Muscles gain strength by working against resistance. In the case of your core muscles -- the stabilizing muscles of the abs, pelvis and back -- the resistance is most often your own body weight. Using an ab wheel -- a wheel with handles on either side that you role out in front of you -- and doing traditional crunches are both effective abdominal strengthening exercises when used within a core workout routine. Learning the differences between the two helps you make the best choice for your body.
Your body is in two different positions during the crunch and the ab wheel roll-out. You are lying on your back with your knees bent and hands behind your head during the crunch, and the movement involves exhaling as you raise your head, shoulders and shoulder blades off the floor. With the ab wheel, you kneel on the floor or a mat with your hands on the bars, the wheel in front of your knees and your arms extended. You then roll the wheel away from you until your torso is flat or almost flat to the ground with your head, back and pelvis aligned.
You begin both exercises by contracting your abdominal muscles, tightening the transverse abdominis muscle to pull your navel toward your back. Both the crunch and the ab wheel also strengthen your rectus abdominis, the muscle located down the center of your abdomen, but in different ways. As you raise your torso during the crunch, the rectus abdominis contracts and greatly shortens to lessen the distance between your ribs and hips. With the ab wheel, your rectus abdominis performs an isometric, static, contraction throughout the exercise. As you roll the wheel toward you, your rectus abdominis contracts and slightly shortens to close the distance between your ribs and hips. Both exercises engage the obliques -- the sides of your core -- as stabilizers.
The most effective ab exercises avoid using other muscles to perform the movement, such as the hip flexors located on the fronts of your upper thighs that are sometimes engaged during abdominal exercises. Researchers at the Biomechanics lab at San Diego State University measured muscle activity in 13 different abdominal exercises. The results showed the most effective abdominal exercises were the ones that used the least amount of hip flexor activity. Hip flexor activity occurs when your hip angles decrease. During the crunch, your hip flexors remain the same, as your knees are bent and unmoving, however, during the ab wheel, your hip angles increase and decrease as you roll in and out, signifying greater assistance from your hip flexors.
The crunch is a calisthenic exercise you can perform anywhere and anytime. It is an easy, effective exercise for beginners and will improve the strength of your core. The ab wheel is more challenging than the crunch and requires you to already have a strong upper body in order to perform the movement properly as your shoulders, chest, back and wrists support the weight of your upper body as you roll the wheel back and forth. In contrast, during the crunch, the majority of your weight is supported by the floor. Both allow you to progress as your fitness level improves. For example, you can perform the crunch on an exercise ball to increase the challenge. To increase the resistance with the ab wheel, perform the exercise on your feet instead of your knees.