How to Perform the Hammer Curl
The bicep brachii comprises roughly one-third of the upper arm and is comprised of the long and short heads. The brachialis, also referred to as the side arm or lower bicep, is commonly considered part of the bicep muscle group even though it's technically different.
The biceps are primarily built through pulling movements such as chin-ups and elbow flexion exercises such as curls. Most bicep movements are performed in vertical plane although the torso angle may also be slightly backward (e.g. incline dumbbell curls) or forward (e.g. concentration curl) depending on the exercise.
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The hammer curl is an isolation pull exercise targeting the brachioradialis, also referred to at the upper-outer forearm. The biceps brachii (short and long heads) and brachialis (lower bicep) act as supporting muscle groups during this movement. 
Supporting muscle groups assist the target muscle group in completing the movement. The anterior or front deltoid, middle and upper trapezius, levator scapulae (rear neck), flexor carpi radialis (wrist flexors), and extensor carpi radialis (wrist extensors) act as stabilizers during this exercise.  Stabilizer muscles help maintain a posture or fixate a joint by contracting without significantly moving. 
The hammer curl is an excellent exercise for building both forearm and bicep size and strength.
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How to Perform the Hammer Curl
Approach the dumbbell rack and select the appropriate working weight. This initial working weight may be slightly higher compared to what you use for dumbbell bicep curls. Ensure you select two dumbbells of equal weight. don't lift 30lbs with your left arm and 50lbs with your right arm. Uneven loading won't improve your gains and will likely lead to an injury.
If this is your first time performing the exercise then pick a conservative weight that you can safely lift for 8 to 12 repetitions. This write-up will discuss the dumbbell variation of the hammer curl but this exercise can also be performed using a cable apparatus or hammer curl bar.
Once you've selected the appropriate working weight grasp the dumbbells, pick them up and bring them to your sides. Take a traditional neutral or hammer grip (thumbs wrapped over the fingers with your palms facing in and towards each other).
At this point you have two options - perform the movement either seated or standing. If performed standing then take a hip-width stance, ensuring your glutes and abdominals remain squeezed throughout the entire movement.
If you prefer to be seated ensure your upper back and glutes remain on the padded supports throughout the entire movement. In both instances ensure your chest is high, shoulders are down and away from the ears, torso is upright, and arms are straight (but not hyperextended). This is your starting position.
After setting your starting position take a deep breath, brace your abdominals, squeeze the dumbbells as hard as possible and begin pulling your hands towards your shoulder. During this arc motion your elbows and upper arms should remain in a fixed location. Do not allow the upper arms or elbows to drift out of place; doing so will take the stress off the target muscle groups.
Continue curling the dumbbell until your forearms are in between parallel and perpendicular with the ground. The precise top position will vary with the individual, depending largely on your upper arm strength, form, and shoulder flexibility.
While some trainees prefer to curl the dumbbell until the forearm is perpendicular with the ground, this may require your elbow to drift forward or shoulder to roll-in or raise. This change in elbow and shoulder location will take stress of the target muscle groups and if not corrected during the lower portion of the repetition, will quickly lead to form breakdown.
Your goal should be to curl the weight towards your shoulder as much as possible, stopping just before your elbows and shoulders change positions. This cue will maximize the engagement of the target muscle groups. Continue squeezing the dumbbells and flexing the biceps, holding at the top position for 1 to 5 seconds.
Once you've held the dumbbells at the top of the movement for the desired duration lower the dumbbells in a slow and controlled motion back to the starting position. The movement pattern for the lowering portion should be the exact reverse of the pulling portion. Complete for the desired number of repetitions.
You can perform this movement curling one dumbbell at a time (alternating) or curling both dumbbells at the same time (simultaneous). Some lifters choose to exhale while curling the dumbbells, at the top of each rep, or in between in each repetition. Choose a breathing pattern that feels the most natural and comfortable for you.
This exercise can be performed using straight sets, pre-exhaust sets, drop sets, rest-pause sets, supersets, trisets, giant sets, paused reps, partial reps, forced reps, or slow negatives. As with any exercise, the two most important components are high-quality form and progression. Progression can take a variety of forms (e.g. more weight, sets, or reps, decreased rest period, improved rep quality, etc?) but strive to improve every training session.
Hammer Curl Form TipsMinimize Elbow Flaring - In addition to using conservative working weights on this exercise ensure your upper arm and elbows stay close to the torso throughout the entire movement. If you find yourself flaring your elbows and upper arms to complete the movement then the weight is too heavy. Keeping your elbows in-line with your shoulders will maximize brachioradialis and bicep stimulation.
Avoid Momentum - Perform the hammer curl in a controlled full-range of motion. Check your ego at the door and don't immediately attempt 100lbs. Stay tight throughout the movement (abdominals and glutes squeezed) and don't allow your shoulder to roll-in or move up towards your ears.
Do not use momentum to swing the dumbbell from the bottom to the top position. This momentum dramatically increases the likelihood of injury and minimizes the stimulus of the target muscles.
Hold the Squeeze - Increase intensity by holding the top position of the dumbbell hammer curl for 5 to 10 seconds. Really focus on squeezing the biceps and gripping the dumbbell as tightly as possible.
This will increase time under tension and the burn in the brachioradialis and biceps. Increased time under tension is an excellent variable to adjust for progressive overload and enhanced muscle growth.
ReferencesGriffing, James, et al. "Dumbbell Hammer Curl." ExRx.net. N.p., 2015. Web.
Griffing, James, et al. "Kinesiology Glossary." ExRx.net. N.p., 2015. Web.