Hamstring curl with stability ball

Flex the knee by drawing the heel in the required direction, then control the return. Keep the pelvis from shifting to create extra range. Prioritize control over speed. Maintain useful tension through the range and deliberately reset the starting position before the next repetition.

Do not turn the final repetitions into fast or partial movements. Aim to load abdominals, hamstrings, glutes and hip flexors and lower back without sacrificing position; at beginner level, progress control first and then add load, repetitions or complexity. On unstable surfaces, master the position with light resistance first; stability should limit difficulty before load does.