Biomechanics of Falling

Abstract

In the current issue of the Mayo Clinic Proceedings, the article by Sinaki et al provides an excellent example of how an understanding of basic biomechanics can serve as a foundation for improving patient care. Specifically, the authors applied the principles of biomechanics to help women with kyphoscoliosis reduce the risk of falling. The prospective trial compared treatment in 12 osteoporotickyphotic women to 13 healthy matched controls—all older than 60 years—before and after an intervention strategy that consisted of wearing a spinal weighted kypho-orthosis (WKO) along with participating in a dynamic exercise program. The WKO is essentially a weighted “backpack” that adds weight posterior to the spine to counterbalance some of the load of the torso that is concentrated anterior to the spine. By performing the exercise program after implementing the WKO device, the subjects would theoretically receive the compounded benefits of altered biomechanics (from the WKO) and the known, independent benefits of the exercise program, plus any added interaction between the 2 treatment modalities. The study by Sinaki et al identified benefit from the intervention. After 4 weeks of treatment, significant changes were noted in balance and gait parameters, back extensor strength had improved, and back pain had decreased significantly. These improvements have implications for reducing the incidence of falls in the described study participants. Falls present a serious risk of injury in the elderly population. Falls can also lead to permanent changes in lifestyle, such as hospitalization, long-term rehabilitation, and the inability to function independently, which can cause further declines in health. The relationship of improving the incidence of falls as a result of biomechanical alterations, as suggested by Sinaki et al, seems highly logical. Biomechanics is concerned with the application of mechanical principles to biological systems. A fall is a biomechanical event, in that an external force, gravity, destabilizes the body’s alignment of the torso over the legs. A fall occurs when the center of gravity of the trunk moves outside the base of support provided by the feet against the floor. The center of gravity is an imaginary point at which all the weight of the torso can be considered concentrated. If the center of gravity moves outside (anterior, posterior, or lateral) the base of support, a fall will result. Various aids can be used to assist with balance problems and to help prevent falls. An obvious aid is the walker, which provides a greater base of support and also allows the person the use of his or her arms to support some of the weight of the torso located anterior to the spine. As a simple illustration of the function of the spinal WKO, consider a book being pushed toward the edge of a shelf. Assuming that the book’s center of gravity is at its geometric center, as the book is pushed it protrudes over the edge, and its center of gravity moves closer to the edge, which is the limit of its base of support. As the distance between the center of gravity and the edge of the shelf decreases, the book starts to become less stable. Ultimately, when the center of gravity is moved beyond the edge of the shelf, its limit of support, the book will fall off the shelf. Just as keeping the book on the shelf requires some distance between the center of gravity and the edge of the shelf (the edge of support), the person’s trunk has a center of gravity, located at the level of the sternum and anterior to the spine, that must be kept within its base of support, the feet. (This center of gravity is separate from the wholebody center of gravity located within the pelvis.) The edge of support for forward movement (anterior flexion) of the trunk is approximately a line passing through the tarsometatarsal joints of the foot. If the torso flexes forward so that the center of gravity of the trunk is located beyond this edge, the person will fall. In simple terms, to remain upright requires a balance between gravity pulling the torso downward and the reacSee also page 849