Anatomy & Physiology

Anatomy & Physiology Study Guide

©2018 Achieve Page 110 There are three major types of skeletal muscle fibers: slow fibers, intermediate fibers, and fast fibers. Most of the skeletal muscle fibers in the body are called fast fibers and reach peak twitch tension in 0.01 sec or less after stimulation. Fast fibers fatigue rapidly because their contractions use ATP in massive amounts, and there are relatively few mitochondria to generate ATP. Slow fibers have only about half the diameter of fast fibers and take three times as long to reach peak tension after stimulation. They continue to contract for extended periods of time due to specialized fibers. Most properties of intermediate fibers are between those of fast fibers and slow fibers. In appearance, intermediate fibers most closely resemble fast fibers, for they contain little myoglobin and are relatively pale. They have a more extensive capillary network around them, however, and are more resistant to fatigue than fast fibers. Muscle Hypertrophy and Atrophy Muscle fibers will develop more mitochondria, a higher concentration of glycolytic enzymes, and larger glycogen reserves because of repeated stimulation. Such muscle fibers have more myofibrils of 368 Muscle Tone Even when the entire muscle is not in a state of contraction, some motor units are always active. Their activity does not produce enough contraction to cause movement, but they keep the muscle tense and firm. This resting tension in a skeletal muscle is called muscle tone. A muscle with little muscle tone appears limp and flaccid, whereas one with moderate muscle tone is firm and solid. Resting muscle tone stabilizes the positions of bones and joints. For example, in muscles involved with balance and posture, enough motor units are stimulated to produce the tension needed to maintain body position. Muscle tone also helps prevent sudden, uncontrolled changes in the positions of bones and joints. In addition to bracing the skeleton, the elastic nature of muscles and tendons lets skeletal muscles act as shock absorbers that cushion the impact of a sudden bump or shock. Isotonic nd I ometric Contractions Muscle contractions can be classified as isotonic or isometric by their pattern of tension production. Isotonic Contractio s In an isotonic contraction, tension rises and the skeletal muscle’s length changes. Lifting an object off a desk, walking, and running involve isotonic contractions. Two types of isotonic contractions exist: concentric and eccentric. In a concentric contraction , the muscle tension exceeds the load, and the muscle shortens. In an eccentric contraction , the peak tension developed is less than the load, and the muscle elongates owing to the contraction of another muscle or the pull of gravity. Isometric Contractions In an isometric contraction, the muscle does not change length, and the tension produced never exceeds the load. Although cross-bridges form and tension rises to peak values, the muscle cannot overcome the resistance of the weight and so it cannot shorten. Examples of isometric contractions include carrying a bag of groceries and holding the head up. Many of the reflexive muscle contractions that keep the body upright when standing or sitting involve isometric contractions in opposition to the force of gravity. Types of Skeletal Muscle Fibers