Position body on seat with back pressed against the back pad. Keep knees slightly bent and feet approximately shoulder width apart. Sit in good body alignment (abs tight, chest up, back straight). In a controlled motion, extend knees and push out. Keep knees in line with toes; avoid drawing knees in or out. While maintaining controlled motion, return to starting position (avoid locking out knees). Do not allow muscles to relax before next repetition.
Here are a few suggested alternative exercises:Ball Leg LungeBall LungeBall Wall SquatBand LungeBand Lying Leg ExtensionBand SquatBand Standing Leg ExtensionBarbell Bulgarian Split SquatBarbell Lateral LungeDead LiftDumbbell Dead LiftDumbbell Hack SquatDumbbell Lateral LungeDumbbell LungeDumbbell Lunge CrossoverDumbbell Lunge WalkingDumbbell SquatDumbbell Sumo SquatFront SquatHack Front SquatHack SquatLunge - BarbellLunge - Smith MachineLunge BackwardLunge CrossoverLunge DiagonalLunge ForwardSquat - Barbell