who would work on the bike if technical issues?
Btw, used to live in ATL for a few years…fun city.
Best,
Em
Hi Em,
From what I can see, the bike checks for updates every time you launch the app. Actually updates, I’ve seen a few since I got it last year. I never looked at the changelog, however, so I don’t know what changed.
If the company goes bankrupt, you can continue to use the bike in “free ride” mode. On other words, you can use it much like a traditional spinning bike.
Cheers,
Michael
Hey Michael,
This was a fantastic review. I’ve actually had the opportunity to use the CAROL bike at a small alternative gym in Napa, CA where I used to live. It was fantastic – especially paired with ARX. I now live in Santa Fe, NM and no CAROL or ARX anywhere to be found. Hence I think I’m going to take the plunge and purchase a CAROL to pair with at-home high-intensity resistance band training.
I’d love to see other people review this bike as well – yours is the only one I’ve seen.
Thanks for the feedback Alex, I really appreciate it! Coincidentally, I just got off the bike 10 minutes ago :)
Regarding other reviews, I think Ben Greenfield did one but I don’t recall if it was a video or blog post.
Hi Michael,
great review, just bought the bike using your code :). You seem very fit, can you share what your VO2 Max is? I am trying to figure out how to estimate my VO2 max based on their octane score. I am a bot claustrophobic, do not want to do traditional mask test.
Thanks for the great review!
M.
Thanks, M!
Frankly, I haven’t kept track of my VO2max because it’s not a crucial metric for what I’m doing (CrossFit).
Sorry that I don’t have a better answer for you but I hope you’ll enjoy the bike!
Cheers,
Michael
Michael, Thank you for this very comprehensive review of the CAROL bike :) Best review I’ve see out there by a long shot!!
I’ve read up a lot on REHIT lately, the research is solid and I’m seriously considering this bike.
My only concern is that the company is quite young and I can’t seem to find any actual honest numbers on how many of these AI Software Bikes they’ve sold (500? 1000? 10,000?). If anyone knows an actual number I’d love to find out :)
I’m a little hesitant to spend so much on a technology that is this heavily dependent on Software Program Updates to function. If the company goes out of business the bike loses all it’s technology and functionality and becomes a very expensive clothing rack!! If anyone knows anything about numbers of bikes out there I’d be super interested and it would put me a bit more at ease for buying one!!
Hey Bobby,
I don’t have official sales numbers either. I’ve only been told that the company was doing well. And if the use of my discount code is any indication, I tend to believe that statement :)
Cheers,
Michael
Thanks for your extensive and compelling review, Michael. I’ve been considering the possibility of getting this bike. I’m looking for ways to reduce blood pressure without drugs as mine is starting to creep up there. What does “Blood pressure — 196% better” translate to in real terms for someone whose blood pressure is sometimes reading 143 over 90? Can you point me to a link that indicates the level of improvement I can hope to achieve if I follow the 40-second routine at least three times a week along with a good diet?
Thanks!
Linda
Hi Lisa,
Here is a study that shows some actual numbers in terms of improvement: https://www.acefitness.org/education-and-resources/professional/certified/may-2019/7267/ace-sponsored-research-what-are-the-acute-and-chronic-responses-to-reduced-exertion-high-intensity
I’d definitely pair exercise with a good diet — by good I mean low-carb high-fat and wholesome sources of food. My recommendation would be a paleo/keto diet – see https://michaelkummer.com/keto-paleo/
Thanks for the in depth and honest review, there aren’t too many out there.
Like Laine, it sounds too good to be true, but I want to believe it will help.
I’ve been diswayed from buying because everyone I tell about the bike laughs at me and says its fake news.
Hey AJ,
I got the same reaction when I started paleo…and then keto…and intermittent fasting. Sometimes, people who don’t want to do their own research to confirm if something is legit or not, brush stuff they don’t understand off as fake news. But it’s basic science at the end of the day :)
If you happen to be in the Atlanta area, feel free to drop by the house and give my bike a try.
Cheers,
Mike
I am seriously thinking about getting this bike, but it sounds too good to be true. It’s been 3 months since you posted this blog. How is it going? Are you seeing benefits? I wish there was one of these near me so I could try it but I don’t know of any and I haven’t been able to find a way to search.
Hey Laine,
thanks for stopping by! I still like this bike and I use it as I had planned — on those days where I don’t have time to go CrossFit. I recently wrote an article about the impact of different diets on exercise performance and learned a ton more about how the body uses glycogen.
Everything I’ve learned backs up the claims CAR.O.L makes regarding its benefits. If you’re in the Atlanta area, you can try my bike anytime :)
Cheers,
Michael