Alternating arms allows you to work each side of your body separately, which helps bolster muscle imbalances and forces your core to work even harder.
Grab a pair of dumbbells with an underhand grip (palms facing away from your body), and stand with your feet shoulder-width apart. Bend at your hips and knees, and lower your torso until it's almost parallel to the floor. Your lower back should be naturally arched. Keeping your abs in tight, bend your left elbow and pull the weight to the side of your torso. Slowly lower your left arm as you bend your right elbow and perform a row on the other side. That’s one rep. Continue alternating until you've completed 15 reps total.