Core training is a staple in the training program of most athletes and general fitness goers. “Core” stability training arrived around the end of the 1990s and was largely derived from studies that demonstrated change in timing of activation of the trunk muscles in lower back pain. Core stability, the argument went, was the key to relieving chronic lower back pain. This has led to worldwide teaching of trunk bracing and “tummy tucking” for lower back pain and injury prevention.
Over the years big holes began to appear in this argument. Dr. Eyal Lederman published an insightful critical review on the topic in 2010 in which he breaks down the several assumptions this approach is based on:
- Certain muscles are more important for spine stabilization, in particular transversus abdominis (TA . . . think planks and side planks);
- Weak abs lead to back pain;
- Strengthening trunk muscles can reduce back pain;
- Groups of core muscles working independently of other trunk muscles; and
- Relationship between stability and back pain.
The real role of the transverse abdominis
The “core” receives a lot of attention through mainstream media and personal trainers alike. Generally, when referring to the “core,” the TA is the main muscle being touted. The TA has several functions in upright posture. Stability is one, but this is not on its own and functions in synergy with every other muscle that makes up the abdominal wall and beyond.
How essential is the TA for spinal stabilization? During pregnancy, the abdominal wall undergoes dramatic stretch which have been associated with force losses and inability to stabilize the pelvis. It has been shown that pregnant women lose the ability to perform a sit up due to these stretch and force loss. However, no correlation has been found between sit up performance and back pain. In other words, the strength of abdominal muscles has not been shown to be related to back pain.
Despite this, core stability exercises are often prescribed as a method of retraining the abdominal muscles and ultimately as a treatment for lower back pain during pregnancy.
Another interesting period is immediately after giving birth. It takes about 4-6 weeks for the abdominal muscles to reverse back to their normal length and about 8 weeks for pelvic stability to normalize. It would be expected there would be minimal spinal support/stabilization during this period and potentially increase the risk of back pain. However, it has been shown that out of 869 pregnant women suffering from back pain during pregnancy, 635 spontaneously recovered within a week of giving birth. How can back pain improve when it takes a minimum of 4 weeks just for the abdominal muscles to return to their normal length?
Core strength and back pain
The forces required for the trunk muscles to stabilize the spine are very low. During standing and walking, trunk muscles are minimally activated with an average activity of 2% of maximal voluntary contraction (MVC) for the rectus abdominis and 5% MVC for the external obliques. When 32kg is added to the torso during standing, co-contraction of the trunk flexors and extensors (both abdominal muscles and back muscles contracting together) rise from an estimated 1% MVC up to 3% MVC. With a back injury, it is estimated these values only rise 2.5% greater and during bending and lifting a 15kg weight, co-contraction only increases by 1.5%.
This raises the question why strength exercises are prescribed so often when such low levels of forces are needed for functional movement. It also suggests that strength losses are unlikely to ever be an issue for spinal stabilization.
Isolation and integration of trunk muscles
One of the principles of core stability training is to isolate the TA through pulling your belly button into your spine when performing “core” strengthening exercises. But to specifically activate the core muscles in this way, you’d have to override natural patterns of trunk muscle activation. Meaning, more than just your trunk muscles do work!
Furthermore, single muscle activation is very difficult and muscle-by-muscle activation does not exist due to being distant from conscious control. For example, bringing your hand to your mouth requires your nervous system to “think” hand to mouth rather than flex the biceps etc. It is doubtful following an injury only one muscle group or single muscle was affected. It has been well documented other muscles are involved in chronic lower back pain; multifidus, hip flexors, diaphragm, pelvic floor muscles and gluteals . To identify the cause of chronic lower back pain as weak or “inactive” abdominal muscles seems like a potentially naive assumption to make.
When training the trunk muscles, there is an element of specificity involved. If you train to contract your abdominals while lying on your back, there is no guarantee that his would transfer to standing, running, bending, lifting etc. In essence, is it possible to train trunk control for a specific activity? Yes. Just train the activity and don’t worry about the trunk.
Training implications and examples
So let’s break this down:
- We can’t divide the trunk into a core and global muscle system. Everything works together.
- Weak or dysfunctional abdominal muscles will not lead to back pain.
- Core stability exercises are no better than other forms of exercise in reducing chronic lower back pain.
Now I’m not saying you should throw away all core training exercises. What I want to get across is the over emphasis of “core” training as a fix for all back pain and performance improvements. I hear it very often and have always had a hard time believing it especially when the person is sedentary. So to pin it just on weak abdominals seems far-fetched. Or an athlete has a weak core yet can perform a multitude of complex and loaded movements well.
Specific core training exercises do have their place in athletic performance where strengthening the ability to resist rotation, extension and lateral flexion are all important in a 3D, unpredictable sporting environment. Furthermore, core training is present within most movements where it’s the ability to create whole body stiffness and/or transfer force from the ground through the body to the upper extremities. While MVC of the abdominals is low during functional movement, during heavy loading or complex unpredictable sporting situations, a greater MVC of the trunk musculature in synergy with the rest of the body may be needed. Some basic video examples of these exercises are below.
I find training the trunk in conjunction with other movements makes sense in the context of this research review. Unilateral lifts are also some of the best at forcing your body to work as a whole and resist forces in multiple directions. Further, in collision based sports, “core” training must also take place outside the weight room. Various grappling exercises require the athlete to apply and resist forces throughout the entire body. Robust running also help train the core in context. As stated earlier in this article, just train the activity and don’t worry about the trunk!