Planks and crunches both work core muscles; however, the plank uses more muscles overall than the crunch does. These two core exercises use different types of muscle contractions, which can shape the muscles differently. Also, an exerciser with core weakness or joint limitations may struggle attempting the plank. There are benefits and drawbacks to both crunches and planks.
Benefits of the Plank
The plank is superior to the crunch in that it uses more muscle groups. This is true if you perform it correctly. Perform the plank on the balls of your feet and with your pelvis tilted downward to actively solicit 20 muscles in an isometric contraction. In isometric exercise, the muscles are held in contraction without motion in your joints. The benefit of isometric contractions is that you tone without bulking. This exercise provides you with strength in your legs, buttocks, back, stomach, shoulders and chest. Most importantly, strengthening your core stabilizes your spine, and your spine's health is vital to your overall health.
Drawbacks of the Plank
Joint limitations of the elbows, shoulders and feet can negatively affect your ability to correctly hold a plank position. If you are out of correct position, then you have compromised the effectiveness of the plank exercise. Furthermore, if you are weak in your abdomen and sag through your lower back while holding a plank, you have lost the toning of your core, the very thing you are trying to obtain. Because the plank uses isometric muscle contraction, it provides core muscle tone but does not provide as much muscle definition. The plank would not be your first choice if muscle definition is your goal.
Benefits of Crunches
Crunches solicit six main core muscles of the abdomen and legs -- seven if you are able to scoop your navel to your spine throughout the motion. The crunch is user-friendly because it can be modified to meet the needs of each individual. For instance, you can simultaneously lift your pelvis and torso to engage the lower muscle fibers of your abdomen. Add a medicine ball, held to your chest, for additional overload and enhanced muscular strength. Furthermore, the upward motion of your torso in a crunch is a concentric contraction: The muscle shortens during the action. The downward motion of your torso — if your torso is lowered slowly — is an eccentric muscle contraction: The muscle lengthens while under contraction. This type of muscle work produces more muscle definition; for some individuals, that would make the crunch exercise superior.
Drawbacks of Crunches
Crunches do not actively engage opposing muscle groups like the plank does. The crunch works your tummy and not your back. The health of your core and spine require strength in both. Therefore, if you are doing crunches, you need to perform an additional back exercise to receive similar core benefits. Another disadvantage of crunches is that they engage only six to seven muscles, while the plank engages 20 muscles. Crunches can be used as a progressive exercise to the plank. Then these two exercises would complement each other.