Slow and fast twitch muscle myths

I read an article by top triathlete Samantha McGlone the other day. In it she discussed how a short-distance athlete can prepare for and adapt to iron-distance. She started out by highlighting her career as a short distance athlete and remarked how odd it was that she is doing so well in short events given her body type.

That body type, we were told did not have "a single fast-twitch muscle fiber" in it. Furthermore, Samantha claimed she knew this because tests had been performed and these tests had returned said result. The article further implied that short distance events, such as sprint and Olympic triathlons relied on fast twitch muscle, whereas longer distances were the purview of slow twitch muscle.

While I have great respect for Samantha and I certainly have to admit that she is a fantastic performer who could beat me anytime, there is no doubt she is sorely misinformed and her understanding of physiology is incorrect. In that respect however, she is not alone. The slow-fast twitch misconception is quite common among athletes and even some coaches and doctors. Along with the lactic acid buildup this is one of the most common misunderstandings in the sport.

Some background:

Mammals have three types of muscle, smooth, striated and cardiac. The latter is a special kind of striated muscle. The terminology (smooth, striated) refers to the transverse stripes seen under light microscopy. Smooth muscle has no stripes or striations, while the other types do. Smooth muscle is also known as involuntary muscle and it lines the gastro-intestinal tract, the blood vessels, the airways, the genito-urinary tract, and it is present in the hair follicles in the skin. It is not under conscious control. Smooth muscle can show burst of activity like other muscle, but most often it maintains near constant, quasi-permanent contractions that are responsible for the "tone" of various structures.

The heart is a pump made of muscle and connective tissue rings and patches. It is not under voluntary control either. It consists of special fibers that look somewhat similar to striated muscle. Heart muscle contracts in bursts like skeletal muscle does. The wall of any heart chamber works as a single unit with all cells contracting simultaneously. Heart muscle has an intrinsic rhythm and it contracts regularly even in the absence of outside stimuli.

Skeletal or striated muscle (so-called "meat") is voluntary, and consists of long thin cells, called fibers that are arranged in parallel bundles. Some cells can be as long as the muscle they are in. Skeletal muscle is connected to bone via specialized connective tissue called tendons. Most muscle contracts rapidly but some "postural" elements also maintain a certain tone and are less suited for rapid movement.

Skeletal muscle contracts in short bursts that alternate with longer relaxation periods. Functionally, the muscle is divided into "motor units" or groups of cells under the control of a single nerve fiber. There are thousands of motor units in a given muscle. There is also a range of small, intermediate and large motor units. Units work independently, and while some are active, others relax. As force requirements go up, more and larger units are recruited up to a maximal contraction. In maximal contractions, which can only be held for a few seconds, all units work together.

Muscle consists of two types of fibers, that have a different appearance. Type I, known as slow twitch, or red muscle is dense with blood vessels and consist of small fibers loaded with mitochondria and myoglobin. Mitochondria are specialized sub-cellular structures responsible for cellular oxygen use. Cells with high metabolism are packed densely with mitochondria.

Type I fibers contract rather slowly but they also tire slowly. They engage almost exclusively in aerobic power output. Myoglobin is an oxygen carrier protein similar to hemoglobin, the transport molecule in red cells. It is responsible for the red color. Type I fibers make up the dark meat common in beef, and chicken legs.

Type II fibers are fast twitch fibers. There are two subtypes, IIa, the aerobic fast twitch and IIb, the anaerobic fast twitch fibers. IIa has larger cells that appear reddish and contract faster than type I. They also tire more easily. Type IIb is known as white muscle because it has very little myoglobin or mitochondria and it is largely anaerobic. Its rate of contraction is very fast. Humans do not have IIb fibers, but they are common in small mammals whose flesh is white or pale.

Humans have an intermediate type of fiber, sometimes called IIx. It has less mitochondria and less myoglobin and can contract faster than type I. IIx tires quickly and can become painful quickly too. That pain is called the "burn" and is incorrectly attributed to a lactic acid buildup. It is quite clear however that the burn has little to do with lactic acid. Many manuscripts also incorrectly label these fibers as type IIb or fast twitch muscle. Humans do not have real fast twitch muscle.

Human muscle is about 50/50 type I and IIx, with some rather minor genetic differences between individuals. In theory, individuals with more IIx fibers would be better suited at anaerobic events such as the 100m sprint or weight lifting, whereas others with more type I fibers would be better endurance athletes. 

Note that in this context, any event that lasts more than 10-15 minutes is considered an "endurance" event favoring type I muscle. The time-frame of a sprint triathlon is many orders of magnitude greater than the time-frame used to describe the various muscle types and any connection between better performance on short versus long distance triathlons and the composition of one's muscle is pretty meaningless and irrelevant.