You can describe someone who's fairly thin and athletic as wiry. While a linebacker on a football team is typically big and broad, a marathon runner is more likely to be wiry.
The adjective wiry can be used in two different ways: first, to mean "lean and strong," like a wiry seven year-old gymnast. The second meaning is "resembling wire." You could describe your thick, impossibly curly hair as wiry, or the crazy coat of your terrier as wiry. In the late 1500s the word wiry meant "made of wire."