Stand close to a vertical bar or any edge like a wall.
Keep your arm fully extended, with the thumbs up, and press your fingers against the
bar or wall, as shown in the picture).
Turn your body away from the wall, until you feel a stretch in the biceps.
Hold stretch for 20-30 seconds, then repeat with the other arm.
Tips
This exercise is difficult to grasp by beginners. Experiment with different
positions of the hand, and different turning angles, until you feel a stretch in the
biceps.