Stand holding a pair of dumbbells by your sides using a neutral grip (palms facing in).
Pull your shoulders back.
Exhale as you slowly curl one dumbbell up towards your shoulder. As the dumbbell rises, supinate your forearm gradually so that your palm faces your shoulder by the top of the curl.
Once your elbow is fully flexed, allow your elbow to move forward a little (shoulder flexion), just until your forearm is vertical.
Hold for a count of two and squeeze your biceps.
Inhale as you slowly reverse the movement at half the speed and return the dumbbell to the starting position.
Repeat the same action with the opposite arm, and then alternate repetitions.
Comments and tips
Keep your back straight and your body still. Avoid swinging the dumbbells.
The biceps brachii flexes the elbow, supinates the forearm, and weakly flexes the shoulder joint. The above execution (steps 1 and 2) incorporates all three movements, thus (arguably) promoting maximal biceps recruitment.
The biceps is the primary elbow flexor when your forearm is supinated. When pronated or in neutral grip, your brachioradialis is the primary elbow flexor. Therefore, some instructors advise that you supinate early on the way up and return to neutral grip late on the way down. This, they argue, ensures that you keep the tension on the biceps instead of the brachioradialis. However, others argue that if you supinate gradually, as described above, the supination has to fight against more gravitational resistance, thus making the biceps work harder.
Instead of alternating your arms, as described above, you can complete all the reps for one arm before performing the reps with the other arm. In this case, start with the weaker arm, and don’t perform more reps with the stronger arm. This will promote the development of equal strength.