Not only does supination and pronation go on at the wrist end of the radioulnar joint, but the radiocarpal joint is an ellipsoidal joint that is capable of circumduction—i.e., you can flex and extend your hand at the wrist and also move it from side to side as in abduction and adduction. But the book only covers flexion and extension at the wrist, so here's that:
What movements happen at this joint?
Flexion is when you pull your hand toward the inside of your forearm, decreasing the angle between the two segments—i.e., you bend your wrist. Extension is when you straighten your wrist. That is, you increase the angle between your forearm and your hand by pulling your hand toward the back of your forearm.
What muscles make these movements happen?
The muscles that flex your hand at the wrist are your flexor carpi radialis, flexor carpi ulnaris, and palmaris longus and the muscles that extend your hand at the wrist are your extensor carpi radialis longus, extensor carpi radialis brevis and extensor carpi ulnaris. Very easy to remember and understand, the flexors are anterior muscles that pull the hand toward the inside of the forearm when they contract and the extensors are posterior muscles that pull the hand toward the back of the forearm when they contract.
What exercises make these muscles work?
Wrist flexors can be exercised with wrist curls and grip strengthening exercises, and wrist extensors can be exercised with reverse wrist curls and racquet sports such as tennis.
My wrist flexors and extensors are pretty weak, I know this because they're what burn out when we have to carry those boards loaded with skatecourt tiles down the stairs for track teardown.
