Stability-Ball One-Arm Triceps Kickback
Sit on top of a stability ball with your feet and legs about hip-width apart, and hold a single dumbbell in your right hand. Hinge forward from your hips with a straight back and rest your left arm across your thighs for support. Lift your right elbow to align with your torso, and hold your upper arm in place as you extend your elbow to press the weight toward the sky. Slowly lower to the start. Complete all reps on one side, then switch.