The human body can withstand much greater loads than its sheer material strength would suggest. Sprinters can push an Achilles tendon to more than 75 percent of its tensile strength, while weightlifters lifting a heavy barbell can push themselves to over 90 percent of the spine's load capacity. Individual collagen fibers tear, and the body repairs them before the entire tendon breaks down. "The true biological system is adaptive," says Sean Sun, Whiting School of Engineering professor of mechanical and biomedical engineering. "If you lift weights, you build muscle. That process adapts to whatever load your body is experiencing."
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