The dumbbell curl exercise involves flexing and extending your elbows to move dumbbells from near your hips to in front of your shoulders and back down. Performing any variation of the exercise on a regular basis, as part of a comprehensive resistance-training program, strengthens the muscles in the upper arm while also providing some toning benefits to the forearm.
The brachialis is known as "the workhorse of the elbow," according to Susan Hall, author of "Basic Biomechanics," because it serves as the primary elbow flexor. This muscle works just as much when your forearm is pronated, as with reverse dumbbell curls when your palms facing downward, as with traditional dumbbell curls with your palms facing upward. The muscle attaches to the lower half of the humerus bone of your upper arm on top and to the ulna bone of the forearm, just below your elbow joint, on the bottom.
The biceps brachii muscle contributes to elbow flexion most effectively when your forearm is supinated, according to Hall. Therefore, it helps the brachialis considerably when you perform the traditional dumbbell-curl exercise, which, is also referred to as the biceps curl. The biceps brachii is also the most externally visible of the elbow flexors and thus receives a lot of attention from the bodybuilding community. The muscle is divided into long and short heads, with both attaching to the scapula bone, just inside the shoulder joint, on top, and to the radius bone of your forearm on the bottom.
The brachioradialis muscle contributes to elbow flexion when your forearm is in a neutral position -- between full pronation and supination. Therefore, the hammer curl, which involves flexing your elbows with your palms facing each other, is a dumbbell-curl variation that targets the brachioradialis. The muscle attaches to the lower potion of the humerus bone, just above your elbow joint, on top, runs down the length of your forearm and reattaches to the radius bone at your wrist.
The pronator teres muscle, which is primarily responsible for pronating the forearm, also contributes to elbow flexion, playing an assisting role when you perform dumbbell curls. Like the biceps brachii, the pronator teres consists of two heads: the humeral head, which attaches to the bony protrusion on the inside of your elbow, and the ulnar head, which attaches to the ulna bone of your forearm, just below your elbow joint. Both heads combine at the opposite end, attaching about halfway down your forearm to the outside surface of the radius bone.
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