First off, few can accurately define "core" as it’s often used in the common vernacular interchangeably with “abs”. “Abs” (itself an often misused term) is just part of the “core”. Core is most properly defined as muscles of the torso. So, in fact, a bench press is a “core” exercise as it uses, primarily, the pectoral muscles. More functionally “core” should be defined as muscles of the torso that “stabilize a body segment so that another body segment can generate power.”(1) Understanding this is key to an optimal approach to core training.
For the purposes of this post I will refer to core as meaning specifically muscles of the abdominal wall (primarily external & internal obliques, rectus & transverse abdominals).
What is the primary function of these muscles? Once we determine that we can then identify the exercises that most address this need (functional training!).
Functions of the Core
We ask a lot of our spine. It needs to bear significant load and be flexible enough to allow for optimal upper body function. As Dr. Stuart McGill of Waterloo University points out:
The spine is a stack of vertebrae that is asked to bear loads, yet it is flexible. A design engineer will tell you that you can’t design a structure to be good at both. A steel beam that is straight and stood on its end is stiff, and can bear loads that try to compress, shear and twist it. So the beam can bear load but it can’t move. A flexible rod that allows movement will bend and buckle under load, but absorbs shock. Our spines do it all. (2)
Stability If you look at human skeleton we have a fairly large structure in the thoracic cage and shoulder complex and a large structure in the pelvic bone. Connecting the two you have nothing but the lumbar spine – so the primary purpose of the abdominal wall is stability.
…..The spine is this beautiful structure that is flexible and allows flowing movement, but requires a 3‐dimensional guy wire system to stiffen and stabilize it when it is require to bear loads. Analysis of the muscular system, together with its associated fascia sheets reveals a clever guy wire system that creates balanced stiffness eliminating the possibility of buckling and injury. The concern is that modern living does not “tune” and train this guy wire system. In many people it lapses into complacency. (3)
Rotation The oblique muscles rotate the spine. Therefore second purpose is rotation.
Flexion Lastly, and almost incidentally, is flexion. But I, and many others, would argue that the spine flexes in order to be flexible and allow for other upper body movement. It is not meant to be a prime mover and therefore should not be trained as such.
So when analyzing which “core” exercises are best we must apply three aspects:
- Effectiveness – does the exercise target the correct muscle(s)? Typically we put too much emphasize on this factor; failing to account for other factors.
- Functionality – does the exercise work the muscle is a manner optimally desirable for functionality (does it apply for real-world needs)?
- Safety – does the exercise have any acute or chronic negative impact?
So, to crunch or not to crunch?
Crunches are easily the most popular core exercise. Firstly if done correctly (a big “if” actually) the crunch does indeed target the primary muscle responsible for spinal flexion – the rectus abdominus. Electromyographic analysis shows significant activation of this muscle during the crunch. So in terms of activating the desired muscle the crunch is effective.
Functionality is questionable. The spine flexes as a matter of necessity, in other words it’s flexible to allow for other upper body movement (can you imagine if it was stiff?). The primary muscle of spine flexion is the rectus abdominus. It, along with other muscles of the abdominal wall, is meant primarily as a stabilizer (its role is to stiffen and protect), and not as a prime mover. Therefore, should not be trained as such.
So are crunches (and other spinal flexion core exercises) safe? No. Training with significant, and often ballistic, spinal flexion movement is dangerous (4) – in his book Low Back Disorders Dr. Stuart McGill explains that “repeated spinal flexion even in the absence of moderate loads will lead to troubles. Evidence shows that this leads to annulus damage, and an eventual disk bulge.”
Note the emphasis on the load required. Contrary to much counter-argument spinal flexion does not need to occur with significant load in order to increase risk of injury – low resistance (i.e. body weight) is enough as “when you perform a crunch or a sit-up, you are mimicking the exact motion for a disk bulge or a herniation.“(5)
Additionally, once we establish the function of a muscle training it in a manner outside that functionality can lead to dysfunction and increase risk of injury (e.g. training a stabilizer muscle as a prime mover). Though, to be certain, the primary safety concern with spinal flexion exercises is the effect on the vertebrae.
As an aside, spinal flexion exercises such as crunches, sit-ups, leg raises etc have the additional detriment of strengthening the hip flexors (in some cases, more so than the targeted rectus abdominus). Why is this bad? The hip flexors (primarily the iliopsoas complex) already tend to be overworked and hyperactive due to being in a constant state of flexion. If you think about how the average person spends their day: seated at a desk, in the car, and on the couch. The hips and knees are flexed for several hours a day, this results in tight (“short”) and/or hyperactive hamstrings and hip flexors. So not only is it ill advised to do spinal flexion exercises due to the impact on the vertebrae but they also exacerbate the chronically tight/hyperactive hip flexors.
Concept and benefits of “Core Stiffness”
Back to the statement related to the purpose of the core muscles at the beginning of this article, core muscles “…stabilize a body segment so that another body segment can generate power.” What does this mean? Exercises that “stiffen” the core benefit us in two ways. So not only does having a strong “guy wire” system increases spinal health (decreases risk of injury) in also makes us stronger and faster.
Want to run faster? Increase your bench press? Stiffen your core!
Dr Stuart McGill:
How does core stiffness enhance limb speed and strength? Consider the pectoralis major muscle – it attaches the rib cage at its proximal end, crosses the shoulder joint, and attaches at its distal end to the humerus of the upper arm. When muscles contract they try to shorten. Consider the specific action here – the arm flexes around the shoulder joint moving the arm from muscle shortening at the distal end. But the same shortening also bends the rib cage towards the arm at the proximal end of the muscle. Thus simply using the pec muscle would not result in a fast nor forceful punch. Now stiffen the proximal end of pec muscle attachment – meaning stiffen the core and ribcage so it can’t move. Now, 100% of pec muscle shortening is directed to action at its distal end producing fast and forceful motion in the arm. In the same way a stiffened core locks down the proximal ends of the hip muscles producing fast leg motion. A loss of core stiffness causes the torso to bend when sprinting, and a loss of speed ‐ some force was robbed that should have been expressed in leg velocity. Thus, a universal law of human movement is illustrated – proximal stiffness enhances distal mobility and athleticism.(6)
In conclusion exercises that best strengthen the stabilizing “guy wire” system is what your core program should focus on. My favourites:
- Plank variations
- Stir the pot
- Plank with Ball Roll
- Side plank
- Bird dogs
- Pallof press
- Pallof press variations
- Press with arm raise
- Twist (deliberate pace)
Core exercises, which make use of isometric postures and static bracing, like the ones mentioned above, create muscular activation while minimizing spine loads and injury mechanisms linked with movement.(7)
4) Dr. McGill points out that spinal flexion exercises without load is okay. Example Cat/Camel exercise https://youtu.be/hipHP1qd_tA