Getting To Know - Your Muscle Physiology

I find it surprising how few athletes really understand their own physiology. They treat their bodies like a magical black box that they throw training stimuluses into and expect phenomenal results to come out the other side. If you can start to understand what's really going on inside that black box, you will become much better at putting the right kinds of things into your training plans to get the results deserving of all your hard work.

Let's open the black box and poke around inside.

A Pain-Free Muscle Biopsy

Muscle biopsies are nasty. Think of a big bore needle. Think of sticking that deep inside your thigh and then extracting a chunk for examination. Okay, we don't need to go there any further. Thankfully, willing subjects have come before you and allowed their muscles to be studied under a microscope for your benefit. Their pain is your gain. Understanding the types of muscle fibers inside your bursting quads will make you a better cyclist.

So, what's inside there? It turns out there are 2 major types of muscle fibers, with one of those types consisting of 2 sub-types. In all then, I like to talk about there being 3 types of fibers. Your unique combination of these types is genetically determined. Training won't really change your quantities of these fibers, but it can change what you're able to do with them.

Type I Muscle Fibers

Type I muscle fibers are also called Slow-Twitch fibers because they are slow to contract. But they're also very slow to fatigue, as long as you meet their fueling needs. These are the fibers that allow you to ride long distances. They have a large number of blood capillaries to supply oxygen and nutrients and to remove carbon dioxide. Their energy cycle is aerobic, meaning their fuel needs are supplied by fat and carbohydrate metabolized in the presence of oxygen.

Fat packs a huge wallop of energy - one molecule of fat produces 100 molecules more adenosine triphosphate (ATP -the basic molecule a muscle cell uses for its energy needs) than one molecule of carbohydrate. But fat metabolism is slow, so it isn't very useful for quick bursts of muscle power. Carbohydrate is better for that.

The faster cycle of converting carbohydrate into ATP makes it the superior energy nutrient in your body for moderate to intense exercise, which is what most competitive cycling is all about.

At less than 60% of your maximum effort, Type I fibers are mainly in charge and fat is their primary fuel source. As you leave the 60% effort level and climb higher, carbohydrate becomes the primary fuel source and you start to recruit the Type II, or Fast-Twitch, muscle fibers. Type II's are the fibers we sub-divide into IIa and IIb.

Type IIa Fibers

Type IIa fibers have 5-times the contraction strength of Type I fibers. They are somewhat fatigue resistant, capable of maintaining a high power output for several minutes. These fibers are usually your race-winning fibers, allowing you to put the major hurt on a climb, to initiate a race winning break or to power away in the last few kilometres before the finish. They also start to get recruited on long distance endurance rides as the Type I fibers eventually fatigue.

Type IIa fibers will use both fat and carbohydrate for their energy needs, but most comes from carbohydrates. They are also capable of anaerobic metabolism when you can't supply the oxygen needs for the basic carbohydrate-to-ATP cycle. You'll know when you start reaching that point as your breathing gets heavy and your muscles begin to feel a burning sensation.

Lactic acid is a byproduct of anaerobic metabolism. Your body is always producing some lactic acid, but usually can recycle it for further energy needs. But when your body can't recycle the lactic acid fast enough back into available energy it leads to an increase in blood pH. Your body can only stand a rise in pH for so long before self-preservation kicks in. You begin to feel fatigued, start to lose power and and are forced to settle down to an effort level that allows you to return your body pH to normal levels.

Type IIb fibers enter the picture when you approach and exceed 80% of your maximum effort.

Type IIb Fibers

These guys are the sprinter's fibers. Type IIb fibers don't use fat and only get their energy needs anaerobically from carbohydrates. They have twice the contractile strength of Type IIa fibers - 10 times the force of Type I fibers. But they fatigue very quickly, usually within about 15 seconds of effort. They can be used again, but need a period of complete recovery.

Fiber Recruitment Pyramid

Type IIb
Type IIa
Type I

The fiber recruitment pyramid is very important to know. Type I fibers are used for all long, low intensity efforts. As effort increases, muscles continue to use the Type I fibers with the Type IIa fibers being added to the effort. As the effort increases still further, the Type IIb fibers are added as well. The fibers on the bottom of the pyramid don't get shut down, but the fibers higher up get added to the effort. What's this mean? Two take-home messages. One about your diet and one about your training.

Your Two Take-Home Messages

First: your aerobic metabolism is always being used. As you increase your effort, the anaerobic metabolism is added to the fold. As your efforts increase still further, the proportion of energy needs being met anaerobically starts to outweigh those being met aerobically. Fat can only be used in the aerobic metabolic process, but carbohydrate can be used in both the aerobic and anaerobic processes. Therefore carbohydrate in your diet is supremely important in order to perform quality high power training and racing efforts.

Second: since most race winning moves last longer than 15 seconds, but shorter than 10 minutes (meaning it usually takes less than 10 minutes to establish a race winning break at the end of a race or during a race before a more sustainable effort is returned to), the Type IIa fibers are the race winning fibers you must train. All of your training of the Type IIa fibers will also train the Type I fibers because the Type IIa fibers are added to the effort - the Type I's are not shut down. Focus your training on the Type IIa fibers and you'll get yourself in the most fit shape to win races.

If you find yourself genetically blessed with a good degree of Type IIb fibers (and you'll know if sprinting seems to be your forte), then by all means train at sprinting and use that strength to win at the end of races. But the majority of cyclists will benefit most from training the Type IIa fibers. The best bang for your training buck will come from training those IIa's.

How to train them? Stay tuned and I'll get into that as this blog progresses.

Photo "Rund um dem Henninger Turm 50" by: tetedelacourse


  1. Thanks Kevin,

    An excellent and well researched post that should benefit any cyclist (and other athletes as well).

    As a Registered Nurse, I have extensive training in physiology, but I certainly don't have this depth of practical application knowledge.


  2. Thank you,

    It was amazing to know what kind muscle fibers are