Abdominal crunches are a classic core exercise. Try a set today! Lie on your back. Place your feet on a wall so that your knees and hips are bent at 90-degree angles. Tighten your abdominal muscles. Raise your head and shoulders off the floor. To avoid straining your neck, cross your arms on your chest rather than locking them behind your head. Hold for three deep breaths. Return to the start position and repeat.