The bicep brachii muscle at work.
You can also learn about how to stretch the biceps (not well known) and the various exercises to train them.
The short head of the bicep is the “peak of the biceps” and can be seen when performing a in which one contracts the biceps with the wrists facing up. The long head of the bicep is the part of the bicep that can be seen more when standing without flexing the arm.
