CAROL REHIT Bike Review [After Two Years of Use]

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 workout is only 8 minutes and 40 seconds long, including two 20-second sprints.

Despite that short workout duration, CAROL provides the same benefits as much longer workouts, and studies have confirmed that riding CAROL three times per week (for a total of 26 minutes of exercise) yields 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’ve had the CAROL bike for over two years, and in this review, I explain how effective the bike has been for improving my cardiovascular fitness and related metrics.

To see what exercising with CAROL looks like in action, check out a video (below) of one of my training sessions.

HIIT vs. REHIT Training

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 7-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 (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 (storage) 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 workout session lasts only eight minutes and 40 seconds, including warmup, recovery and cooldown. In other words, you have to exert yourself for only 40 seconds per ride (two 20-second sprints), paired with eight minutes of casual bike riding. Anyone can do that, regardless of their level of fitness!

The CAROL REHIT Bike

So what is CAROL?

CAROL 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 a 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 rapidly empty the glycogen stores in your thigh muscles through two 20-second 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 Does the CAROL Exercise Bike Work?

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.

What 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.

CAROL 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 2024, 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 from 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 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 37.8 to 48.5 (a 28% improvement) since my first CAROL ride.

Workouts/Training Modes

CAROL offers the following seven workout modes you can choose from, including a free ride mode.

• Intense
• Energizer
• Fat Burn 30
• Fat Burn 60
• Constant Power
• Continuous Ramp Up
• Free Ride (including support for Peloton workouts and Apple Fitness+)

Intense is the primary program and the one I use most often. It consists of two 20-second sprints, plus a two-minute warmup, a three-minute recovery, and a three-minute cooldown. So, my total time investment is eight minutes and 40 seconds for each indoor cycling workout.

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 two 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.

As you can see in the image above, the two 20-second sprints trigger a cascade of chemical reactions in your body that result in the creation of new mitochondria (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.

Energizer is a shortened version of Intense that consists of two 10-second sprints. I choose Energizer when I don’t feel like “killing” myself, or on days when I feel sore and I already worked out (CrossFit).

Fat Burn 30 and Fat Burn 60 are both excellent workouts if your fitness goals include burning fat and/or losing weight. These two workouts consist of a two-minute warmup, 30 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 — but effective, if burning fat and calories are your primary goal.

In Constant Power mode, CAROL will adjust the resistance automatically to maintain the chosen power.

Continuous Ramp Up is a testing protocol that you can use to measure your VO2max. During the test, CAROL will increase the resistance by three watts every three seconds until you fatigue.

Free Ride, as the name implies, allows you to use the bike like a traditional ergometer. You can freely set the resistance and other parameters and ride for as long as you like. If you have a Peloton membership, you can use this mode to participate in traditional Peloton group workouts.

You can also use the Free Ride mode to participate in Apple Fitness+ sessions.

CAROL Hardware 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 5-10 business days. I got my bike wrapped in a large cardboard box.

CAROL features a commercial-grade stainless steel frame and a heavy flywheel. The weight is necessary for stability reasons. The good news is that 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.

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.

If you have fewer than five riders in your household, I recommend using a strong password to protect your CAROL account and storing it in a password manager, such as 1Password*. The reason why I recommend using a randomly-generated password is because the CAROL app allows you to remained signed into the bike without having to enter your password every time you want to ride the bike.

However, if you have more than five riders in your household or if you use CAROL in a commercial setting, I recommend picking a strong password that’s easy to type because the bike can’t remember more than eight accounts at a time.

To learn how I create strong passwords that are easy to remember, check out this article.

Competing With Others

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.

How Much Is CAROL?

For consumers, CAROL costs $2,395 plus a $12 monthly subscription fee per household. Corporate users (such as gyms) also pay $2,395, but they’re required to purchase additional licenses for their end-users.

If you want to give CAROL a try, you can use discount code MK100 to get $100 off the purchase price.

Check Price*

It’s worth noting that CAROL offers 0% APR financing for 12 months. That means you’d pay $199.58 per month for 12 months to own the CAROL bike.

Additionally, CAROL offers various bundles that come with accessories, such as a chest strap, tablet holder, floor mat, bottle holder and an extended warranty.

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.

1. Make sure the bike seat and handlebar are adjusted properly for your height.
2. Use biking shoes.
3. 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.

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 longer.

Here’s a side-by-side comparison of the different types of workout each of these platforms offers:

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).

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. 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.

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 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.

Technical Tidbits

CAROL’s touchscreen is an Acer tablet running the Android operating system. That means CAROL’s software is simply an Android app that communicates with the bike’s sensors.

Considering that we’re an all-Apple household, CAROL is the first (and probably last) Android device in our home.

Since I don’t necessarily trust Android (or any of the bloatware that might run on it), I quarantined the bike on its own VLAN so that it doesn’t have direct access to any of the other devices that are connected to my home network.

I know that’s probably overkill, but since I have the necessary networking infrastructure, I decided to go down the rabbit hole.

Frequently Asked Questions

CAROL Review Summary

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. (They’ll even pay for the return shipping to send it back.)

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!