Biomechanics of Falling
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Mayo Clin Proc. • July 2005;80(7):847-848 • www.mayoclinicproceedings.com 847 EDITORIAL Address correspondence to Allan F. Tencer, PhD, Department of Orthopedics, Harborview Medical Center, 325 Ninth Ave, MS 359798, Seattle, WA 98104 (e-mail: [email protected]). © 2005 Mayo Foundation for Medical Education and Research Mayo Clinic Proceedings July 2005 Volume 80 Number 7 Biomechanics of Falling I n the current issue of the Mayo Clinic Proceedings, the article by Sinaki et al 1 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 osteoporotic- kyphotic 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 imple- menting 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 signifi- cantly. 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 popu- lation. Falls can also lead to permanent changes in lifestyle, such as hospitalization, long-term rehabilitation, and the inability to function independently, which can cause fur- ther 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. Biome- chanics 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 whole- body 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 tarso- metatarsal 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 reac- See also page 849 For personal use. Mass reproduce only with permission from Mayo Clinic Proceedings. For personal use. Mass reproduce only with permission from Mayo Clinic Proceedings.
Transcript of Biomechanics of Falling
Mayo Clin Proc. • July 2005;80(7):847-848 • www.mayoclinicproceedings.com 847
EDITORIALEDITORIAL
Address correspondence to Allan F. Tencer, PhD, Department of Orthopedics,Harborview Medical Center, 325 Ninth Ave, MS 359798, Seattle, WA 98104(e-mail: [email protected]).
© 2005 Mayo Foundation for Medical Education and Research
Mayo ClinicProceedings
July 2005Volume 80Number 7
Biomechanics of Falling
In the current issue of the Mayo Clinic Proceedings, thearticle by Sinaki et al1 provides an excellent example of
how an understanding of basic biomechanics can serve as afoundation for improving patient care. Specifically, theauthors applied the principles of biomechanics to helpwomen with kyphoscoliosis reduce the risk of falling. Theprospective trial compared treatment in 12 osteoporotic-kyphotic women to 13 healthy matched controls—all olderthan 60 years—before and after an intervention strategythat consisted of wearing a spinal weighted kypho-orthosis(WKO) along with participating in a dynamic exerciseprogram. The WKO is essentially a weighted “backpack”that adds weight posterior to the spine to counterbalancesome of the load of the torso that is concentrated anterior tothe spine. By performing the exercise program after imple-menting the WKO device, the subjects would theoreticallyreceive the compounded benefits of altered biomechanics(from the WKO) and the known, independent benefits of theexercise program, plus any added interaction between the 2treatment modalities.
The study by Sinaki et al identified benefit from theintervention. After 4 weeks of treatment, significant changeswere noted in balance and gait parameters, back extensorstrength had improved, and back pain had decreased signifi-cantly. These improvements have implications for reducingthe incidence of falls in the described study participants.
Falls present a serious risk of injury in the elderly popu-lation. Falls can also lead to permanent changes in lifestyle,such as hospitalization, long-term rehabilitation, and theinability to function independently, which can cause fur-ther declines in health. The relationship of improving theincidence of falls as a result of biomechanical alterations,as suggested by Sinaki et al, seems highly logical. Biome-chanics is concerned with the application of mechanical
principles to biological systems. A fall is a biomechanicalevent, in that an external force, gravity, destabilizes thebody’s alignment of the torso over the legs.A fall occurs when the center of gravity ofthe trunk moves outside the base of supportprovided by the feet against the floor. Thecenter of gravity is an imaginary point at which all theweight of the torso can be considered concentrated. If thecenter of gravity moves outside (anterior, posterior, orlateral) the base of support, a fall will result. Various aidscan be used to assist with balance problems and to helpprevent falls. An obvious aid is the walker, which providesa greater base of support and also allows the person the useof his or her arms to support some of the weight of the torsolocated anterior to the spine.
As a simple illustration of the function of the spinalWKO, consider a book being pushed toward the edge of ashelf. Assuming that the book’s center of gravity is at itsgeometric center, as the book is pushed it protrudes overthe edge, and its center of gravity moves closer to the edge,which is the limit of its base of support. As the distancebetween the center of gravity and the edge of the shelfdecreases, the book starts to become less stable. Ultimately,when the center of gravity is moved beyond the edge of theshelf, its limit of support, the book will fall off the shelf.
Just as keeping the book on the shelf requires somedistance between the center of gravity and the edge of theshelf (the edge of support), the person’s trunk has a centerof gravity, located at the level of the sternum and anterior tothe spine, that must be kept within its base of support, thefeet. (This center of gravity is separate from the whole-body center of gravity located within the pelvis.) The edgeof support for forward movement (anterior flexion) of thetrunk is approximately a line passing through the tarso-metatarsal joints of the foot. If the torso flexes forward sothat the center of gravity of the trunk is located beyond thisedge, the person will fall.
In simple terms, to remain upright requires a balancebetween gravity pulling the torso downward and the reac-
See alsopage 849
For personal use. Mass reproduce only with permission from Mayo Clinic Proceedings.For personal use. Mass reproduce only with permission from Mayo Clinic Proceedings.
Mayo Clin Proc. • July 2005;80(7):847-848 • www.mayoclinicproceedings.com848
EDITORIAL
tion force from the floor upward through the feet, whichcounteracts gravity. With this background in persons withkyphoscoliosis and anterior displacement of the center ofgravity, the use of the WKO, as studied by Sinaki et al,makes sense. Adding a weight to the posterior aspect of thetorso moves the center of gravity of the torso rearward.This alone would reduce the tendency to fall. The person’scenter of gravity moves rearward, away from the edge ofsupport defined by the joints of the toes. The additivechanges of the 4-week exercise program were not sepa-rately examined in the report by Sinaki et al; however, onewould envision that this pr ogram would provide severaladvantages, including improving the person’s motor skillsand muscle memory in response to a new center of gravity,strengthening the requisite muscles for improving balance,and accelerating the process of anatomical alterations thatmay result from use of the WKO device.
Regarding this latter benefit, in addition to an immediatechange in weight distribution, the WKO also adjusts pos-ture, reducing kyphosis. Consider the playground seesawas an example. One child sitting far away from the centerbar, about which the seesaw pivots, can hold up an adultsitting much closer but weighing more than the child. Thisis an example of the requirement of the balance of mo-ments, the engineering term for a bending force. A weightacting over a distance creates a bending moment. (A leverallows the smaller force applied by the worker further awayfrom the pivot point to be transferred to the larger weight,closer to the pivot point, that is being lifted.) If the lowerlumbar spine is considered the fulcrum, the center of grav-ity of the torso acts at a certain horizontal distance anteriorto it, causing the torso to bend forward. This is counter-acted by the pull of the spinal extensor muscles acting
posterior to the spinal fulcrum. Providing a relatively smallweight (eg, 1.0 kg, as used in the WKO device) but at agreater distance posterior to the spinal fulcrum adds to themuscle-bending moment, counteracting the kyphosis, andallows the person to stand more erect.
Therefore, the WKO has 2 beneficial effects. It dis-places the center of gravity posterior to the edge of sup-port along the joints of the toes, increasing stabilityagainst incidents that would cause the person to pitchforward and fall. Also, the WKO allows the person tostand more erect because of its addition to the extensionmoment generated by the back muscles; thus, the personis in a better position to look ahead and avoid obstacleswhile walking.
In summary, the report by Sinaki et al validates theefficacy of using the WKO and a dynamic exercise pro-gram to improve biomechanics in persons with kypho-scoliosis. This in turn can reduce the possibility of theperson falling and prevent the downstream, potentially life-altering consequences of falling. Further research is nowneeded to determine the feasibility of using this interven-tion in large populations of kyphoscoliosis patients, so thatissues of patient compliance plus treatment effects andadverse effects can be better elucidated.
Allan F. Tencer, PhDDepartment of OrthopedicsHarborview Medical CenterSeattle, Wash
1. Sinaki M, Brey RH, Hughes CA, Larson DR, Kaufman KR. Significantreduction in risk of falls and back pain in osteoporotic-kyphotic womenthrough a spinal proprioceptive extension exercise dynamic (SPEED) program.Mayo Clin Proc. 2005;80:849-855.
For personal use. Mass reproduce only with permission from Mayo Clinic Proceedings.For personal use. Mass reproduce only with permission from Mayo Clinic Proceedings.
