We ended the previous installment with an introduction of the star of the show, the COG and BOS relationship. The article also introduced the notion that this relationship was determinant, in other words it would be the most influential on function. This article continues to develop the point and sets the stage for the change efforts in upcoming installments.
Each of the different COG BOS relationships is imposing a new demand on the neuromuscular system as the kinetic energy is increased as the COG moves away from the BOS and the need for the production of internal stability is increased as the BOS changes to a less stable form. Within each of these snap shots in time of the COGBOS relationship, there are critical tasks that must happen in order to progress to the next more difficult COG BOS position. Again, the bridge is presented as an example.
Bridging, the COG will start out in the BOS which is the length of the body requiring the critical task of attaining a neutral position. This speaks specifically to the need for a degree of flexibility and mobility to attain this and future positions. The BOS will then change as the person attains hook lying position from the full body BOS to a 3 point BOS. Again a critical task is presented for the person must be able to attain the hook lying position and be able to maintain this position. To fail this task, makes the next more complicated task unattainable. The COG then is dynamic as the body raises the COG above the 3 point BOS and the BOS further changes from the trunk to only the upper spine, scapulae, neck and head in contact. After attaining maximal rise, the lowering begins, requiring a different muscle contraction and level of control, until the beginning level is returned to. Critical tasks during the rising and lowering are similar, however the difference is the concentric versus eccentric muscle actions. Ability to do one and not the other, provides actionable data for the clinician in selecting the plan of care outcomes.
Each COG BOS shot in time can then be further translated into body segment positions. A body segment position will either be associated with the COG or the BOS. Inadequacies in either will result in the critical task being unattainable and would likely be observed clinically as a compensatory movement as the body seeks another way to complete the critical task. An example of this is the anterior weight shift present in the hip hinge during sitting to standing. A functional body, capable of participating in life ought to be able to perform a sit to stand without the use of upper extremities for propulsion up or balance. A thoracic spine which is kyphotic and flexed will result in a poor anterior weight shift of the COG over the BOS in preparation for standing. As a result the patient will compensate with a rotation as the patient uses both hands then one hand torsion the body up. This is an incredibly inefficient compensation and often leads to loss of balance.
With this patient, a further close up shot of the thoracic spine mobility would take place as each segment of the functional unit would be examined at the joint level and antagonist muscles to Thoracic spine extension would be assessed for adequate mobility. A change effort would begin for the change of the position and mobility to promote thoracic spine extension. When this is completed, we are not done yet, as the goal is to be able to sit and stand.
Here is another examples, this time with the BOS as the area of inadequacy. The patient in this case is able to attain and maintain the feet apart, knees apart position to establish the BOS required to perform the sit to stand motion. However during the rising portion of the sit to stand, the knees adduct, essentially blocking the COG from forward translation and essentially pushing the COG posterior to the BOS. This is an inadequacy of the BOS as it impairs the movement of the COG. As a result, the patient is stuck during the rise phase and eventually falls back into the chair, with an uncontrolled descent as there lower extremities are overwhelmed by the task at hand.
How often today do PTs, OTs and other providers asses a person’s ability to stand up from a seated position as poor and then make the conclusion that what is needed is more lower extremity strength. It would be easy to draw that conclusion when the assumption that sitting and standing is a function of lower extremity strength. In this problems solving structure, the clinical decision making would go something like this: Problem is patient cannot stand up on their own, impairment is lack of lower strength, solution is to make legs stronger. Exercises prescribed would include SLR with weights, SAQ with weights, possibly bridging exercises and prone hip extension exercises all designed to promote the extension capacity of the lower extremities.
The proof that it is not lower extremity strength but the inadequate position of the COG behind the BOS which is causing the poor performance of sit to stand can be performed by the reader. Simply sit in a chair place your feet forward such that the knees are flexed to 75 degrees. Now slump and try and stand up… What happened to your leg strength? Is this some sort of remote impairing of leg strength, or is it simply a misalignment of the COG and the BOS. Certainly a lack of lower strength could be part of the what is being observed. However, that lower extremity strength is best built in a pattern that is intensification of function. In the upcoming article we will further examine how this relationship will guide the selection of interventions in the change effort, and how that change effort will be sequenced.