Double crunches, which require no special equipment, are calisthenics that strengthen and tone the abdominal, or ab, muscles.
This exercise is the basic ab crunch combined with a hip-lifting movement. Beginners should first become proficient at ab crunches before gradually working up to double crunches.
To do double crunches, lie on your back with your ankles crossed, knees bent and apart. Place your fingertips behind your ears, lightly supporting the head. Throughout the exercise, keep your chin off your chest, as if you have an orange under your chin.
Exhale as you contract your abs to lift head, neck, shoulders, and torso up and forward. Move until your shoulder blades are off the floor. Simultaneously, contract your abs to lift, or "curl," your hips until your buttocks and tailbone are off the floor. Pause briefly, then slowly lower yourself to the start position. Breathe regularly throughout the exercise. Each double crunch should take about four counts. Repeat, building to 20 repetitions.