Science

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How CAROL Bike's 5-min Reduced Exertion HIIT (REHIT) workout works

When CAROL Bike launched in 2018, you could get the health and fitness benefits of a 45-minute run in just 8 minutes 40 seconds. 4 years later—it's just 5.

06 Jul 2022

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How does REHIT work?

In simple terms, REHIT makes your body go into fight or flight mode—increasing its energy demands 100-fold.

Your body also has no idea how long you will need to sprint for, so your muscles go to work to mobilize more glycogen (stored sugar) than you actually need. This rapid depletion of energy reserves tells your body to get fitter and stronger.

Tap into the science of muscle glycogen depletion


CAROL is the only exercise bike that offers true REHIT because its AI personalizes your workouts, making sure you hit your personal maximum intensity every time. Or as health and exercise researcher at the University of Stirling, Dr Niels Vollaard, puts it:

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.

CAROL Bike’s new 5-minute REHIT workout: the science

CAROL’s original 8-minute 40-second REHIT Intense ride worked like this:

  • Warm up – 2 minutes
  • Sprint 1 – 20 seconds
  • Recovery – 3 minutes
  • Sprint 2 – 20 seconds
  • Cooldown – 3 minutes


Research shows REHIT’s physiological impact comes from the maximum effort sprints. That got us thinking: could we shorten the non-sprint periods while protecting the health and fitness benefits?

Here's Dr Vollaard: “The health and fitness benefits of REHIT are triggered by two 20-second maximum intensity sprints. Warm up duration has no impact on the effectiveness of REHIT.”

And what about recovery between sprints? “To get the full health and fitness benefits of REHIT your recovery between sprints needs to be long enough to allow for a strong second sprint.”

How long is ‘long enough’? That’s up to you. As a rule, it’s sensible to wait until you’ve caught your breath, but it’s simply whenever you feel ready to go all-out again.

We kept the cooldown at 3-mins. Here's CAROL CEO and co-founder, Ulrich Dempfle on why: “It’s important for blood pressure to normalize again after the sprint. Research shows light pedaling does this faster than just sitting or standing would. That’s because keeping the muscles active assists in pumping blood back to the heart and speeding up the removal of ‘waste’ products from the muscle, reducing the imbalance between the concentration of such particles inside and outside the muscle cell.”

Put simply, the health benefits of REHIT are in the all-out sprints. And while cooldown is essential to get your body back to normal, warm-up and recovery duration are up to you.


CAROL Bike’s new 5-minute REHIT workout: how to do it

  • Hop on your bike and log in to the CAROL app
  • Choose Intense ride
  • From 20 seconds into the warm up and one minute into the recovery phase, a checkered flag will appear on the RPM dial
  • When you see that flag and feel ready, simply pedal faster to begin the sprint

Got a question? Let’s book a call.

All our experts have MSCs in Exercise Science, and they’re here to answer your questions. Whether it’s about these workouts, or the science behind CAROL Bike, whatever’s on your mind—we’re here for you.

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