CAROL is a stationary bike built to deliver a training protocol called REHIT (Reduced-Exertion High-Intensity Interval Training). Instead of a long steady ride, you do two all-out sprints of 10 to 20 seconds inside an easy ride of under 8 minutes total. The bike sets the resistance for each sprint to your own effort, so the work lands where it needs to for your body.
How to Increase VO2 Max Without Hour-Long Cardio
You didn’t skip cardio because you were lazy. You skipped it because 45 minutes on a treadmill, four days a week, is not something your calendar can hold. So it became the thing you’d get back to. Then a few years went by. The CAROL bike is the quick cardio workout that removes that trade-off. It runs a protocol called REHIT (Reduced-Exertion High-Intensity Interval Training), built around two all-out sprints of 10 to 20 seconds each, inside an otherwise easy ride of under 8 minutes total. The rest of the session is gentle pedaling. The sprints do the work. And the bike is AI-powered: it learns how you ride and sets the resistance for you, session after session, so every sprint is calibrated to you.
In the founding REHIT trial, adults riding three times a week raised their VO2max by 12 to 15 percent in six weeks. Later research found two sessions a week is enough to get there.
That is the entire ask. Two sprints, twice a week.
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The CAROL Bike Process
Step 1: Get Ready
Set your seat height and handlebars, log in so the bike knows it's you, strap on your heart-rate monitor, and choose your ride. It takes a minute the first time. After that, the bike remembers your setup.
Step 2: Your Ride
The ride starts easy: a couple of minutes of light pedaling to warm up. When the bike prompts you, you sprint as hard as you can for 10 to 20 seconds, then ease back down. A few minutes later you do it once more, and that's the workout. You're off the bike in under 8 minutes.
Step 3: Save & Compare
Save your ride and check your stats. Then see how you stack up against other riders, locally and around the world. You've got a number to beat next time, and over the following weeks those numbers are how we track your progress.
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What Is REHIT? How the CAROL Bike Works
Two 10–20 second all-out sprints per session
12–15% higher VO₂max in six weeks (REHIT trials)
28% better insulin sensitivity in men (trial)
Two sessions a week is enough
Under 8 minutes on the bike
Rated only "somewhat hard" overall
No impact: it's a seated bike
Every sprint is all-out, relative to you
Works across a wide range of fitness levels
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Benefits of the CAROL Bike
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Two sprints, twice a week, under 8 minutes on the bike. That’s a schedule you can actually hold for years, and that matters far more than any single workout. Nothing to pack, nothing to change into, no hour to find.
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The sprints are genuinely all-out. They’re also only 10 to 20 seconds long. In published REHIT research, people rated these sessions as tolerable and felt good afterward, across a range of starting fitness levels. That’s the opposite of what most people expect from the word sprint.
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It’s a seated bike. No impact, no pounding, nothing overhead. If you’re managing a knee, a hip, a back, or a rebuilt anything, it’s a way to train your heart hard without asking your joints to absorb it.
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The CAROL bike is the cardio half of FastFit, our paired session with Miha Bodytec whole-body EMS. Full-body strength work and real cardiovascular conditioning in one visit, under 30 minutes. EMS builds the strength; CAROL handles the cardio EMS can’t.
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CAROL is AI-powered. It tracks how you ride and adjusts the resistance to you, session after session, so the bike is always set exactly where it needs to be to get the most out of your sprints. You don’t manage any of it. You just pedal as hard as you can.
Ready to Get Your Cardio Back?
Tell us your goals and we’ll build your FastFit plan around them: your body, your schedule, your week.
Frequently Asked Questions About the CAROL Bike in Chicago
Yes. That’s the whole point of REHIT, and it’s why people call it a quick cardio workout rather than a shortcut. You’re on the bike under 8 minutes, and only 20 to 40 seconds of it is hard. The two all-out sprints drive the aerobic adaptation; the easy pedaling around them is setup and recovery. As part of FastFit, the full visit, EMS and CAROL together, still stays under 30 minutes.
The two sprints are genuinely maximal, and they’re meant to be. They’re also very short, and everything around them is easy. In the research, people rated these sessions as no worse than somewhat hard overall and reported feeling good afterward. Most people find the anticipation harder than the sprint.
Two sessions a week is enough. The original research used three, and a later trial found that dropping to two didn’t reduce the gains in aerobic capacity. We usually build it in twice a week and adjust from there.
No. The sprint is all-out relative to you, not to anyone else. The bike sets resistance from your own effort. That’s why it works across very different fitness levels. What matters is that you’re cleared for high-intensity exercise, which is what your consultation is for.
Whatever you can move in. There’s no suit, no liner, and nothing to rent. Everything you need for the EMS half of FastFit is provided.
The sprints are brief but maximal, so heart rate and blood pressure climb quickly. That’s exactly why we screen first. If you have a heart condition, uncontrolled blood pressure, or any history of cardiac symptoms, get cleared by your cardiologist before starting this or any high-intensity exercise. Every session at the MedGym is supervised by our staff.
Often, yes. It’s a seated bike, so there’s no impact and no loading through your spine or knees the way running or jumping would. It depends on your specific condition, so bring it up at your consultation and we’ll tell you honestly whether it’s the right tool for you.
A spin class is 45 minutes of mostly moderate work, set to music, in a group. The CAROL bike is under 8 minutes, most of it easy, with two timed, measured sprints. A spin class asks for your time; CAROL asks for under a minute of real effort. Both are exercise, but they’re not doing the same job.
FastFit is our paired session: Miha Bodytec whole-body EMS for resistance, then the CAROL bike for cardio. Full-body strength and cardiovascular conditioning in the same visit, under 30 minutes.
REHIT is studied mainly for aerobic fitness and blood-sugar control, not fat loss, and we’re not going to oversell it. That said, an all-out effort like this keeps your metabolism elevated for hours after you step off the bike. That’s the “afterburn” effect, so you keep burning a little extra while you recover. Cardio is still only one input. If fat loss is the goal, the plan also needs strength, nutrition, and, where it fits, the medical side. Your consultation is where we sort out which of those you need.
That is what the protocol was built to do. In the founding REHIT study, adults who had not been exercising raised their VO2 max by 12 to 15 percent in six weeks. VO2 max is one of the clearest measures of cardiovascular fitness, and it is usually the first thing to slip once long cardio stops fitting your week. Most of our members ride twice a week as part of FastFit.
Source: Metcalfe et al., Eur J Appl Physiol, 2012 (three rides a week for six weeks).
No. A CAROL bike is a real investment to own, and most people want to know it works for them before they commit. At the MedGym by Balanced Flow in Bucktown, CAROL is part of FastFit: whole-body EMS for strength, then CAROL sprints for cardio, coached, in one session. No purchase, no setup, no bike taking up a room at home.
The MedGym is at 1755 N Damen Ave in Bucktown, a short walk from Wicker Park and an easy trip from Logan Square or Lincoln Park.
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