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Why it's key: While the biceps curl machine, which has rigid arms, works your upper arms in a fixed path of motion, "a cable machine allows for multiple planes of motion and therefore uses more of those muscle fibers, which is better for developing strength," says Keith Spennewyn, owner of FitU Studio in Minneapolis. Also, standing up to do curls requires you to keep your balance, helping you tone your core.
How to use it: Set the adjustable pulley on the machine so it's as close to the floor as possible and curl the bar up toward your chest. Do a full extension (lowering the bar all the way down to your thighs), then a half extension (lowering it to waist level, so your elbows are bent 90 degrees), and count it as one rep, Esco suggests. "This will fatigue biceps quicker and ensure that the only thing moving is your elbow."