Leg extension and arm 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. Use a steady tempo: control the return phase in particular and avoid relying on momentum. Exhale through the hardest part and inhale as you recover the starting position.

Check that the movement stays stable in both directions and that the torso is not compensating. The goal is controlled loading of abdominals, glutes and hip flexors and lower back; match resistance and range to a beginner level and never force a painful repetition. On unstable surfaces, master the position with light resistance first; stability should limit difficulty before load does.