What do the biceps do?
The biceps runs down the front of the upper arm. It bends the elbow and turns the palm to face up, the movement you use to turn a key or a screwdriver. Two other muscles, the brachialis and the brachioradialis, help it bend the elbow.
How do you train your biceps?
Any curl trains the biceps: bending the elbow against resistance while the upper arm stays still. Rows and pull-ups also work the biceps hard, so if you already do those, a couple of curl variations are enough. Palm-up curls favour the biceps, and thumbs-up hammer curls bring in the brachialis and forearm.