In today’s fast-paced world, finding time to work out can sometimes seem nearly impossible.

Using machine learning, the CAROL (CARdiovascular Optimization Logic) bike aims to solve this problem, claiming to be the fastest way to get fit — all you need is just 8 minutes and 40 seconds, 3 days a week.

Unlike other stationary bikes that are designed for home use, the CAROL bike has a stronger build that’s certified for professional and commercial use, which is important since the workouts are meant to be pretty intense.

What’s more, after just six rides, the software automatically sets your resistance level for a personalized workout experience.

However, while a personalized and effective workout in under 10 minutes sounds like a dream, you may wonder whether it’s too good to be true.

This article provides an in-depth and hands-on review of the CAROL bike to help you decide if it’s worth purchasing.

Developed by scientists, the CAROL bike uses a relatively new exercise method called reduced exertion high intensity training (REHIT).

You may already be familiar with high intensity interval training (HIIT) — a popular workout method that involves short but intense bursts of exercise, followed by a recovery period.

REHIT is similar to HIIT, just with fewer and shorter intervals.

The goal of REHIT is to quickly deplete muscle glycogen stores — the body’s preferred fuel source — as rapid depletions of muscle glycogen have been linked to several benefits, such as improved aerobic fitness and insulin sensitivity (1).

In fact, a small 2012 study including 8 men found that completing 3 REHIT sessions per week for 6 weeks resulted in a 28% improvement in insulin sensitivity (2).

So how does REHIT work?

The idea behind REHIT is to provide an optimal amount of resistance during each 10–20 second sprint in order to deplete your glycogen stores just as quickly as if you’d exercised for 45 minutes or longer at a moderate intensity.

While REHIT has been studied in lab settings, the CAROL bike is the first to make this time-efficient workout available to consumers.

According to the company, using REHIT, the CAROL bike’s Intense Workout could deplete muscle glycogen stores by 25–30% in your quadriceps, though more research is needed (3).

While it may seem impossible that a 10-minute workout can be as effective as a 45-minute or longer ride, studies support the effectiveness of REHIT and the CAROL bike.

Before taking a look at the research, it’s important to understand the following two terms (4):

  • V̇O₂ max. This measure of cardiovascular fitness indicates the maximum amount of oxygen that the body can use during exercise. Improvements in V̇O₂ max allow for better endurance and performance during cardio workouts.
  • MetS Z-score. This measure of cardiometabolic health takes your blood pressure, waist circumference, blood glucose, and blood fat levels into account.

One study from the American Council on Exercise (ACE) found that the REHIT workout on the CAROL bike was more effective than moderate intensity continuous training (MICT) at improving waist circumference and V̇O₂max after just 8 weeks (4).

Furthermore, the average improvement in the REHIT group’s MetS Z-scores was 62%, compared with just 27% improvement for the MICT group (4).

In another small, 5-week experiment performed by the British Broadcasting Corporation (BBC) for live TV, 6 adults performed the CAROL bike’s intense workout 3 times per week. All of them improved their V̇O₂ max, though there was no control group to provide a comparison (5).

A more recent study from ACE explored the metabolic responses that resulted from REHIT performed on the CAROL bike, particularly the excess post-exercise oxygen consumption (EPOC) following these workouts. You can think of EPOC as the “after-burn” that occurs after exercise (6).

In this study, 20 healthy adults, ages 25 to 76, performed two rides on the CAROL bike, one intense workout and one fat burn workout, about 9 minutes and 15 minutes long, respectively.

Additionally, each participant performed two treadmill workouts, one 30 minute moderate intensity workout and one 20 minute vigorous intensity workout.

The results showed significantly greater EPOC from the bike rides when compared with the treadmill workouts, especially following the 15-minute fat burn ride (6).

This means that participants experienced a greater calorie burn over a longer period of time following their workout on the CAROL bike.

This study further supports the theory that the CAROL bike could offer an effective workout in just 10 minutes. However, more research, specifically larger human studies, are still needed.

The CAROL bike ranges in price from $2,395–$2,795, depending on the bundle option.

Here are the current prices and warranties:

  • Essentials Bike Bundle: $2,395, 1-year warranty
  • Standard Bike Bundle: $2,595, 2-year warranty
  • Premium Bike Bundle: $2,795, 3-year warranty

According to the company’s website, the warranty covers any defects that arise during the 1–3-year coverage period. However, there are specific conditions under which the bike cannot be repaired or replaced, such as if it’s improperly assembled or used.

To make the bike more affordable, the company also offers special financing options for qualified customers, allowing the cost to be split into 12 monthly payments with 0% APR.

Monthly payments range from $199.58 per month for the Essentials Bike Bundle, up to $232.92 per month for the Premium Bike Bundle.

Shipping is free within the United States and the United Kingdom, with professional assembly available for an additional $150.

Finally, the bike requires a $12 monthly membership for access to the workouts and user profiles.

I was fortunate enough to receive the bike fully assembled, so I’m not able to speak firsthand on assembling the bike. However, most reviewers agree that the assembly is straightforward and should only take about 45 minutes, though you’ll want the help of a friend.

Photo credit: Ellen Landes, MS, RDN, CPT

Once the bike is assembled, adjusting the seat and handlebars of the bike is simple and can be done by tightening and loosening a handle.

New customers receive an invitation to meet virtually with CAROL’s customer success manager. If you’re new to cycling, you may benefit from getting in touch, so you can get the bike set up appropriately for your height.

However, as a general rule, the seat of the bike should be about hip level.

When I first started with the bike, I followed the on-screen prompts to connect to Wi-Fi and create my profile by entering a password and providing my age, height, and weight.

Once my account was set up, I was ready to get on the bike for my first ride, which consisted of two 10-second sprints to introduce me to the workout.

After the first three rides, you get the option to stick with 10-second sprints or increase them to 15 or 20 seconds for the next three rides.

After six rides, you get access to all the bike’s workouts — a benchmark I found to be motivating, as it made me want to get on the bike regularly so that I could unlock the full workout library.

The CAROL bike has a heavy flywheel, leveling feet, and a sturdy, stainless steel frame, all of which are important for stability during maximum intensity sprints.

It also features a silent belt drive, which provides a smooth and quiet ride, and transport wheels for easy moving and storage.

Pedals and seat

Unlike many competitors, the CAROL bike includes dual-sided pedals, which allow you to use either SPD clips or regular athletic shoes.

I personally don’t own spinning shoes, so I used the foot straps on the other side of the pedal. The straps are easy to adjust as needed. However, SPD clips typically allow you to pedal faster and more efficiently, so I would recommend investing in a pair of spinning shoes.

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Photo credit: Ellen Landes, MS, RDN, CPT

The seat is comfortable enough throughout the workout, but, as with any bike seat, it can get a little uncomfortable over time. Though, as you’re only on the bike for a short duration, this wasn’t a dealbreaker for me.

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Photo credit: Ellen Landes, MS, RDN, CPT

Display screen

The CAROL bike is also equipped with a 10-inch (25.4-cm) tablet, which displays your heart rate and performance metrics. It’s also where you’ll find prompts for sprinting and slowing down, along with the breath pacer for recovery.

Additional workout features

The CAROL bike offers six styles of music to choose from, integrated speakers, and Bluetooth connectivity for playing music using your favorite app.

The bike allows for eight user profiles and access to all of CAROL’s guided workouts, though it requires a monthly subscription.

Without a subscription, the bike functions as a traditional stationary bike. Therefore, if you don’t plan on buying the monthly membership, you’re better off saving money and choosing a different machine.

It’s also important to note that the bike doesn’t include trainer-led classes. However, if you’re a Peloton member, you can install the app directly on the CAROL tablet, allowing you access to Peloton’s extensive class library.

If you do want to install the Peloton app to your CAROL tablet, you’ll need to get in touch with CAROL’s customer service to get it set up.

Finally, one of CAROL’s newer features is a global leaderboard that allows you to track your progress and compete with other CAROL users around the world. I really enjoyed being able to see how I compared to other riders out there and found it motivating to work harder.

CAROL bike accessories

In addition to the special features listed above, CAROL offers bike bundles that include accessories, such as a water bottle holder and heart rate chest strap.

Here’s what’s included with each bundle:

  • Essentials Bike Bundle: CAROL bike
  • Standard Bike Bundle: CAROL bike, chest strap, and tablet holder
  • Premium Bike Bundle: CAROL bike, chest strap, tablet holder, floor mat, and water bottle holder

Safety features

Given that you’re moving at high speeds, it’s nice to know that the bike has many safety features, including a freewheel safety clutch and computer-controlled magnetic breaks.

These features make it easy to increase and decrease speed and to stop pedaling and hop off safely, even if the wheel is still spinning.

Finally, the heart rate monitor, which is located on the handlebars, keeps a constant read on your heartbeat and will alert you if your heart rate gets too high during your ride, but this does require you to keep your hands on the handlebars throughout the workout.

If you purchase one of the bundles that includes the chest strap, you’ll have a more accurate read on your heart rate during your workout. This is nice, especially if you’re like me and prefer to take your hands off the handlebars during the recovery periods.

I had the opportunity to try the CAROL bike. Over the course of 2 months, I’ve been getting in about three rides per week, which has been easy to fit into my schedule since the workouts are so brief.

Before I could start cycling, I first had to connect the bike to Wi-Fi and create my profile by entering my age, weight, and height.

Next, I had to complete the first six rides before getting full access to the bike’s workouts.

These initial rides are designed to help you practice the maximum intensity sprints and give the bike the opportunity to assess your fitness level and ability so that it can adjust accordingly.

More specifically, the first three rides are called ramp-up rides and include short sprints — just 10 seconds each — allowing you to practice the maximum intensity bursts.

While the 10 second sprints were definitely challenging, I found these rides to be a little too easy, and when I finished, I felt like I needed a little more. Of course, these are meant to be practice rides and not the full workout just yet.

The next 3 rides are considered calibration rides. During these rides, you choose between 10-, 15-, and 20-second sprints, and the bike automatically sets the resistance to ensure that you reach your maximum intensity.

After the initial six rides, you’ll have access to all of the programmed workouts, though the bike will continue to learn and adjust the resistance based on your progress over time.

The bike offers several workout options, including fat burn, energizer, endurance, and free ride. However, the main workout that has been studied along with the benefits from REHIT is the intense workout.

The intense workout is structured as follows:

  • 2-minute slow warmup
  • 20-second maximum sprint
  • 3-minute recovery
  • 20-second maximum sprint
  • 3-minute cooldown

The bike’s integrated display screen prompts you through each workout, with the bike automatically adjusting your resistance as your speed increases.

While 20 seconds may not seem very long, keep in mind that the sprints are meant to be very challenging and the faster you pedal, the more resistance you’ll feel.

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During the recovery periods, an on-screen breath pacer will guide you in getting your breathing back under control. You’ll also see a graph of your power output from the previous sprint so that you can try to improve on the next one.

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The CAROL bike also includes a “free ride” mode that allows you to use the bike like a traditional stationary bike. In this mode you can manually adjust the resistance level and even stream classes from popular fitness apps.

Furthermore, the CAROL bike’s website offers suggested training plans based on your fitness goals. These plans include recommendations on how often you should use the bike and which workout programs to choose.

What I liked about the CAROL bike

Overall, I enjoyed working out with the CAROL bike. I appreciated being able to work out in such a short period of time, and I found it easy to fit it into my current workout routine.

Even after trying the bike out for a couple of months, the 20-second sprints still feel incredibly challenging. This is because the bike continues to adjust to your fitness level.

The on-screen breath pacer seemed like a really cool idea, but I didn’t often follow it. Instead, I found it easier to catch my breath in my own rhythm. But some people may find it to be a very helpful tool for recovering between sprints.

If you’re a numbers person, like me, you’ll likely enjoy viewing your workout metrics and seeing where you’re falling on the leaderboard.

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Photo credit: Ellen Landes, MS, RDN, CPT

On the topic of the data, it can be a little confusing at first.

I found that speaking with CAROL’s customer success manager was very helpful in understanding all the metrics. He explained to me that after each ride, I can click on the “i” in the upper right corner of each metric for an explanation of my results.

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I also like that the bike didn’t leave me super sweaty or sore. It was a challenge, but it was over in record time and didn’t affect my ability to keep up with my other workouts as planned.

It was also easy to hop on in the middle of the day without worrying about getting too sweaty before getting back to work.

Lastly, I appreciate having the mobile app to access my stats and progress any time without having to view them on the bike itself. However, there doesn’t appear to be a way to look back on individual workouts. Instead, you’ll see a summary of your total progress overall.

Potential downsides of the CAROL bike

Overall, my experience with CAROL was positive, but there were a few drawbacks as well.

For starters, unlike many other bikes, there’s only one handlebar grip. I prefer to have vertical handles along with the horizontal handlebars for a more comfortable grip. This is a personal preference that may or may not be a drawback for you.

Photo credit: Ellen Landes, MS, RDN, CPT

Additionally, I tried riding out of the saddle during a free ride a few times. As a former spinning instructor, I liked adding in a climb out of the saddle for some variety.

However, I noticed that, while pedaling was super smooth in general, it felt like the pedal stuck a bit at the bottom of the rotation when I came out of the saddle. Applying more resistance helped, but didn’t totally get rid of the stickiness.

That said, this might not be an issue if I had been using SPD cleats, rather than athletic shoes.

If you plan on using the bike with the Peloton app, I recommend investing in a pair of spinning shoes, as several Peloton rides include climbs out of the saddle.

The CAROL bike offers an incredibly time-efficient workout that’s backed by science. While more research is needed, results have been promising.

So, if your biggest hurdle to working out is time, it’s worth considering the CAROL bike. According to the website, the “minimally effective dose” is just 3 intense workouts each week, which is less than 30 minutes total.

Furthermore, as the workouts are highly personalized, the bike is suitable for riders of any fitness level and adapts as you become stronger.

That said, as it’s designed specifically for REHIT workouts and includes limited workout options, exercising on the bike could become monotonous for some — especially if your main workout is cycling.

Similarly, if you prefer more traditional indoor cycling workouts, the CAROL bike isn’t the best fit, especially as there are several more affordable smart bikes on the market.

Finally, keep in mind that, while REHIT may help you improve your cardio fitness, other workouts — yoga and strength training — should also be included in your exercise routine.

Here’s how the CAROL bike compares with some of its main competitors:

CAROL BikePeloton Bike+NordicTrack s22i BikeBowflex VeloCore
Warranty1–3 years1-year• 10-year on frame
• 2-year on parts
• 1-year on labor
• 2-year on parts
• 1-year on labor
Size45.5 × 22 inches
(115.6 × 60 cm)
59 × 22 inches (150 × 60 cm)63 × 22 inches (160 × 60 cm)60 × 24 inches (152.4 × 61 cm)
Max user weight331 pounds (150 kg)297 pounds (134.7 kg)350 pounds (158.8 kg)325 pounds (147 kg)
Trainer-led classesnoyesyesyes
Automatic resistanceyesyesyesyes
App subscription$12 per month$39 per monthincluded for 12 months, then $33 per month$19.99 per month
Pedalsdual-sided, offering SPD clips and toe cagesDelta-compatibledual-sided, offering SPD clips and toe cagesdual-sided, offering SPD clips and toe cages
Display10.1-inch (25.7-cm) touchscreen24-inch (61-cm) swiveling touchscreen22-inch (60-cm) swiveling touchscreen 16-inch or 22-inch (40.6-cm or 60-cm) touchscreen
Other features• short REHIT workouts
• machine learning resistance
• live and on-demand classes using the Peloton app
• rotating display
• live and on-demand classes using iFit
• rotating display
• streaming capabilities
• leaning mode

Overall, the CAROL bike is smaller than its competitors, which might be ideal if you’re limited on space.

It’s also the only bike that offers REHIT workouts.

However, it has the smallest touchscreen and is one of the most expensive options. It also lacks many features offered by other smart bikes, including trainer-led classes.

The CAROL bike is suitable for riders of varying fitness levels, because it offers a personalized experience that will adapt as you improve.

Over the past 2 months, it’s been a great complement to my workout routine, as it’s made it easy to sneak in an effective cardio session on busy, mostly sedentary work days.

However, if you’re interested in long rides or trainer-led classes, the CAROL bike isn’t your best option, especially given its high price point.

That being said, if you’re looking for an effective solution to cardio fitness, in as little time as possible, the CAROL bike could be a great solution.