Balance is defined as the ability to maintain the center of gravity within the boundary of stability, which is the base of support. Already here we see the beginning of the confusion between stability and balance. This is a much larger issue, which we will get back to in later posts here on this blog. Postural control is the task of controlling the body´s position in space for the dual purposes of stability and orientation (Palmieri, Ingersoll, Stone, & Krause, 2002). Once again the word stability surfaces, and yet again we will deal with this in later posts and publications.
In order to quantify balance the center of pressure (COP) is often used. Many variables based upon the movement of the COP are used such as the maximum and minimum excursion away from and average point. One should be careful in defining good and not so good balance based upon the amplitudes of the COP since there are many issues concerning this measure. One simple issue would be if foot position is standardized between subjects.
What I really wanted to write about today relates to the velocity of the COP. An increase in COP velocity is thought to represent a decreased ability to control posture, whereas a decreased velocity represents an increased ability to maintain upright balance (Palmieri, et al., 2002). What is of interest then is when a group of female athletes with ACL-deficient knees where compared to a control group. The group of ACL-deficient knees had lower COP velocity than the control group, thus better balance (Davids, Kingsbury, George, O’Connell, & Stock, 1999).
From a functional perspective this makes perfect sense. I would consider better balance to be an increased velocity to the boundary and come back, rather than small amplitudes of the COP well within boundary of stability. An analogue to this would be when you learn how to drive. You start out in the middle of a parking lot at low speeds, but this is not where you stay. One can argue that is good driving since there will be no accidents, but it does not get you anywhere either.
Jessica, Ali and Ola
Davids, K., Kingsbury, D., George, K., O’Connell, M., & Stock, D. (1999). Interacting Constraints and the Emergence of Postural Behavior in ACL-Deficient Subjects. Journal of motor behavior, 31(4), 358-366.
Palmieri, R. M., Ingersoll, C. D., Stone, M. B., & Krause, B. A. (2002). Center-of-pressure parameters used in the assessment of postural control. Journal of Sport Rehabilitation, 11(1), 51-66.