The human body evolved expressly to move. We run, jump, and climb as soon as our tiny limbs allow it. But a select few thrill-seekers go much further, hungry to push beyond our species' more-mundane corporeal capabilities. The most extreme among us fling, squeeze, and stretch their bodies in the name of guts, glory, and even entertainment. Here's what all that abuse does to their agile-but-fragile frames.