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The key to building muscle is protein synthesis and cellular repair. Skeletal muscle is made up of long fibrous chains, containing proteins.
Bundles of thousands of fibers make up the muscle itself. When an athlete engages in a heavy workout this structure is damaged, causing microtears in the muscle fibers. The body naturally repairs this damage by mending the torn fibers. The result is a muscle that's larger and stronger than before. And with each additional workout, the athlete adds bulk, strength and speed.