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Stability-Ball Single-Dumbbell Skullcrusher
Lie faceup on a stability ball with your upper back and shoulders in the center and your hips lifted to align with your knees and shoulders. Hold the ends of a single dumbbell with both hands over your chin. Bend your elbows and slowly lower the weight toward your forehead. When your elbows make 90-degree angles, extend your arms to return to the start.