The biceps curl is a fundamental exercise targeting the biceps muscles, the muscles on the front of your upper arm. No other muscles are directly involved in a biceps curl, although several muscles assist during the movement. During the curl exercise, the biceps muscle shortens then lengthens to control the movement of the weight.
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The biceps curl can be done with a barbell, resistance band, dumbbells or a cable machine. Although the type of resistance changes, the movement is the same. The exercise starts with your arm straight down at your side, your palm facing forward. Bend your elbow to curl the weight up until your palm is almost touching your shoulder. Your upper arm should stay locked in place next to your side with your elbow pointing toward the floor.
A muscle contracts when the muscle fibers create tension against an external resistance. Your biceps muscle contracts both concentrically and eccentrically during the biceps curl. When you curl the weight up, the biceps muscle creates enough force to overcome the external resistance. During this phase of concentric contraction, the muscle fibers shorten, pulling at the elbow joint and lifting your forearm. When you lower the weight, the muscle fibers do not create enough force to overcome the external resistance. The fibers lengthen during this eccentric phase to control the descent of the weight.
The biceps curl is a single-joint, isolation movement that only targets the biceps muscle, which runs from your shoulder to your forearm. However, other muscles, including your brachialis and brachioradialis, small muscles in your upper arm and forearm, assist the biceps muscle. Muscles in your upper back contract without shortening or lengthening in order to stabilize your torso, and muscles in your forearm contract without movement to stabilize your wrist joint.
In order to complete a proper bicep curl and fully isolate your biceps, your shoulder should stay stable throughout the movement. If you lift the weight from your shoulder, this involves the anterior deltoid, or front shoulder muscle, in the movement. Keep your torso upright during the movement. Don't arch your lower back to propel the weight up, as this can cause lower back pain, and it diminishes the work of the biceps muscle.