Weighted seated bicep curl (on stability ball)

Sit on a stability ball with your feet flat on the ground and your back straight. Hold a medicine ball with an underhand grip, palms facing up, and let your arms hang down by your sides. Keeping your upper arms stationary, exhale and curl the medicine ball up towards your shoulders. Pause for a moment at the top, squeezing your biceps. Inhale and slowly lower the medicine ball back down to the starting position. Repeat for the desired number of repetitions.