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Chronaxie varies across different types of tissue: fast-twitch muscles have a lower chronaxie, slow-twitch muscles have a higher one. Chronaxie is the tissue-excitability parameter that permits choice of the optimum stimulus pulse duration for stimulation of any excitable tissue.
“The proportion of slow-twitch to fast-twitch muscle fibers in a particular muscle varies depending on the function of the muscle and the type of training in which an individual participates.
Type I, slow-twitch, slow oxidative, or red muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity. Type II, fast-twitch muscle, has three major kinds that are, in order of increasing contractile speed: [7] [8]
The fast twitch fibers rely on a well-developed, anaerobic, short term, glycolytic system for energy transfer and can contract and develop tension at 2–3 times the rate of slow twitch fibers. Fast twitch muscles are much better at generating short bursts of strength or speed than slow muscles, and so fatigue more quickly. [36]
[9] [10] This contributes by increasing the muscle's shortening velocity. Other studies have shown that particular muscle fiber types are favored in sprinters versus non-sprinters, as well as within different levels of sprinters. Faster individuals tend to have a greater percentage of Type II (fast-twitch) muscle fibers.
The soleus muscle is composed of "red" muscle which was revealed to indicate that muscle fibers were fatigue-resistant but created small forces when contracting. The gastrocnemius muscle is heterogeneous, composed of both "red" and "pale" muscle, and thus containing fast-twitch high force fibers.
Learn how fast-twitch vs slow-twitch muscle fibers influence your performance. Plus, what to know about the role your genetics and training play.
The muscle fibers belonging to one motor unit can be spread throughout part, or most of the entire muscle, depending on the number of fibers and size of the muscle. [ 2 ] [ 3 ] When a motor neuron is activated, all of the muscle fibers innervated by the motor neuron are stimulated and contract.