Spine Stability and the Balancing Stick

Study Title: Spine stability: Lessons from a balancing stick
Authors: NP. Reeves, KS, Narendra & J. Cholewicki
Journal: Clinical Biomechanics
Date: 2011

Summary:

• Reeves does it again! This paper here was a continuation of his "Spine Stability" series of papers and although not an actual study, was an excellent "lecture" piece. Coming off of his Six Blind Men & the Elephant paper where he looked at the role and importance of robustness (or variability) for both static and dynamic stability , Reeves uses a simple object, a stick, to provide us with a conceptual, but comprehensive lesson in spinal stability. In this paper, he walks us through five different "lessons", all pertaining to a stick (or two) being balanced, to help us look further into the information necessary to stabilize the spine, noise in control and spine performance, spine evolution for stabilization and efficient control, control delays and spine performance, and the role of goals for spine control. All pertaining to feedback control.
• His first experiment examined the role tracking plays in stability through positive and negative feedback. It was discovered that inorder to balance a stick in the hand effectively, one must possess both position and velocity related feedback. The lesson from this was that the central nervous system monitors both position and velocity during spinal movement for each degree of freedom (rotation and translation about each axis) it possesses. Its significance? That muscle spindles, especially of the deep musculature, play an important role by measuring both muscle length and rate of change for spinal stability.
• His second experiment examined tracking into more detail by looking at precision. Using similar concepts to the image above, he placed a weight on the top of the stick for one trial and down at the bottom for the next to understand angular displacement and velocity. It was realized that balancing the stick with a weight at the bottom created a larger "gain" (or necessary force) to maintain equilibrium and as such, the lesson was learned that impairments in tracking the spine will impair control and may lead to non-optimal recruitment patterns. Therefore, the relevance of this was that not only is position-related feedback important but also velocity related feedback for spinal control.
• The third experiment looked into controllability and observability. Using two different trials of stick balancing (one with two sticks in series - on top of each other - and the other with two sticks in parallel - beside each other), it was revealed that multiple masses will have different movement characteristics. The lesson from this experiment was that independent control of the various spine segments through its physical make up and neural recruitment may be important for feedback control and subsequent stability, as well as metabolic efficiency.
• His fourth experiment examined delays in feedback control. By utilizing multiple trials with various stick lengths, it was revealed that shorter sticks make balancing more difficult due to an increase in size and rate of oscillation. He attributed this to delays in feedback control. The relevance? In rapid dynamic environments, feedback control may negatively influence spine control, especially in the low back pain population.
•  The final experiment was more of an observation looking at various control strategies. Realistically, there are many different ways to balance a stick and so for any given task, there will likely be an infinite number of possible feedback gains to provide stability. Ultimately, the objective should be to respect the goals of the system (or the person) whether it be for maximizing performance or minimizing costs. The takeaway from here is that the central nervous system should learn the dynamics of the spine system and that neuromuscular retraining may be valuable to help "reset control to a more desirable strategy".

• We must remember, however, that this was a "lecture" piece rather than an actual clinical trial or review. The principles in this paper are quite sound but as always, more rigorous research in these topics must be performed especially with humans as subjects.

Reeves, NP. et al. (2011). Spine stability: Lessons from a balancing stick. Clinical Biomechanics, vol 26; 325 - 330