Synergists: Biceps Brachii (especially the outer or long head), Brachioradialis
Sit on a bench with your legs apart, with a dumbbell in front of you.
Pick up the dumbbell with your right hand and rest your right arm against the inside of your right thigh. Your elbow should be lower than the top of your thigh so that your elbow doesn’t pivot, and your right arm should be almost fully extended, with your palm facing away from your right thigh.
Rest your left arm on your left thigh to support your back.
Exhale as you curl the dumbbell up towards your shoulder.
At the top of the movement, hold the contracted position and squeeze your biceps for a count of two.
Inhale as you lower the dumbbell to the starting position.
Repeat for the desired number of repetitions.
Repeat with your left arm.
Comments and tips
Keep your back straight and upper arm still. Only your forearm should move.
Do not swing the dumbbell.
Make sure that your elbow doesn’t pivot on your thigh and doesn’t fully lock out when you extend your arm.
Most people think that you isolate your biceps with the concentration curl. In fact, the exercise targets your brachialis. Your biceps (especially the long head) acts as a synergist, along with your brachioradialis.
The three flexors of your elbow are your biceps brachii, brachialis, and brachioradialis. If you curl with your elbow in front of your body, as in the concentration curl and the preacher curl, you target your brachialis, and your biceps and brachioradialis act as synergists. If you curl with your elbow by your side, as in the dumbbell curl, you target your biceps, and your brachialis and brachioradialis act as synergists. And if you curl with a neutral grip (palm facing to the side) or reverse grip (palm facing down), as in the hammer curl and reverse curl, you target your brachioradialis, and your biceps and brachialis act as synergists.
Your brachialis is the strongest flexor of your elbow. While most of it sits on the outside of your arm, part of it lies under your biceps. This means that if you build your brachialis, it will prop up your biceps and make it look bigger. It will of course also offer better assistance to your biceps.