It's a depressing fact, but shortly after age 50, you begin to lose nearly 2 percent of your muscle fibers each year. Figure 1 is a graph of the average number of quadriceps muscle fibers present in adults ages 18 to 82.1 Looking at the center of the graph, you can see how the number of muscle fibers remain stable through your 30s and 40s. After that, muscle mass declines rapidly, dropping from 600,000 muscle fibers at age 50 to 320,000 muscle fibers by age 80. 

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