Synergists: Biceps Brachii (especially the short or inner head), Brachioradialis
Attach a stirrup (handle) to a shoulder-high cable pulley.
Grasp the stirrup with one hand and step backward until the cable is pulled taut.
Stand with your loaded side to the pulley, with your arm horizontal, and your palm facing upward (supinated).
Keeping your upper arm horizontal, exhale as you flex your elbow and curl the stirrup toward your shoulder.
Hold for a count of two and squeeze your biceps brachii.
Inhale as you extend your elbow and return the stirrup to the starting position.
Repeat for the prescribed number of repetitions.
Repeat the exercise with your opposite arm.
Comments and tips
The one-arm overhead cable curl targets your brachialis, not your biceps brachii, which acts as a synergist, along with your brachioradialis.
The short (or inner) head of the biceps brachii is activated more than is the long (or outer) head. Since the short head contributes mainly to the width of your biceps brachii, use the one-arm overhead cable curl to build the width of your biceps brachii.
Start with your weak arm, and do not perform more repetitions with your strong arm. This will promote equal contralateral size and strength (equal size and strength on both sides of your body).