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CAROL REHIT Bike Review

Published:
Last Updated: Oct 05, 2021

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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.

The CAROL Bike

Michael Kummer

CAROL - Stationary HIIT Bike
Build Quality
Convenience
Effectiveness
Training Programs
Community

Summary

CAROL is a 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 but it provides the same health benefits as a 45-minute run.

4.6

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

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.

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.

The CAROL Bike uses a commercial-grade steel frame.
The CAROL Bike uses a commercial-grade steel frame.

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 REHIT Bike Review on YouTube
Watch my CAROL Bike Review on YouTube.

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 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 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

Michael's CAROL PR of 1469
The new CAROL app makes it easy to see your progress.

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.

CAROL - Fat Burn 30 Workout
Fat Burn 30.

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

CAROL - Constant Power Mode
Constant Power mode.

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.

CAROL - Continuous Ramp Up
Continuous Ramp Up mode.

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 - Free Ride
Free Ride mode.

CAROL Hardware and Setup

The seat of CAROL is fully adjustable
CAROL comes with a comfortable and adjustable seat, but it can also be replaced with a bike seat of your choice.

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

Michael's Latest Octane Score
My latest Octane Score.

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

CAROL Global Leaderboard.
CAROL Global Leaderboard.

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.

Video: Bryan vs. Michael Compete on CAROL
Video: Bryan and Michael compete on CAROL!

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

Me doing one of the sprints on the CAROL bike.

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

CAROL vs. Other Exercises
CAROL vs. other exercises.

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

CAROL offers the same benefits of other workouts with require much less time.
CAROL offers the same benefits of other workouts with require much less time.

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

CAROL vs. Spinning
CAROL vs. a spin bike.

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:

CAROLPeloton Bike+NordicTrack s22i
Live streaming fitness classesYesYesYes
Live streaming gaming/racing contentComing soonNoNo
Moderate-intensity continuous training (MICT)YesYesYes
Reduced exertion high intensity training (REHIT)YesNoNo
Comparison of workouts available on these three platforms.

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).

CAROLPeloton Bike+NordicTrack s22i
EN ISO 20957 certificationClass SA: professional and commercial useClass H: domestic useClass H: domestic use
Silent belt driveYesYesYes
Frictionless magnetic brakeYesYesYes
Motorized brake systemYesYesYes
Freewheel safety clutchYesNoNo
Cycling shoes optionalYesNoNo
Hand heart rate monitorYesNoNo
Touchscreen monitorYesYesYes
Hardware differences.

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.

CAROLPeloton Bike+NordicTrack s22i
Initial purchase price$2,395.00$2,495.00$1,999.00
ShippingFree (UK and U.S. only) FreeFree
Monthly membership$12.00$39.00$39.00
Pricing comparison.

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?

Kensington Town Hall employee on CAROL
Kensington Town Hall employee on 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

What is high-intensity interval training (HIIT)?

If you stumbled across this article, chances are that you already know what HIIT means. For those of you who don’t, let me quote Wikipedia:

“High-intensity interval training (HIIT), also called high-intensity intermittent exercise (HIIE) or sprint interval training (SIT), is a form of interval training, a cardiovascular exercise strategy alternating short periods of intense anaerobic exercise with less intense recovery periods, until too exhausted to continue.”

One of the most famous HIIT workouts is Tabata. During Tabata, you perform an exercise, such as pushups, at high intensity for 20 seconds before resting for 10 seconds. Then you repeat that cycle for eight rounds.

If you did the math right, you’ll realize that one round of Tabata takes only four minutes to complete. If you repeated that exercise at the proper intensity — for a total of eight minutes — you should be on the ground trying to catch your breath.

What are the benefits of HIIT?

While high-intensity interval training is relatively new, there are several scientific studies that show the benefits of such cardiovascular exercise strategies, including improved athletic capacity and condition, improved glucose metabolism, lower insulin resistance, a reduction of fat and weight loss, improved brain power and cognitive function, a decreased risk of developing cardiovascular disease, and an increased VO2 max (maximal aerobic capacity).

From a practical perspective, HIIT sessions often have a duration of 30 minutes or less. That makes them attractive alternatives for busy people who don’t have time to go for a long run or spend two hours in the gym.

The combination of short duration and high effectiveness is why the 7-Minute Workout became so popular a few years ago. In fact, before I started CrossFit, I included P90X and the 7-Minute Workout as part of my fitness routine.

What are the drawbacks of HIIT?

The downside of HIIT is what makes it so effective: its intensity. HIIT usually requires a high level of motivation to push through those high-intensity intervals, and many people don’t have the willpower for it.

As a result, some people give up early and abandon HIIT, or they perform the exercises with less intensity, which significantly reduces the workout’s effectiveness.

That’s why I was so excited when I stumbled across CAROL, which effectively reduces the workout to two 20-second bursts. Everyone can push it for 20 seconds, even if you don’t have the strongest willpower when it comes fitness.

What are other types of traditional HIIT programs?

There are numerous types of HIIT regimens that differ mostly in length, type of exercise and intensity. However, based on a study from 2016, there wasn’t a significant difference in outcome and benefits between them.

Here are a few popular HIIT regimens that you might have heard about (each of which carries the name of its respective inventor): Peter Coe regimen, Tabata regimen, Gibala regimen, Zuniga regimen and Vollaard regimen.

I won’t go into the details of these HIIT workouts, but if you’d like to learn more, just click on the respective links above.

How difficult is it to assemble CAROL?

I’m not the most gifted handyman, but it took me less than 30 minutes to assemble my CAROL bike. All the instructions are pretty 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.

The photo below shows how I have them now.

CAROL - Foot straps

Can I use my own seat or pedals with CAROL?

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).

Where can you try CAROL?

CAROL has a few showrooms in the U.S. and across Europe where you can try out the bike. I tried the bike at the Paleo f(x) show* in Austin, Texas, and got hooked on the platform.

If you’re a gym or business owner, you might also consider offering CAROL to your customers and employees. CAROL is particularly well-suited for office environments because employees can complete a workout over lunch in business attire without breaking a sweat.

Can I use this stationary exercise bike to lose weight?

You absolutely can! CAROL has two workout routines specifically designed for fat loss. In free ride mode, CAROL even tracks calories burned. However, if you eat well and exercise regularly, there’s no need to count calories.

Is CAROL better than a recumbent exercise bike?

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.

How does such a short workout do anything?

It’s the principle of quality over quantity. CAROL’s workouts do essentially the same thing as a longer workout; they just 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 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 you burn fat (spurring weight loss) and build muscle.

Can you use CAROL without an internet connection?

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.

What happens to the bike if the company behind CAROL goes bankrupt?

In the unlikely event that the company that makes CAROL goes belly up, you can continue using the bike as a regular spinning bike. Specifically, you can use the “Free Ride” mode and manually adjust the resistance, time and other parameters.

How is CAROL different from an Assault bike?

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 couldn’t simply replicate CAROL’s REHIT workouts on such a bike, considering that it costs less than CAROL.

The thing with 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, which 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 would from CAROL.

What’s CAROL’s Q Factor?

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.

Can I use CAROL to prepare for a triathlon or marathon?

While CAROL can certainly support your training efforts by improving your cardiovascular fitness (VO2max), it’s not meant to replace your training routines. For example, you need to practice running to become a more efficient and better runner.

Getting better in any sport requires technical skills you need to practice, and swimming, running or riding a bike for hours are such skills. But rest assured, including CAROL in your training regimen will improve your overall fitness and make it easier to sustain your target speeds in races.

Does the CAROL bike support third-party heart rate monitors?

Yes, you can use your own chest strap with the CAROL bike. If you don’t have one, the handlebar of the bike features a built-in HRM that’s fairly accurate, based on my experience.

Can I use CAROL to participate in Peloton group classes?

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.

Can I use CAROL in combination with Apple Fitness+?

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 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!

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.

29 thoughts on “CAROL REHIT Bike Review”

  1. 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!

    Reply
    • 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

      Reply
  2. 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!

    Reply
    • 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.

      Reply
  3. 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,

    Reply
  4. 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?

    Reply
    • 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

      Reply
  5. 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

    Reply
    • 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

      Reply
  6. 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.

    Reply
  7. 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

    Reply
    • 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

      Reply
  8. 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

    Reply
    • 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

      Reply
  9. 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.

    Reply
    • 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.

      Reply
  10. 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.

    Reply
    • 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

      Reply
  11. 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!!

    Reply
    • 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

      Reply
  12. 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

    Reply
  13. 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.

    Reply
    • 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

      Reply
  14. 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.

    Reply
    • 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

      Reply

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