Sit on a bench with your legs apart and a dumbbell in one hand. Rest your elbow on the inside of your thigh, near your knee, with your palm facing up. Curl the dumbbell towards your chest, focusing on contracting the bicep. Slowly lower the dumbbell back to the starting position. Perform the exercise on one arm, then switch to the other. For maximum effect, keep your upper arm stationary and avoid using your body to create momentum.