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		d. Effect of exercise Training can alter the composition of a muscle. Weight training will cause the preferential hypertrophy (thickening) of FG fibers. This thickening is caused by the increased production of contractile proteins and the fusion of additional satellite cells. In addition, there are actually 2 different fast myosins in humans (IIa and IIx). Fibers that contain IIx are even faster than those that contain IIa. Certain training regimes can cause a switch in expression of these 2 genes. A regime of vigorous resistance-training followed by a relatively inactive period stimulates the production of IIx myosin and very fast muscle fibers. e. Injury and age SO fibers appear to require continuous stimulation by neurons; most of the loss of muscle mass in paralyzed patients comes from loss of SO fibers. Aging aso causes a loss in muscle mass (approzimately 50% by age 80). This is caused by a loss of muscle fibers to cell death. At the same time, the distribution of FG and SO fibers changes. Instead of being interspersed, they occur in patches. One explanation for this is that when a motor neuron dies, its associated muscle fibers also die, unless they can form a synapse with an adjacent motor neuron. However, SO and FG fibers are associated with different classes of neurons and this is part of their identity. If they form a synapse with the wrong type of neuron, they start to produce the alternate form of myosin, creating a mixed fiber. A recent study showed that, while fewer than 5% of fibers from young volunteers were mixed, about one third of fibers from elderly subjects were mixed. Discussion Question Myoglobin also contains a heme group, so it makes muscle red when living and dark when cooked. Which type of fibers contain myoglobin? Why is the breast meat of geese and ducks dark while that of chickens and turkeys is "white"? 4. Types of muscles in fish SO and FG fibers are even more segregated in fish who have two separate muscle systems - red and white muscles. The red muscles lie parallel to the body axis, while the white muscles wrap around the fish's body in a spiral. The two muscle systems are used for 2 completely different swimming motions.