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Everything you need to know about CAROL and muscle glycogen

How CAROL's short, effective workouts deplete muscle glycogen to improve aerobic capacity.

01 Jun 2022


You may have heard that CAROL’s two 20-second maximum intensity sprints, in between a gentle warm-up, recovery and cool-down periods, has been shown to be the most efficient way to deplete glycogen and improve your aerobic capacity.

Yet what is muscle glycogen and what is it about a CAROL workout that makes it such a fantastic way of getting fitter faster? Let’s find out.

What is muscle glycogen?

Glycogen is a stored form of carbohydrate found in skeletal muscle (found within the cytosol of skeletal muscle fibers, to be precise). The average adult has about 500 grams of skeletal muscle glycogen content.

Skeletal muscle performance, such as completing a CAROL intense ride workout, requires a constant supply of energy. The body’s immediate energy source for this type of exercise is Adenosine triphosphate (ATP).

Given that ATP is essential for repeated muscle contraction, you’d assume that there would be large stores within skeletal muscle, but that’s not the case. In the absence of an unlimited supply, when ATP levels are depleted during high-intensity exercise, your body must find an alternative energy source.

And that’s where glycogen comes in. Its location within the muscle means it can be easily accessed as an energy source during intense exercise.

What is muscle glycogen depletion?

Our body can store enough glycogen to power up to 15 seconds of additional exercise. After the body uses all of the glycogen, we say it is “depleted.”

Glycogen depletion stimulates molecular changes that lead to improved fitness. This is due to a part of human evolution known as the “fight or flight” mechanism, which is triggered when all-out physical performance is needed, and after exercising to the point of exhaustion, our body starts using glycogen as fuel.

Working beyond glycogen depletion will improve your aerobic capacity, which in turn will make you leaner, fitter, and healthier.

What happens to muscle glycogen levels during CAROL rides?

Workouts performed on CAROL include short “supra-maximal” sprints, which is where you work beyond your maximal aerobic capacity for very short durations.

The real magic of CAROL takes place with two 20-second sprints and what happens during the recovery timeframe between workouts. Research has consistently found with this frequency and duration of sprints that muscle glycogen stores are depleted 20 to 30 percent in the quadriceps. Even more fascinating is that the maximal rate of glycogen breakdown is also achieved with this frequency/duration combination of sprints.

The rapid muscle glycogen depletion that occurs during a CAROL intense ride sets in motion two important metabolic events that persist into recovery:

  • Activation of signalling pathways: Along with glycogen depletion comes the release of a glycogen-bound enzyme called AMPK. The activation of AMPK turns on an important signalling cascade that will eventually lead to more mitochondria.
  • Activation of transporter proteins: Following CAROL sprints muscle glycogen stores need to be replenished. Glycogen synthesis requires bringing glucose from the bloodstream into the muscle cell via a transporter protein known as GLUT-4. Increased GLUT-4 transporter protein activity persists for up to 24 to 48 hours post-exercise. This metabolic event has tremendous implications for those trying to manage their blood sugar levels.

All of which means that more sprints and/or longer sprints do not elicit a greater rate of glycogen breakdown. This is one reason why you don’t have to exercise a long time with CAROL to get optimal benefits.

Quite simply, CAROL is the quickest, most straightforward way to deplete glycogen and improve your aerobic capacity.

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