With the recent explosion in popularity of Pilates exercise programs, terms such as core stability have begun to pass into common language. The uninitiated may be aware that this has something to do with the abdominal muscles, but for Physiotherapists the growing awareness of core stability has revolutionized the way they look at back pain. So what actually is core stability?
Let me just say from the outset that doing heaps of sit-ups and crunches will not give you core stability. These exercises target the wrong muscles and work them in the wrong fashion (more on this later). It is convenient at this point to divide the notion of core stability into two distinct areas – the specific exercises used by physiotherapists in the rehabilitation of low back pain (which forms the subject of this article), and Pilates, which was originally designed as a holistic system of strengthening exercises incorporating specialized equipment or floor classes. Until fairly recently Pilates was the domain almost exclusively of dancers and gymnasts.
Core stability refers to the ability of certain, specialized muscles to prevent uncontrolled and potentially injurious movement of the low back, trunk or lumbar spine. It’s important to have adequate core stability, as powerful muscles attaching to the trunk initiate practically all sporting movements and most everyday ones. These movements are more efficient if these power-generating muscles have a stable base to work from. Forces transferred up the legs or arms (e.g. from running or hitting a tennis ball) are also absorbed and transferred in the trunk. If these aren’t controlled it can injure your back as well. Ever jarred your back when you missed a step? That’s force transfer at work.
It’s for this reason that a physio may look at core stability in an individual with seemingly unrelated problems – a hamstring strain, for example, where an inability to properly control spinal movement can contribute to reinjury or delayed recovery.
The problem is these muscles become inhibited (think of it as being similar to a software crash) when you have back pain, and significantly, they don’t necessarily automatically switch back on once the pain settles. They lose their memory and timing, such that if they do come back on, their protective abilities can be delayed. Moreover, research has shown that if the core stabilizers don’t work well, the back can be predisposed to further or recurrent injury. This has been studied and well-documented in physiotherapy literature.
An analogy: imagine that you are carrying five milk crates (your vertebrae), stacked on top of one another, with only a loose piece of string attaching them to each other (ligaments). Without some form of external bracing or buttressing (your core stability muscles), it would be difficult to maintain balance of the crates, and, say, run at the same time. The string linking them together would help, but only once they’d become unbalanced. Your five lumbar vertebrae on their own are inherently unstable. The role of the core stabilizer muscles is to provide the external support that maintains balance of the whole system.
The two primary core stabilizer muscles are Multifidus and Tranversus Abdominis. These muscles comprise the innermost layer of muscles in the trunk.
*ABOVE: Different configurations of the lumbar multifidus (from Bogduk N, Twomey L, (1991) Clinical Anatomy of the Lumbar Spine, 2nd Ed. Churchhill Livingstone)
Multifidus is a small muscle that has a variety of configurations (see above). Some run from one vertebra to the next, others cross a couple of levels at once. Multifidus lies very close to the centre of rotation of the spine, and is therefore ideally placed to limit sloppy or uncontrolled movement of the spine when it is activated.
Transversus Abdominis (TA) resembles a corset insofar as it wraps around the trunk from back to front, and when it tenses it creates tension in a broad piece of tissue called the thoracolumbar fascia, which runs across the low back. It also increases intra-abdominal pressure (IAP), which also helps to brace and support the spine.
The combination of TA contraction and increased IAP convert the otherwise soft abdomen and wobbly spine into a rigid cylinder. Your pelvic floor muscles also play a role in maintaining the cylinder.
A word on muscles – not all muscles are best trained by lifting weights at a gym in 3 sets of 10. The lumbar stabilizers are considered to be slow twitch (or type 1) muscles, that is, they produce low amounts of force but have very low rates of fatigue. Their role primarily is to control or limit movement. To do this, they work isometrically (like flexing your bicep – the muscle contracts but there’s no movement). There are slow twitch muscles all over the body – the rotator cuff group in the shoulder, gluteus maximus in the hip and the VMO in the knee are examples. Type 1 muscles tend to be found in the deeper layers of muscles. Slow twitch muscles are also known as tonic muscles, and the exercise they respond to best are those with low grades of contraction, held for prolonged periods, and often while other movements are performed on top. This closely resembles how they work in real life.
Doing weights suits fast twitch or type 2 muscles (examples include the gastrocnemius in the calf, pectoralis major in the chest/shoulder). These are the power muscles, which can work hard but require time to recover. Your sit-up muscles – the rectus abdominis muscles are this second type (which is why sit-ups don’t help core stability!). Type 2 muscles tend to be located more superficially in the body.
ABOVE: The abdominal wall has four layers of muscles, of which Transversus Abdominis (highlighted) is the innermost. (from Clemente C, (1987) Anatomy: A Regional Atlas of the Human Body, 3rd Ed. Urban & Schwarzenburg
Physiotherapists are skilled in the teaching of core stability exercises, and today they are used as an important component of rehabilitation from back pain and injury.
Incidentally it is the TA and multifidus muscles (along with the other tonic muscles in the body), when well conditioned, contribute to good erect posture.
Core Stability Basics by Derek Mansfield, Physiotherapist