Core training is common terminology thrown around in fitness circles today.  However, not much research has specifically addressed more advanced stability ball exercises and muscle activation until now.

A recent article released in the May 2010 Journal of Orthopedic & Sports Physical Therapy looked at 8 stability ball exercises and maximal voluntary isometric contraction (MVIC) versus traditional bent knee curl ups and crunches.

The 8 stability ball exercises studied with EMG were:

  1. Roll out
  2. Pike
  3. Knee up
  4. Skier
  5. Prone hip extension left
  6. Prone hip extension right
  7. Decline push-up
  8. Seated march right
Pike (end position)

Pike (end position)

 

All exercises were performed with a cadence of 1/1/1.  A metronome was used to ensure uniform repetitions and holds.  Researchers concluded that the pike and roll-out were the most effective exercises based on EMG activation.  However, keep in mind that these also require the greatest effort and pose a high degree of difficulty.

Why is this stuff important?  Research done by Cholecki and VanVliet concluded that no single core muscle can be identified as the most important for spinal stability.  Additionally, they believe that the relative importance of the muscle varies based on the direction and magnitude of the load imposed on the spine.

We have known for years the spine is least stable and most vulnerable in trunk flexion (as in the knee bent curl up), and that no one muscle contributes more than 30% to overall spine stability.  Choosing more demanding core exercises also typically increases spinal compressive forces as well.  This may be contraindicated in some populations.

Therefore knowing your client and condition is essential.  For example, flexion is often contraindicated with active disk pathology, whereas it may be indicated in those with facet arthropathy or spondylolisthesis.  Over the past decade or so, much attention has been placed on the transversus abdominus muscle. 

The prevalent thinking has been that it is a major contributor to spinal stability, although this is somewhat controversial and has not been unequivocally validated with science.  Another flaw here is that isolated contractions of it have not been demonstrated in higher level activities (e.g. sports). 

So, where is the functional tie in here?  The transeversus abdominus has shown similar activation patterns (within 15%) to the internal obliques with exercises similar to those in this study.  The highest activity from internal obliques was during the pike, roll-out, knee up, skier and hip extension left exercise.  This may indicate that transveresus abdominus activation is also high, but further research will need to be done.

The last critical piece of data involves looking at hip flexor activation as the psoas generates remarkable spinal compression and anterior shear forces at L5-S1.  This can be especially troubling for individuals struggling with lumbar disk pathology.  The exercise in this stud that demonstrated moderate hip flexor activation were:

  • Bent knee sit-up
  • Pike
  • Seated march
  • Hip extension exercises

So, if you or your client has a weak rectus abdominus and/or obliques or lumbar instability, these exercises may be contraindicated.  In the end, know that the stability ball provides much greater muscle activation compared to traditional bent knee sit-ups and crunches on the floor.  The caveat is identifying which ones are appropriate and most efficient in your case.

As a general rule, I suggest that you avoid long lever arm action with the legs in the presence of active disk pathology and instability.  You may opt for stability based exercises in a neutral spine position like planks as there is minimal shear and compressive loading here until clients develop more stability and strength.  With healthy and mroe advanced clientele, many of the stability ball exercises studied would be good alternatives to traditional crunch work to build muscle strength for the core.