Abdominal crunches are a popular exercise designed to strengthen the abdominal muscles. To perform a crunch, lie on your back with your knees bent and feet flat on the floor. Place your hands behind your head or across your chest, then lift your upper body towards your knees while keeping your lower back on the ground. This movement primarily targets the rectus abdominis, the muscle responsible for the "six-pack" appearance. Crunches can be modified for different fitness levels, making them accessible for beginners and more advanced individuals alike. Regular practice can improve core strength and stability.