When I was in graduate school, I stumbled across a picture of a prone bridge abdominal exercise (i.e. "plank"), and the caption said it was a "Core Stabilizing Exercise". The next day I saw another magazine with the same plank exercise, but they called it a "Core Strengthening Exercise." Well, which is it? Strength and stability are two completely different ways of training.
As a result — in 2007, I wrote an exhaustive, research review article called "Core Training: Stabilizing the Confusion," in hopes of clarifying some of the common misperceptions with Core training.
And, here we are again. This time, the newest misperception is that abdominal crunches are the best way to strengthen the Core. I am not sure where this one came from, but until the Core is clarified these types of misperceptions will keep happening. So, here we go – hold on to your Core.
What is the Core?
The "Core" musculature can generally be defined as the 29 pairs of muscles that support the complex interactions of the lower back, pelvic and hip joints. The goal of the Core is to stabilize, support, and move this lumbo-pelvic-hip complex during movement.
The idea of training the Core came from a line of revolutionary research on low back pain. As a quick summary, in people with low back pain, the inner muscles that stabilize the spine were slow to activate, thus were unable to properly protect the spine during movement. Subsequent training with exercises that targeted these inner, stabilizing muscles helped rehabilitate low back pain.
The Core's Two Systems: Stability Versus Movement
This next distinction of the Core into TWO systems is important for deciding whether or not "crunches" are the BEST Core exercise.
The Stability System
The Stability System includes all the muscles that are primarily responsible for stabilizing the spine, such as the transversus abdominis (TrA) and multifidus. These were the same muscles that were highlighted in the original low back pain research mentioned above.
Exercise Choice: Low-intensity, endurance exercises with little to no movement of the spine and pelvis (e.g. planks, bridging). Designed to help with protecting and stabilizing the spine, and helping prevent/treat low back pain.
The Movement System
The Movement System includes the muscles that are in charge of producing movement and torque of the spine, such as the rectus abdominis, external obliques, and erector spinae. Though these muscles can help increase stability of the spine through intra-abdominal pressure, their primary goal is moving the spine.
Exercise Choice: Higher-intensity, dynamic exercises with movement of the spine through a full range of motion (e.g. rotations, wood chops, trunk flexion). Designed to translate to functional activities and performance — real world application.
Conclusion and Clarification
A common crunch exercise is done with low resistance and little movement of the spine. Thus, in this aspect — and if done properly, with good posture — the crunch can be a good exercise choice for training the Stability System (the small muscles that help keep the spine stable under little movement).
However, many choose a bigger, situp–like crunch exercise to target the outer, superficial muscles (e.g. rectus abdominis = "six pack") — trying to train the Movement System. It is difficult to keep good posture during this exercise, as many have to pull their head and neck forward. Also, it does not translate well to most functional movements needed in life and sport (except getting up off the floor, out of bed, etc).
Thus, in this way, the crunch is not a great choice for the Movement System, as it pales in comparison to other well-known Movement System exercises.
Finally, others choose a different type of Crunch®, which is not a great choice for the Core.