Knee extension supported on a stability ball

Extend the knee progressively and control the resistance on the way back. Keep the pelvis supported and avoid using torso momentum. Keep the same range from repetition to repetition instead of speeding up as you fatigue. Breathe continuously and end the set before technique starts to break down.

Choose a resistance that lets you preserve technique from start to finish. You should feel most of the work in abdominals, glutes and hip flexors, quadriceps and lower back; if you feel sharp pain, joint pinching or a clear loss of control, reduce the load or range and stop if symptoms persist. On unstable surfaces, master the position with light resistance first; stability should limit difficulty before load does.