Most biceps curls do recruit the shoulders, but as stabilizers.
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If you're looking to specifically strengthen your shoulders, biceps curls won't be the most effective exercise for you to incorporate into your workouts. During most curls, one of the major shoulder muscles is recruited, but it acts as a stabilizer. However, the primary muscle recruited during biceps curls is also recruited during some movements at the shoulders. Therefore, while biceps curls shouldn't be the primary exercise you use to strengthen your shoulders, they can be incorporated in a rotator cuff- and shoulder-conditioning program.
While bicep curls tone your upper arms, they aren't the best choice for strengthening your shoulders.
Look at Biceps Curls
Biceps curls are most commonly performed from a standing position and with either a pair of dumbbells or a barbell. Stand and hold your arms down by your sides, with your palms facing forward. If you're using a barbell, grip the bar with your hands at shoulder-width. Keep your elbows into your torso as you bend them to lift the weights up to your shoulders. Straighten your arms to lower the weight back to your legs and repeat.
Assess the Muscles
The biceps curl exercise, whether performed with a barbell, dumbbells or a cable pulley unit, primarily works your biceps brachii, which is the major muscle at the front of your upper arm. Your brachialis and brachioradialis, two muscles in your arms, assist the biceps in bending your elbows against resistance.
Recruit the Shoulders
The major muscle in your shoulders is your deltoid, which features three separate heads. The anterior head of the deltoid is located at the front of your shoulder and it's primarily responsible for flexing your shoulder, or lifting your arm forward. During most biceps curls exercises, the anterior deltoid is recruited to isometrically contract so that it can help keep your shoulders in a static position as you curl the weights. However, the anterior deltoid is not recruited during dumbbell concentration curl or preacher curls. This is because during these exercises your elbow is pressed against either your thigh or the arm pads, securing the static position of your shoulders.
Give Some Credit to the Biceps
While your biceps brachii muscles are primarily responsible for bending your elbows, they also contribute to shoulder flexion. According to ExRx.net, the biceps muscle is a weak shoulder flexor, but it is recruited to help out your anterior deltoid as you lift your upper arm in front of you. This is likely why the American Academy of Orthopaedic Surgeons recommends incorporating biceps curls into a rehabilitation workout program designed to strengthen the rotator cuff and shoulder.