Sugar, Sugar, Sugar! - A Rhyme to the Reason for Why We Need It

Sugar, Sugar, Sugar! - A Rhyme to the Reason for Why We Need It
By Coach Tim Snow
Have you ever wondered why we need to consume sugary drinks, gels and bars when we’re training? The short answer is that our muscles need them. But why?  
The muscles in our bodies have three types of fibers: Type I, known as our slow-twitch muscle fibers; and Type IIA and Type IIX, our fast-twitch muscle fibers. 
Type I muscle fibers are loaded with capillaries and mitochondria, so they have plenty of blood supply (capillaries) and they are extremely efficient at using the oxygen in the blood that the capillaries bring (mitochondria). These muscle fibers are up for just about any task below Lactic Threshold (LT). Type I muscle fibers are like your Toyota Priuses. They can go very, very long distances on very little fuel. 
Type IIA muscle fibers…I am going to skip these for a moment and then come back…
Type IIX muscle fibers are your ‘Fast-Twitch Glycolytic’ fibers. These fibers don’t have much in the way of capillaries or mitochondria, so there isn’t much blood supply being brought to them.  (Even if there were (which there isn’t), they wouldn’t have the tools to do much with the oxygen that blood supply brought.)  What they DO have are special enzymes which utilize glycogen really, really well. These muscle fibers are called upon once your effort crosses over into Critical Power (CP) territory. These are your sports cars. They are designed for speed and are extremely powerful, but have terrible gas mileage, and so run out of energy quickly.
Okay, back to Type IIA muscle fibers, which have blended qualities of Type I and Type IIX muscle fibers.  These are your ‘Fast-Twitch Oxidative Glycolytic’ fibers. They have a moderate number of capillaries and mitochondria. They also have some of those special enzymes that utilize glycogen really well. These are the muscle fibers who were friends with everyone in high school.  These muscle fibers are called upon once your effort crosses over Lactic Threshold (LT), and take care of a lot of the horsepower up until CP.  In car terms, these are your Toyota RAV4s or your Honda CRVs. A little peppy when you need them to be, but good on fuel efficiency.
Thank you to Dr. Phil Skiba for this great graphic, outlining how our different muscle fibers are recruited. [I HIGHLY recommend his book Scientific Training for Endurance Athletes.] 
As we operate below LT, our Type I fibers handle the workload, and do so very efficiently.
As we cross over LT we begin to recruit more and more of our Type IIA muscle fibers. This doesn’t mean that our Type I fibers ‘clock-out’ and go on break! It just means that there are more - and different - hands on deck, helping to carry the load.
As we cross over CP we begin to recruit more and more of our Type IIX muscle fibers. Again, the Type I, and Type IIA fibers are still engaged, but now there are three different work crews all focused on the same task.
So, back to the original question at-hand…Why do we need all of these sugary drinks, gels and bars when we’re training? Because any time that we are using our Type IIA and Type IIX muscle fibers, we are utilizing some level of glycogen to fuel our effort. In the above image, that is everything to the right of LT. And these don’t necessarily have to be sustained efforts.  In a typical Z1/Z2 ride or run, it's likely that we cross over into LT (above Z3) many times, even if just for a few moments.
The next time that you are riding or running - particularly outside, in an uncontrolled environment - pay attention to your power meter. As an example, let’s say your LT is 250w, and your CP is 270w. Although for most of the ride or run, you are utilizing your Type I muscle fibers, which rely on oxygen to burn fat as their fuel, every time that you pop above 250w - even if/when you didn’t intend to - you are calling upon your Type IIA muscle fibers. Pop up above 270w, even if for only a few moments, and you are calling upon your Type IIX muscle fibers. And what do those two fiber types need for fuel? Glycogen. And glycogen is sourced from those sugary drinks, gels and bars. 
So next time your grab for your sugary drink, gel or bar, know that you are doing so for a purpose.
Sugar, Sugar, Sugar! - A Rhyme to the Reason for Why We Need It
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