Muscle Physiology – Oh Those Crazy Muscles!

When the average person takes a look at a bodybuilder all he sees is a series of lumps and bumps of varying shapes and sizes. Upon inspection he may find these odd protrusions to be fascinating, cool, or perhaps grotesquemaybe even a little intimidating. But little does he know how complex and intricate all these “lumps” really are!Let’s take a look inside these lumps, see what they are made of and what we can do to make them bigger and stronger. Ok, first let me stop calling them lumps or bumps and refer to them more properly as skeletal muscles, or just muscles for short.

Muscle is composed of bundles of muscle fibers or myofibers. Each fiber is composed of myofibrils, which, in turn, are composed of myofilaments. The myofilaments are made up of two proteins called myosin and actin. The myosin and actin act within the smallest functional unit of muscle, the sarcomere, to produce a contraction. It is the myosin protein, however, which holds the key to the differences in muscle fiber types. (Complicated lumps these muscles, aren’t they!) In general, there are four different fiber types in skeletal muscle. These four include: Type I, also known as slow-twitch or red fibers; Type IIA, IID, and IIB, also known collectively as fast-twitch or white fibers.

Type I are the slowest, smallest, and have the highest level of endurance of all the fibers. Next come the Type IIA, IID, and finally the Type IIB, which are the fastest, largest, and least endurance oriented in the group.

As you contract a muscle, you’ll recruit those muscle fibers in a specific order. The smallest (lowest threshold) fibers, the Type I, are recruited first. As the speed or force of contraction is increased, you will sequentially recruit the Type IIA, IID, and IIB muscle fibers. However, to recruit the Type IIB fibers it may take over 90% of a maximal contraction!

All people are born with these muscle fiber types. Most muscles contain almost an even split of slow (Type I) and fast (Type II) fibers. There are a few exceptions, however, as the soleus muscle of the calf is predominantly slow twitch, while the gastrocnemius muscle and the hamstrings are predominantly fast twitch. Also, individuals on opposite ends of the athletic spectrum like sprinters and marathon runners for example, may possess a higher percentage of one fiber type.

So what does this mean to all of us bodybuilders? Well, in order to obtain maximal muscle size we must regularly train all of our muscle fibers. A combination of bodybuilding (higher and lower rep), powerlifting, and even a little Olympic-style lifting may be best. Bodybuilding seems to stress the Type I and IIA fibers, while the IIB fibers may be best stimulated through powerlifting and Olympic lifting. The IID fibers are perhaps stressed equally by all three forms of training. I believe that the key to long term progress in bodybuilding can be summed up by one important word: VARIATION!

Doing the same three sets of 8-12 reps over and over will help you to a point. But after that, make a change! By selectively alternating your training volume (total sets/reps), training intensity (weight lifted), training techniques (forced reps, pre-exhaust, supersets, drop sets, etc.), training tempo (rep speed), rest between sets, bodypart arrangement, etc., you’re more likely to avoid injuries and overtraining and therefore maximize your gains. As long as the stimulus presented to your muscles is novel and taxing, they will continue to adapt accordingly.

Just remember that your muscle fibers are not static entitiesthey are dynamic and malleable. Therefore you yourself should not be static-don’t do the same thing over and over and expect a different result. Don’t get stuck in just one type of training program! Be creative! Be dynamic! Your muscles will reward you with continuous gains in size, strength, and endurance. Now, go get to work on those “lumps!”

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Eric Broser (gopro on the forums): Moderater of training, diet & nutrition, and supplementation on, is available for online personal training, dietary guidance, and contest prep coaching. For details send him a e-mail