An abdominal strengthening exercise is a type of exercise that focuses on strengthening the muscles in the abdominal area. Situps and planks are two common exercises used to strengthen the abs. Situps involve lying on your back with your knees bent and feet flat on the floor, then lifting your upper body off the ground while keeping your lower back on the ground. Planks involve holding your body in a straight line while balancing on your toes and forearms. Both exercises help to build core strength and stability.