One day after swimming front crawl, which works the triceps nicely, it all became clear. The triceps muscle has a curious shape at the top on account of its three 'heads' — or connecting origins. And the name derives from tri-ceps from latin meaning three-heads. And so it goes. The biceps is a two-headed muscle attaching with a long and short connection at the top. And the more complicated quadriceps has four heads that attach at the top, though they're a little harder to see.
Beyond this concept finally clicking for me it was fun to emulate some of the anatomical graphical style of Leonardo Da Vinci — perhaps the ultimate at sketchplanations? — which I have long admired. For inspiration, this is a cracking collection: Leonardo Da Vinci the Graphic Works.