The Art of the Contraction

For a muscle to contract, it must receive a signal from the brain that tells it what to do. These signals come from nerve cells called neurons. A neuron and the muscle fibers it controls are collectively known as a motor unit. Muscle fibers and motor units are generally classified into two types, slow twitch (type I) and fast twitch (type II), with fast twitch further divided into type IIa and type IIb. Slow-twitch motor units are used when low force production is required, and they can work for extended periods of time, whereas fast-twitch motor units are capable of greater force production than slow-twitch, but require greater stimulation before they will work, and fatigue much more rapidly. Think of slow-twitch fibers as cheap labor that will work all day but never produce quickly, while fast-twitch fibers need more money before they'll come to work, and they won't stay long but they'll get a lot of work done while they're there.

A key point to understand is that motor units are always recruited in size order, from smallest to largest (or slow twitch first, then fast twitch), regardless of the speed of the movement. Fast-twitch motor units are never used unless the slow-twitch fibers have been recruited first. This phenomenon is known as the size principle. If the force requirements of an activity are low, you will use slow-twitch fibers only, even if it's a quick movement like swatting a fly. If the force requirements are high, as in a maximal effort like trying to push a car (or lift heavy weights), you will use your fast-twitch fibers in addition to your slow-twitch fibers, even if you're moving very slowly.

Bar Talk

Each muscle consists of millions of myofibrils. Myofibrils are muscle fibers that work to contract the muscles of the body. They are what you're trying to build when you lift weights.

Bar Talk

Each muscle consists of millions of myofibrils. Myofibrils are muscle fibers that work to contract the muscles of the body. They are what you're trying to build when you lift weights.

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