Sit upright on a stability ball with your feet flat on the floor and core engaged. Hold a weight (such as a medicine ball, dumbbell, or similar) with both hands at your sides and arms extended. Keeping your elbows close to your body, curl the weight up toward your shoulders by contracting your biceps. Pause and squeeze at the top, then slowly lower the weight back to the starting position under control while maintaining balance on the ball.
Log your weighted seated bicep curl (on stability ball) sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.