Tuesday, October 4, 2011

What is the Core?

In an adult skeleton, there are about 206 bones (1).  Of the 206 bones, only five of them serve as an important skeletal structure that the core muscles press and pull against.  Unlike the thoracic spine, the five disks of the lumbar spine does not have ribs that provide it with additional strength, stability for posture, shock absorption and rigidity against impact.  To overcome this obstacle, the body has to rely on a network of muscles and fascia to take on the demands that both bone and muscle experience, this is the core.  Because the muscles of the core can move the spine through all planes of motion, the lumbar spine is more vulnerable to damage.  Any muscular imbalance will cause the spine to favor a dangerous posture and likely damage the lower back.

The relationship between the muscles of the lumbar spine and the disks can be best interpreted as a guy wire system.  Imagine that the lumbar spine is a five story tower with muscles or "wires" that attach to both sides.  In normal conditions (see figure 1.1), the muscles of the lumbar spine keep the disks aligned correctly- both sides have equal tension.  In unhealthy conditions (see figure 1.2), the disks are pulled out of alignment due to unbalanced muscular strength. As shown on figure 1.2, the wires to the right overpowered the wires to the left and caused the tower to lean out of alignment.  In muscular terms, the wires to the right are now in a tightened position and the wires to the left are in a stretched position.  If no actions are taken to correct this imbalance, low back pain will usually occur.

Figure 1.2 Unbalanced
Figure 1.1 Balanced
Muscular imbalances may be caused by the following:
  1. Poor posture for extended periods of time
    • Example:  Driving, office jobs, watching tv, sitting at a table and waiting at an airport
  2. Poor posture while lifting light or heavy objects
    • Example:  Prepping and cooking food, reaching for objects above shoulder height, moving boxes and lifting suitcases/ luggage
  3. Repetitive activities/ sports dominantly in one plane of motion.
    • Example:  Running, cycling, swimming and rowing.
  4. Poor posture during sports
  5. Poorly designed weight lifting programs. 
    • Example:  Only performing exercises to visibly improve rectus abdominis appearance ("six pack").
There is more to the core than the "six pack" muscle, the rectus abdominis.  In addition to the diaphragm and the muscles of the pelvic floor, there are about 16 muscles- a lot more than this if I were to include every intricate muscle.  If you feel comfortable with the anatomy of the core, skip onto the next section about core exercise design.

Starting with the deepest groups of muscles, there are eight muscles that directly affect the movement of the lumbar spine.  The first group of muscles is the erector spinae group.  This group is responsible for spinal extension and posture.  The second group of muscles is less well known probably because the name is not as easy to remember, the transversospinalis group.  These muscles are small and literally attach from vertebrae to vertebrae.  Their purpose is to provide stabilization of individual vertebrae and prevent excessive rotation, lateral flexion and spinal flexion.  The quadratus lumborum is in a group of its own because it is able to perform two entirely different movements of the spine, spinal extension and lateral flexion.  Through bilateral (at the same time) contractions, this muscle performs spinal extension.  If used unilaterally (one side only), the quadratus lumborum laterally flexes the spinal column and also lifts the hip upward. (4)

Due to its function, the middle layer is usually the most important core muscle group to focus on.  As a group, the muscles/ fascia form a box.  This layer is important because it contains the transverse abdominis, the muscle responsible for providing the compressive forces that takes off stress from the lumbar spine and provides a stable foundation for the upper and lower body to press and pull against.  If the transverse abdominis is weak, more stress is placed on the lumbar spine and the muscles of the extremity will have to work extra hard.  Why is this a problem?  In addition to the risk of low back pain, the increased demands on the extremities means that more stress will be placed on the joints above and below the core.

The outermost layer of the core is responsible for gross movement of the trunk and also provides additional compression.  This group consists of the rectus abdominis, internal oblique, external oblique, latissimus dorsi and the iliopsoas (hip flexor).

Deep layer:
  • Erector Spinae - Spinal extension
  • Iliocostalis, longissimus and spinalis
  • Transversospinalis - Spinal extension, lateral flexion of the spine, rotation of the spine
  • Interspinalis, Intertransversarii, multifidus, rotatores and the semispinalis group (thoracic) 
  • Quadratus lumborum - Bilateral activation = spinal extension , Unilateral activation = lateral flexion of the spine
Middle layer (the box):
  • Diaphragm - "The lid"
  • Pelvic floor - "The bottom"
  • The sides of the box
  • Transverse abdominis - Flexes vertebral column/ compresses abdominal wall
  • Quadratus lumborum - Spinal extension/ lateral flexion of the spine
  • Multifidus - Spinal extension, rotates spinal column
Outermost layer:
  • Rectus abdominis (connected by linea alba) - Spinal flexion, compresses abdominal wall and helps with expiration
  • External oblique
  • Bilateral - Spinal flexion & compresses abdominal wall
  • Unilateral - Lateral flexion of the spine
  • Internal oblique
  • Bilateral - Spinal flexion & compresses abdominal wall
  • Unilateral - Lateral flexion of the spine
  • Iliopsoas
  • Bilateral - Spinal flexion
  • Unilateral - Lateral flexion of the spine
  • Latissimus dorsi (Thoracolummbar Fascia)
By knowing that the core is a group of muscles that works as one unit to stabilize the abdominal area, it is easier to understand why crunches and sit-ups are not effective core exercises.  These exercises target mainly one muscle rather than all of the core muscles at once.  Choose exercises that will make the core work hard to keep the spine positioned correctly.  Because core exercises directly affects the spine, risk of injury is greater; so be sure to consult a fitness professional to ensure that the exercises are performed with correct form.

Core Exercise Tip #1:  Learn how to activate the core and breathe at the same time.
If you noticed above, the majority of the core muscles are responsible for compressing the abdominal wall to provide a stable foundation.  This means that any exercise can become a core exercise as long as core activation is learned.  There are two methods that many use to activate the core, abdominal hollowing and bracing.  McGill stated that in terms of stabilizing the spine, bracing was more effective than abdominal hollowing or "centering", a common method used for yoga/ pilates (3).  Another study also found that bracing was a more effective method to stabilize the spine (2).  In order to brace effectively, I always pretend as if I was about to be punched from the front, sides and back of my abdominal wall at the same time.  While activating the core, remember to always breathe or else blood pressure will rise very fast!

Core Exercise Tip #2:  Choose exercises that require core activation to stabilize the spine throughout all planes of motion.
Because the core is responsible for maintaining good posture by preventing movements such as spinal flexion, extension, rotation and lateral flexion at the spine, it is important to challenge the core this way.  Exercises such as the half kneeling anti rotation is an excellent way to challenge the core through rotational forces.  To challenge the core in the sagittal plane, a popular exercise is the plank and variations on it.  To prevent lateral flexion at the spine, the farmer's walk is an excellent choice.

Core Exercise Tip #3:  Don't forget the lats!
Although it seems like it doesn't belong, the latissimus dorsi contributes to the stability of the core.  Because the lats are located at the core, they will provide additional stability for the deeper muscles to press against.

  1. Bryant, Cedric X., and Daniel J. Green. ACE advanced health & fitness specialist manual: the ultimate resource for advanced fitness professionals. San Diego: American Council On Exercise, 2008. Print.
  2. Gambetta, V. (2007). Athletic Development: The Art and Science of Functional Sports Conditioning. Champaign, Ill.: Human Kinetics
  3. McGill, S.M. et al. (2003). Coordination of muscle activity to assure stability of the lumbar spine. Journal of Electromyography and Kinesiology, 13, 353-359.
  4. McKinley, Michael P., and Valerie Dean Loughlin. Human anatomy. Boston: Mcgraw-Hill Higher Education, 2006. Print.