What is Muscle?
The Oxford Dictionary defines muscle as “a band of fibrous tissue in the body that has the ability to contract, producing movement in or maintaining the position of a part of the body”.
Humans have three different types of muscle fibers: Type I, Type IIA, and Type IIB.
Type I fibers are called slow twitch fibers and are characterized by a slow contraction time and a high resistance to fatigue. Structurally, they have a small motor neuron and fiber diameter, a high mitochondrial and capillary density, and a high myoglobin content. Energetically speaking, they have a low supply of creatine phosphate (which is used for quick movements), a low glycogen content, and are loaded with triglycerides. Functionally, Type I fibers are used for aerobic activities that require low levels of force, such as walking, maintaining posture, and long distance running.
Type IIA and Type IIB are both considered fast twitch fibers. Type IIA fibers have a moderate resistance to fatigue and are like a transition between Type I and Type IIB fibers. Structurally, Type IIA fibers have a large motor neuron and fiber diameter, a high mitochondrial density, a medium capillary density, and a medium myoglobin content. Energetically, they have a large supply of creatine phosphate and glyocogen and a medium amount of triglyceride stores. Functionally, Type IIA fibers are used for prolonged anaerobic activities that require a relatively high force output, such as a 400 meter race.
Type IIB fibers are very sensitive to fatigue and are used for short anaerobic, high force production activities, such as sprinting, hurdling, and jumping. Like Type IIB fibers have a large motor neuron and fiber diameter, but have a low mitochondrial and capillary density and myoglobin content. They have large stores of creatine phosphate and glycogen, but low stores of triclycerides. Type IIB fibers are used for the most activities that require the highest output of force, such as heavy weight lifting.
At any given velocity of movement, the amount of force produced depends on the fiber type that is utilized. During a dynamic contraction, when the fiber is either shortening or lengthening, a fast twitch fiber produces more force than a slow twitch fiber. During an isometric condition, when the length of the fiber does not change while contracting, both fast twitch and slow twitch fibers produce the same amount of force.
Muscles produce force by recruiting motor units. A single motor neuron innervates a number of individual skeletal muscle cells. This is called a motor unit, and a depiction of one is shown below. The number of muscle cells in a motor unit is inconsistent between different muscles, but each motor unit contains the same type of muscle fibers. The force generated by a whole muscle can be varied physiologically by firing different numbers of motor units and by changing the firing frequency of any motor unit
During voluntary contractions, the pattern of recruitment is controlled by the size of the motor unit. Small motor units, which contain slow twitch muscle fibers, have the lowest firing threshold and are recruited first. The largest motor units that contain Type IIB fibers have the largest threshold and are recruited last. This ordered pattern of recruitment is called the size principle.
References:
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Interesting ideas… I wonder how the Hollywood media would portray this?