How to Perform the Drag 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.
Whether you're a top tier athlete or weekend warrior a beastly set of biceps will set your physique apart from the rest.
The drag curl is an isolation pull exercise targeting the biceps brachii, which is comprised of the short and long heads. The brachialis (lower bicep), brachioradialis (upper-outer forearm), posterior or rear deltoid, and anterior or front deltoid act as supporting muscle groups during this movement. 
Supporting muscle groups assist the target muscle group in completing the movement. The middle and upper trapezius, levator scapulae (rear neck), and wrist flexors act as stabilizers during this exercise.  Stabilizer muscles help maintain a posture or fixate a joint by contracting without significantly moving. 
The drag curl is an underrated and underutilized tool for building big and strong biceps.
How to Perform the Drag CurlYou can perform the drag curl using a barbell, EZ bar, cable apparatus, or smith machine. Although this walkthrough will focus on the performing the exercise with a barbell, you can apply the general form and tips to all setup variations.
Place a barbell on the floor or rack pins and select the appropriate working weight. 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 working weight will likely be less than the amount you use for traditional barbell curls.
don't place 10lbs on one side and 25lbs on the other. Uneven loading won't improve your gains and will likely lead to an injury. As a reference point the shorter barbell usually weighs about 30lbs and the longer barbell usually weighs about 45lbs.
Once you've selected the appropriate working weight approach the bar with a hip width stance. Pick up the bar from the ground or racks pins with a shoulder-width supinated grip (palms facing you). Taking too narrow of a grip will cause excessive internal rotation of the shoulders.
An excessively wide grip will decrease the exercises' range-of-motion. You can use a traditional grip (thumb wrapped around the fingers), hook grip (fingers wrapped around the thumb), or a false grip (thumb and fingers on the same side of the bar).
After setting your stance and grip, stand tall with an upright posture, chest high and shoulders retracted. Your arms should be straight but not hyperextended and the bar should be close to but not resting on your hips. This is the starting position.
Take a deep breath, brace your abdominals for impact and begin pulling the bar towards your lower chest. Throughout this pulling movement the bar should be kept close to your body and travel vertically upwards. This is in contrast to the semicircle arc motion performed during traditional barbell curls.
Your elbows should travel backwards and remain horizontally in-line with the wrists and shoulders during the drag curl. Keep your shoulders away from your ears to ensure it remains a curl rather than a shrug movement.
Continue curling the barbell upwards; just before the point where your elbows drift forward or shoulders roll in or raise. For most lifters this will be when the forearms are parallel with the ground and the barbell is slightly below the lower chest.
The term curl is a bit of a misnomer; your elbows shouldn't travel in front of your torso nor should the forearms be perpendicular at the top of the movement. Continue squeezing the barbell and flexing the biceps, holding at the top position for 1 to 5 seconds.
Once you've held the barbell at the top of the movement for the desired duration lower it 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.
Some lifters choose to exhale while curling the barbells, 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.
Drag Curl Form TipsMinimize Elbow Flaring - Ensure your shoulders, upper arms, elbows, and forearms remain horizontally in-line with each other throughout the entire movement. If you find yourself flaring your elbows and upper arms to complete the movement then the weight is too heavy. Minimizing elbow flaring will ensure the biceps are engaged to their maximum potential.
Avoid Momentum - The barbell drag curl provides maximum benefits when it's perform in a controlled full-range of motion. Check your ego at the door and don't immediately attempt to beat your traditional barbell curl repetition maximum.
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 barbell 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 drag curl for 5 to 10 seconds. Really focus on squeezing the biceps and gripping the barbell as tightly as possible. This will increase time under tension and the burn in the biceps. Increased time under tension is an excellent variable to adjust for progressive overload and enhanced muscle growth.
Increase the Bar Diameter - Experiment with thick bars and implements like Fat Gripz, which increase the diameter of the bar. An increased bar diameter improves grip strength, induces additional bicep and forearm hypertrophy, as well as removes stress from the elbows. Many trainees will elbow tendonitis find thicker bars to be extremely helpful in improving arm health.
References1) Griffing, James, et al. "Barbell Drag Curl." ExRx.net. N.p., 2015. Web.
2) Griffing, James, et al. "Kinesiology Glossary." ExRx.net. N.p., 2015. Web.