How Skeletal Muscle Fibers Work

Skeletal muscles are responsible for controlling the various parts of the body so that we can move, run, and jump. Even unconscious movements like breathing and moving our eyes around to scan the environment are controlled by skeletal muscles. There are different types of muscle fibers involved in these movements, depending on whether a sudden, quick contraction is needed or a more gradual contraction with longer endurance.

Skeletal muscle fibers: How muscles create force

No matter what purpose the skeletal muscle serves, all muscle cells use the same process to create force. Thick muscle fibers made of a heavy protein called myosin attach to thinner filaments made of a different protein, called actin. The myosin chains in the heavy filaments are actually enzymes that can connect, slide along the thinner fibers and then detach to repeat the process again. The energy for this process comes from the ATP that is generated by the mitochondria in the cells.

Skeletal muscle fibers: Fast twitch and slow twitch

There are two different types of muscle fibers: Fast twitch or slow twitch. This refers to how fast each type of muscle fiber can contract and generate force. While all muscle fibers are made of myosin heavy protein chains, there are variations in the type of myosin chains used in fast twitch fibers and those used in slow twitch fibers. Besides just the difference in speed, fast twitch skeletal muscle fibers differ from slow twitch in a couple of other ways. Because fast twitch fibers can contract quickly, they are able to create significantly more force than slow twitch skeletal muscle fibers. This increased force comes at a price, however, as fast twitch muscle fibers will fatigue much faster than slow twitch fibers.

Skeletal muscle fibers: Where the different types are used

The body must perform a variety of tasks, which is why it is crucial to have different types of skeletal muscle fibers. Slow twitch fibers are located in areas where endurance is more important than quickly generating power and force. Slow twitch muscle fibers are used for postural control and holding the spine and upper body steady and balanced over the legs. Fast twitch skeletal muscles are used for running, jumping, and other athletic movements that require a quick burst of power.

Most skeletal muscles have a combination of fast and slow twitch fibers, with one type being used depending on the task the muscle is performing. A few areas of the body have muscles where one type of fiber is used almost exclusively. For example, the muscles controlling the movement of the eyes and jaws are made of mostly fast twitch fibers. In the other hand, the soleus muscle in the legs are comprised mostly of slow twitch fibers.