Something muscular, with a tight and stretched toughness, is sinewy. Tennis players' lean arms have a sinewy beauty, all the muscles showing as they hit their smoking serves.
A sinew is a tendon that attaches muscles to bones, and something sinewy has a lot of sinew or shows a lot of built-up tissues. If you're a carpenter or a piano player, you probably have well-exercised, sinewy hands and fingers. Lines in nature, such as tree limbs and roots, can be sinewy too. Being sinewy is attractive when it's in good health, but it can also make you look too stretched or thin, even scrawny. Tough meat can be sinewy and unchewable.