Look closely at the tiny muscle bulging on the forearm just below the elbow. That is the extensor digiti minimi, a small muscle that helps extend the little finger. Anatomically this muscle becomes especially active and visible when the little finger is extended. Notice how Moses is lifting his pinky? That is why Michelangelo sculpted that specific muscle in tension. This kind of insane biological accuracy came from his nights spent dissecting corpses at the hospital of Santo Spirito in Florence, where he traded a wooden crucifix and other artwork for access to dead bodies to master the mechanics of human flesh.