Modified Jackknife Sit-UpMain Muscle: Abdominals Mechanics Type: Compound Lie flat down on the floor with your arms beside your hips and your legs straight together. Using your abdominal muscles, simultaneously lift your arms horizontal to the floor as the legs and back lift off the floor. Pull the thighs toward the chest. Your knees should be at a 90-degree angle and your arms should be even with the lower part of your leg. Slowly lower, and straighten the body, until your arms and legs return to starting position. Focus on proper breathing and do not hold your breath. Do not swing the arms and legs and keep your body tight. Do not bend the arms. Keep the neck relaxed as much as possible.