June 3, 2019
Pondering Postural Control: Stability Limits & Internal Representations
And so here we are on the brink of balance, the place we call stability limits. I’m not sure how often we think about the edges of balance. I first started thinking about stability limits (although not using these words) when I learned about equilibrium reactions in my NDT certification course. We needed to be able to identify the movement components of the reaction that were present, the timing of each component and those components that were missing. We also needed to identify where the stability limit was and how the missing components impacted the quality of the reaction, the quantity of the stability limit and therefore the quality of balance. That was a lot of information gleaned from just one small facilitation task!
I watch my clients closely as they move in space. Are they careful or carefree or careless? Are they secure as we play on stable and unstable equipment and surfaces? What is the quantity and quality of their anticipatory and reactive mechanisms? These observations can provide valuable information regarding stability limits.
Stability limits depend on some of our previously discussed components – musculoskeletal and neuromuscular components, reactive and anticipatory mechanisms, these can all impact these limits. Sensory systems that we will discuss in the coming months are also vitally important – vestibular, visual, proprioceptive and tactile all impact our interpretation of the boundaries of our stability. Unfortunately there is limited research that looks specifically at stability limits in different populations (specifically children who are deaf and those with DCD).
And of course clinically we can assess stability limits formally. The BESTest is one of my favourite standardized assessments precisely because it examines so many of these systems. And recently researchers have proposed the Kids BESTest. Using the pediatric functional reach test is also an option.
In reality, stability limits are indicative of the quality of our internal representations = that part of the parietal lobe that is the somatosensory cortex or as Kim Barthel calls it, the “map of me”. Sensory integration theory tells us that the somatosensory cortex is built by somatosensory input – tactile and proprioceptive information that tells us how long our arm is or what it feels like when our center of gravity moves towards the edge of our base of support.
Ultimately, stability limits are just one piece of quantitative information. Using a dynamic systems model, should there be difficulties with a stability limit, we look at all the pieces of the puzzle. When a child is less stable, why is the most important question. Determining which systems are contributing to the issue will direct effective treatment.
I have a client presently who is a puzzle to me. She is 7 years old, with no diagnosis and has low postural tone in her trunk, increased muscle tone in her gastrocs and tightness in her hamstrings – just like countless other children I have treated over the years. However, some days she is totally confident bench sitting in therapy and reaching to the edges of stability for toys and other days she is terrified to shift her weight even slightly. The same is true at home and at school. In treatment we address emotional regulation, musculoskeletal components, neuromuscular synergies, reactive and anticipatory mechanisms and we have explored movement at the edges of stability. However, we have yet to discern what systems are creating the challenge to consistent stability limits. I suspect the answer may lie in the specifics of her sensory processing.
And speaking of sensory processing, next month kicks off our discussion of the sensory systems as they impact postural control. First up is one of my personal favourites, the Vestibular System!