Voluntary, or skeletal, muscles do work by contracting in response to conscious control. Connected to tendons most, but not all, connect on their ends to bones on opposite sides of a joint. While all skeletal muscles do work via the same mechanism, the sliding of thick (myosin) filaments past thin (actin) filaments, they can be subdivided into groups dependent upon the speed at which the myosin protein works.
Basic Muscle Biochemistry
All muscle cells create ‘tension’ or force through the action of the actomyosin complex. Thick filaments, largely composed of the myosin heavy chain protein, are able to latch onto and slide along thin filaments, which are largely composed of the protein known as actin. Myosin heavy chains are all enzymes. They can attach, slide to generate force, and release to start the cycle over because they utilize their enzymatic activity and the energy stored in ATP, the near universal source of cellular energy.
Fast Twitch Versus Slow Twitch Fibers
While all muscle fibers use actomyosin to contract and generate force, there are differences in the types of myosin heavy chain protein used by different muscle fibers. In direct relationship to the type of myosin heavy chain that is used, muscle fibers can be characterized as either ‘fast twitch’ (or type II) or ‘slow twitch’ (or type I). Muscle twitch type is reflected in the speed of contraction, fast twitch contracts very rapidly, while slow twitch contracts at a slower speed.
Twitch Type Differences Are Not Limited to Speed
While there is a clear difference in the rate at which a slow twitch muscle fiber contracts versus the rate at which a fast twitch fiber contracts, there are more biochemical differences between these two types of skeletal muscle. For example, slow twitch muscle fibers fatigue a good deal more slowly than do fast twitch fibers. And while fast twitch fibers contract more rapidly they typically can generate more force than slow twitch fibers.
Separating Fiber Types in the Body
Not surprisingly, the differences in the biochemistry of different muscle twitch types lend them to be better able to perform different functions. Since slow twitch fibers tire more slowly they are often found in muscles important for postural control, things like holding the spine steady and keeping the legs planted firmly beneath the body when standing. On the other hand, fast twitch fibers are utilized for bursts of power and contraction. When you sprint you use the power generated by fast twitch muscles.
In the human body, individual skeletal muscles tend to have a mixture of twitch types. There are a few examples of where one twitch type predominates over another. The soleus muscle in the lower leg is almost entirely slow twitch fibers while some muscles that control the jaw and those that control eye movements are almost all fast twitch fibers. On the other hand, many of us have practical experience with the separation of twitch types in bird muscle. That Thanksgiving turkey, or that roasted chicken, has very clear separation of twitch types. Slow twitch muscles have many higher numbers of mitochondria and myoglobin and thus appear reddish or dark. This is not true for fast twitch, white meat.
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