balance across lifespan - 1 without attention arson, et al....

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Unformatted text preview: 1 without attention arson, et al. {1998}. ac. Neuropsychola :hological findings. ibsessions, compul- New York: Wiley. 3 vocal cord in the . Movement Disor- , 3c Wiesendanger, 'rilles de la Tourette leptic treatment of (2000). Movement Ieractivity disorder. ntington disease in Development, 16, )take sited in post- ewology, 30, 558m 'ders of the central 1e neuropsychiatric 791. :rlmutter, 8., et al. :h streptococcal in )f Psychiatry, 155, , DOW, S. P., et ai. natric disorders as :natic fever. Ameri— M. M., Roth, S. J., area of the United :6 News, 3, 4543. )lved for those af 3. leuropsychological ?, 65—74. CHAPTER 10 Balance and Postural Eontrol across the lifespan HARRIET G. WILMAMS LAURA HO The ability to regulate and maintain balance is often taken for granted; however, it provides the primary foundation for mobility, for use of the upper extremities, and for ' maintaining overall functional independence throughout life. The development of bal- ' ' ance/postural control is a significant part of early development and changes in the ca- ' pacity to maintain balance are a critical factor associated with aging and loss of func~ tional ability. Children With poor baiance/postural control often have difficulty with acquisition of both gross and fine motor skills and frequently shovv deficits in a variety of visuomotor or eye—hand coordination behaviors. Balance functions are also of con- '. cern to the aging individual; nearly one-third or more of elderly individuals 65+ fall ' each year. Frail elderly fall even more frequently and suffer seriOus injuries and hospi- ' '3 talizations as a result. For all ages, lack of appropriate control of balance and posture f'can have negative effects on both mental and physical health; these effects are mani- fested in a variety of ways and include loss of confidence in ability to perform physical ' tasks, loss of independence, Withdrawal from social activities, and diminished self- image and self-esteem. The health care costs of falls and loss of independence for the elderly alone are estimated to reach some $132 billion by 2030. Add to this the in creasineg greater health care costs of providing services to young children with a vari- ety of developmental needs, and it is clear that it is important to understand the nature of balance/postural control, how it develops and declines, and What factors contribute ' to the development and maintenance of effective control of balance. WHAY ES BALANCE? ' Balance can be defined simply as the ability to maintain the center of gravity over the __ base of support (e.g., Nashner, 1997). The center of gravity in humans is located in the ' pelvic region; the exact location of the center of gravity at any given point in time is dependent on the position of the body (head, arms, legs, etc). The base of Support de~ 211 212 CLINICAL DlSORDERS fines the limits of stability and the limits of stability define the area within which the center of gravity must be maintained to avoid disequilibrium, instability, or a fall (gen- erally this is 121/2 degrees in anterior—posterior and 16 degrees in medial—lateral direc— tions). Balance is also frequently categorized as static or dynamic; static balance refers to the ability to maintain the center of gravity over the base of support during quiet standing or sitting (Woollacott 8c Tang, 1997). Dynamic balance is known as “mov- ing balance” and involves maintaining balance when the center of gravity and base of support are moving (as in reaching up for an object) or when the center of gravity is moving outside the base of support (as in walking up a flight of steps). The processes involved in maintaining balance are anticipatory, reactive, or a combination of the two (e.g., Cordo 8C Nashner, 1982; Haas, Diener, Rapp 36 Dichgans, 1989; lnglin & Woollacott, 1988; Nashner, 1977). Reactive control of balance occurs when perturba- tions or events that contribute to instability (e.g., a slip, push, or trip) are unexpected; anticipatory control of balance occurs when instability is expected or can be predicted and thereby is planned for (e.g., getting into or out of a car and waiking down a flight of stairs). Optimal balance functioning requires both effective reactive and anticipa- tory control applied appropriately to the demands made on the individual as he or she is attempting to maintain stationary balance or stay in control while moving. THE BALANQE/POSTURAL CONTROL SYSTEM The current view of balance control is based on systems models (e.g., Shumway-Cook 85 Woollacott, 2001; Wooliacott 85 ShnmwaymCook, 1990). This perspective of bal- ance describes the body as a mechanicai system with mass, which is subject to external (e.g., gravity and inertia) and internal forces (e.g., muscular contraction). Balance is also a multidimensional process, which involves the interaction and function of a num— ber of physiological systems; these inciude among others, the peripheral nervous sys— tem (PNS) and central nervous system (CNS), the muscular system (e.g., strength and muscular endurance), the skeletal system (joint range of motion, flexibility, bone strength, etc), and the visual, proprioceptive, and vestibular systems (e.g., Yim-Chiplis SC Talbot, 2000). The CNS is integral to maintenance of balance and postural control through its systematic monitoring and integration of information from the three major sensory systems and through organizing the appropriate motor output to activate the correc- tive responses needed to maintain balance. The former is referred to as sensory organia zation of posture/balance, the latter as the motor coordination component of posture/ balance. Input from the three primary sensory systems involved in balance/postural control normally previde redundant information about the state of equilibrium of the body and indicate whether or not a corrective response is needed and what the nature of that response should be. The relative importance of different sources of sensory infor- mation to maintenance of balance appears to differ with age and the context in which the perturbation to balance occurs. For example, young children who are just learning to stand are more reliant on vision than are older children who have had considerable experience in walking (e.g., Woollacott 56 Shurnway—Cook, 1990). In contrast, regard— less of age, when a person is standing on a narrow surface (e.g., a balance beam) and balar sion balar ence, The ' priat dime nal a gani: respi bilitj one: eral, to e) as p are E bod: The are 3 base men lOWI chili gene and and Shu [DOE fOlll Alt. terr tior ulu a within which the ility, or a fall (gen- edial—lateral direc~ ic balance refers to )port during quiet 3 known as “mov— gravity and base of :enter of gravity is :ps). The processes )ination of the two 3, 1989; Inglin 8C irs when perturba~ p) are unexpected; :r can be predicted king down a flight :tive and anticipa- vidual as he or she ile moving. Vi ., Shumway-Cook Jerspective of bal« subject to external iction). Balance is Function of a num— herai nervous sys- (e.g., strength and , flexibility, bone (e.g., Yim—Chipiis ontrol through its :ee major sensory :tivate the correc— as sensory organi- 30nent of posture! e/postural control rinm of the body 'hat the nature of i of sensory infor- : context in which 3 are just learning had considerable l contrast, regard- alance beam) and Balance and Postural Control 213 balance becomes unstableIinput from the ankle proprioceptors is more critical than vi- sion to restoration of balance (Nashner, 1997). An individual’s capacity for effective balance also depends on a number of other factors, such as attention, previous experi- ence, use of medications, state of mind, and fatigue. MOTOR COORDNATEON ASPECTS OF POSTURE AND BALANQE CONTROL: AN GVERWEW The term motor coordination refers to the timing and sequence of activation of appro- priate postural responses to correct for perturbations to balance. Motor coordination dimensions of postural/balance control provide information about the integrity of spi~ nal and CNS functioning, as well as information important in interpreting sensory or— ganization aspects of posture/balance. The timing, sequence, and control of postural responses represents the postural control system’s plan of action for addressing insta- bility, adapting to it, and bringing the body back into balance. Postural synergies are one form of response organized by the CNS to adjust to instability of the body. In gen- eral, evidence indicates that healthy, young individuals (children and adults) respond to external perturbations to balance by activating stereotyped muscle responses known as postural synergies. These responses involve activation of leg and trunk muscles and are specific to the direction of the induced sway. Muscles on the anterior Surface of the body respond to posterior sway; muscles on the posterior surface to anterior sway. The most commonly studied postural synergy is the “ankle strategy” where responses are generally activated first in the stretched ankle muscles and radiate upward from the base of support in a distal—proximal sequence. Thus, in response to platform move— ment creating anterior sway, the stretched gastrocnemius muscle is activated first, fol- lowed by the hamstrings and then paraspinal muscles. Postural synergies are present in children as young as 15 months, undergo dramatic change from 4 to 6 years, and are generally adult-like by 7 to 10 years of age. Head control, headwtrunk coordination, and development of anticipatory postural adjustments continue to develop to 8 years and beyond {see Nashner, 1977; Shumway—Cook 85 Woollacott, 2001; Woollacott, Shumway-Cook, 85 Williams, 1989). There is general consensus that for adults and most children 7 years or older, the timing and sequence of postural synergies are as follows (e.g., Peterka 36 Black, 1990a, 1990b; Woollacott 85 Jensen, 1996): a 100 millisecond (msec): Center of pressure (COP) moves; passive properties of body biomechanics. a 110 msec: Distal muscle contracts. 9 130—140 msec: Proximal muscle contracts (e.g., 20w30 msec later). a 130 msec: Active torque generation occurs. ' a 230 msec: COP reaches peak displacement. e 260 msec: Peak sway is reached and body returns to upright position. Although the foregoing is rather universally observed in the body’s response to couni teracting sway, other factors can and do affect these synergistic responses to perturba- tions of balance. These include support surface conditions, initial body position, stim— ulus velocity (e.g., the speed with which the platform, for example, is moved), 214 CLINICAL DISORDERS displacement amplitude ’(e.g., the distance that the platform is moved), and problems with the vestibular system and/or the inner ear mechanism, as well as availability and accuracy of visual and proprioceptive information. SENSORY ORGANEZATON Oi: POSTURE/BALANCE QONTROE. Integral to the maintenance and control of posture and balance is the capacity to dc- tect disturbances to stability (Assaiante 8C Amblard, 1995). Detection of perturbations to balance is primarily a function of the visual, proprioceptive, and vestibular systems, Under typical circumstances, all three sources are available and provide accurate and redundant information about the state of equilibrium. This information is integrated and synthesized at higher levels of the nervous system. The nature and importance of each source of sensory input for balance control have been examined through use of an experimental paradigm that involves systematic removal and/or modification of inn puts from the three sensory systems. The typical paradigm requires the individual to stand on a measurement platform under several of the following sensory conditions (Shumway—Cook 8C Horak, 1986; Shurnwawaook Sc Woollacott, 1985): (1) all rele~ vant sources of sensory input are available; (2) proprioceptive and vestibular inputs are available (eyes are closed); (3) vision and vestibular information is available (ankle proprioception is removed or diminished); (4) proprioceptive and vestibular inputs are available along with conflicting visual information (e.g., sensory conflict employing the moving room condition: the walls and ceiling move, the floor does not); (5) vestib- ular information alone is available (eyes are closed and ankle proprioception is con~ trolled for or dampened); and (6) vestibular information is available along with erro— neous visual information (sensory conflict is present and ankle proprioception is reduced or controlled for). The effect of these different sensory conditions on posture/balance control is typi- cally assessed through examination of the change in amplitude or extent of sway. The universal outcome of studies using this paradigm is that balance control is best when- all three sources of sensory information are accurate and available (e.g., redundant sensory conditions). In addition, most studies indicate that while there is some increase in sway when vision is removed, significantly greater increases in sway are observed when either ankle proprioception is eliminated or when conflicting visual information is present. The most dramatic effect on posture/balance (e.g., sway) occurs when ves- tibular information and/or vestibular information coupled with erroneous visual infor mation are the only sources available for detecting sv‘vay. Thus, balance control is most effective when all three sources of sensory input are present and accurate, but control of balance is still possible and reasonably effective when at least two of the three sources of sensory information are available and accurate (e.g., Shumway-Cook 85 Woollacott, 2001). DEVELOPMENT OF POSTURE/BAEANCE CONTRQL For a growing child, good balance is important to a variety of aSpects of development including psychosocial interactions with peers, participation in games and sport activi— ties, and self—esteem. Factors that affect optimal development of posture and balance ou kn , and problems ivailability and NTROl. capacity to dc- f perturbations ibular systems e accurate and in is integrated importance of through use of lification of in- e individual to ory conditions 5): (1) all rele- Wstibular inputs vaiiable (ankle ular inputs are lict employing rot); {5) vestib- :eption is con- ong with erro- uprioception is control is typi~ t of sway. The )l is best when 7 .g., redundant some increase y are observed al information :urs when ves- is visual infor— zontrol is most te, but control 0 of the three EWQY‘COOk 56 f development id sport activi- ‘e and balance Balance and Postural Control 215 _'control may have long-reaching effects that are not immediately obvious but appear much later in development Sensory issues Sensory organization of balance responses in children typically has been examined us— ing a “moving platform” paradigm. The individual stands on a measurement platform embedded in the floor; unexpectedly, the platform is moved forward or backward (or rotated, etc.). This creates a disturbance to balance similar to the start or stop of a bus (or other moving surface) on which the individual is standing. In these circumstances, accurate visual cues derived from sway caused by movement of the platform are avail- able and can be used to assess the nature and extent of the disturbance to equilibrium. I Data from studies (e.g., Butterworth 85 Hicks, 1977; Foster, Sveistrup, 8c Woollacott, 1996; Woollacott, Debu, 85 Mowatt, 1987) using this approach have shown that vi- sual input has little or no effect on the timing of postural synergies (latencies: 90*120 " - msec) in young children (as young as 2 years with some experience in standing/walk- __ing) or adults (Sundermeir, Woollacott, Jensen, 5C Moore, 1996,; Woollacott et al., 1987). . What effect does vision have on postural synergies in infants younger than 2 years of age? Sundermier and Woollacott (1998) categorized chiidren by developmental lev- -- els: (1) “pull to stand” (7—9 months old who could pull the body with some assistance to a standing position) and (2) “newly walking” (13 months old with 4—8 weeks of walking experience) and tested them with “eyes open” and “eyes closed” on a moving " platform. They examined both the latency of onset of gastrdcnemius muscle activity and integrated electromyography (EMG). Data indicated that vision had little or no ef— fect on the speed with which postural responses were activated. This suggests that dur- . ing development, vision may be more involved with activation of slower acting path— ways integral to maintaining posture/balance (e.g., those with latencies greater than 200 msec). Results of this same study indicated that there were developmental differences in the effects of vision on amplitude of muscle activity (integrated EMG). Vision had no measurable effect on the amplitude or force of postural responses in 7—9-month—olds; in contrast for “newly walking” children, the amplitude of activity in the gastrocne- mius muscle was significantly greater with vision than without. These data suggest that vision may play an important role in behavioral transitions that involve new chal- lenges to balance associated with, for example, learning to sit, stand, and/or walk in— dependently. Once the child gains experience and has acquired some mastery of these behaviors, the effect of vision seems to be minimized leg, Butterworth 8c Hicks, 1977; Woollacott 85 Sveistrup, 1992, 1994). Since the elevated activity observed in the gastrocnemius in “new walkers” when vision was present was greater than was func- tionally necessary to maintain balance, it may be that vision, under certain circums stances, acts to amplify proprioceptive—mediated responses that are necessary to adapt to new and different perturbations to balance. Visual information seems not to significantly affect the speed of automatic pos— tural responses (latency : 90—100 msec) in children or adults; however, the sway of in- fants just iearning to stand and/or walk is strongly affected by the presence of errone— ous visual cues. Typically, conflicting visuai cues are created through the paradigm known as the “moving room” phenomenon. This environment creates erroneous vie 216 CLINICAL DISORDERS sual information about sway (e.g., with regard to whether or not sway is occurring and the direction of sway). The individual, young or old, typically sways in concert with the room as visual input from the moving room indicates that the body is sway- ing in the direction opposite to the actual direction of Sway. Most children and espe— cially older adults show increased sway under these conditions. In addition, the magm; tude and direction of this sway actually leads to falls in many individuals. This 1 suggests that the young postural control system is reliant on vision and finds it difficult to ignore or suppress visual information even when it is inaccurate and may lead to in- creased instability. Once the chiid has some experience in walking the magnitude of the sway response to conflicting visual cues declines; however, this sway response is still present in adults (cg, Sundermier et al., 1996). Thus, the development of postural control in children is characterized by changes in the contribution of different sensory systems to balance and by enhanced integrative processes (e.g., Butterworth 8C Hicks, 1977', Forssberg 5C Nashner, 1982). Clearly, al~ though vision is an important source of sensory input for postural control in young children, by 4 to 6 years proprioceptive inputs gain influence and integrative processes begin to emerge. With continued growth and development, children also display an ink creasineg greater capacity to substitute one source of sensory information for an— other (e.g., proprioceptive input for vision) (Shumway-Cook Sc Woollacott, 1985; Woollacott, Shumway-Cook, 85 Nashner, 1986) and to resolve “sensory conflict” (e.g., to suppress erroneous information when appropriate). In general, however, most children require more than vestibular input to control balance effectively (Woollacott et a1., 1986) ' Motor Coordination Components Although postural synergies are present in children as young as 15 months, they are not refined until 7—10 years of age. During this time, important changes occur in vari- ous aspects of motor coordination and/or postural synergies. Although the appropriate distal—proximal sequence of muscle activation is present in children 1—3 years of age, latencies of postural synergies are longer than those of adults but shorter than those of 4—6-year—olds. Postural responses in children 14; years of age are also of longer durae tion and more stretch reflex responses are present than in older children. There is a dramatic period of change in postural responses from 4—6 years. Latency of onset of postural responses is significantly slower and more variable during this time. This change is important to keep in mind since reduction in response variability may reflect important changes in the development of the young nervous system. The sequence of activation of muscles is appropriate and reflex responses typically seen in younger Chile dren are no longer present. By 7—10 years of age, latencies of postural responses are comparable to those of adults and the variability seen in 4—6-year-olds has decreased. Duration of muscle activity is now also more proportional to the magnitude of the per turbation to balance than was true earlier in development. All of these changes allow children to adapt to various threats to balance more effectively (sec ShumwayeCook <35 Woollacott, 1985, 2001). Sundermier, Woollacott, Roncesvalles, and Jensen (2.001) also provide evidence that with increasing age and/or developmental level, the muscle activity involved in re— sponses to perturbation of balance increases and is better coordinated. The improved ray is occurring Nays 1n concert e body is sway~ ldren and espe- tion, the magni- Lclividuals. This finds it difficult may lead to in- ie magnitude-of way response is ized by changes nced integrative 82.). Clearly, alw ontrol in young rative processes ;0 display an in :mation for an- )ollacott, 1985; :nsory conflict” , however, most '. 'ely (Woollacott ionths, they are es occur in vari- thc appropriate —3 years of age, :er than those of of longer dura- iren. There is a ency of onset of this time. This )llltY may reflect The sequence of in younger chil- al responses are s has decreased. itude 0f the per— e changes allow imway-Cook (SC rrovide evidence y involved in re- 1. The improved Balance and Postural Control 217 {timing of muscle activity is accompanied by both increased peak torque at the ankle and hip and decreased time to restabilize or recover balance. Both developmentally and chronologically younger children display less synergistic muscle activity, undergo . greater sway, and take longer to stabilize. Overall, motor development level appears to -. 'be a better predictor of motor coordination aspects of balance control than does chro— nological age. Balance control improves and is more robust as mastery of locomotor skills and "-:.regulatory abilities improve. Roncesvalles, Woollacott, and Jensen (2001) classified children as new, intermediate, and advanced walkers, runners—jumpers, gallopers, hoppers, and skippers. Overall, children with more advanced locomotor skills (hop— . pers, skippers) adjusted to increasing threats to stability without stepping or losing balance more effectively than did children with less mastery. They had faster recovery times and relatively larger muscle torques. In addition, responses of children with greater 10comotor mastery were nearly similar to those of adults. _ Development of Gait/locomotion - '- Development of locomotion is a multidimensional phenomenon that involves a num- ber of physiological and biomechanical systems and subsystems. Optimum posture/ balance control is a critical component in the appropriate development of gait and all ' __ the locomotor skills (see Breniere r36 Bril, 1998, Bril 85 Breniere, 1993; Massion, 1992; Roncesvailes et al., 2001; Thelen, Kelso, 8c Fogel, 1987; Thelen, Ulrich, 5C Jensen, 1989). Some aspects of gait/locomotion that change during development inciude (1) relative gait velocity (speed of gait/height; low in children with less than 5 months of independent walking, increases and remains constant from 15—21 months of walking, approaches adult values at 3 years of independent walking); (2) relative step length (step length/height: follows a positive nonlinear pattern characterized by a dramatic in crease in step length in the initial 6—8 months of independent walking followed by pro- gressive but moderate increases thereafter); and (3) center of mass (CM) acceleration {negative prior to 5 years of independent walking and positive after; vertical accelera~ tion of the CM results from hip acceleration and leg muscle capacity to control stabil- ity at foot contact) (Breniere Sc Bril, 1998). Data from Breniere, Bril, and Fontaine (1989) suggest that there are four phases of postural control integral to the develop- ment of efficient locomotion/gait. Phase 1 occurs at the onset of independent walking (vertical acceleration of the CM is negative, the swing phase very short, and duration of the double stance phase very long). This is the behavioral manifestation, in part, of a deficit in the muscular capacity required to counteract the effects of gravity. Phase 2 includes the period from 1—5 months of independent Walking typically described as a period of “walking by falling” (gait velocity increases, stripe/step length increases). Phase 3 usually occurs between 5 and 8 months of independent walking and is a reflec- tion of the system’s attempt to control the “fall” aspect of locomotion. Because muscle strength at joints involved in locomotion is not yet sufficient to fully modify this “walking by falling” pattern, gait continues to be somewhat problematic. Phase 4 rep— resents 4+ years experience in independent walking. The pattern of locomotion in this phase is similar to that of adults; the system now exhibits increased capacity to coun- teract the force of gravity and the inertia induced by movement. There are also accom— panying changes in arm position andjor action as control of gait increases. During CLINICAL DISORDERS early weeks when newly walking children InOVe with a wide base of Support, the arms are held in fixed postures at high guard. As balance control improves (the base of sup, port narrows), arm movement and arm—foot opposition become incorporated into the gait pattern. Arm action thus plays the dual role of stabilizing the body and also con,- tributing to forward movement (Ledebt, 2000). Shortly after the child walks, he or She begins to explore other modes of locomotion and run, jump, gallop, hop, and skip pat- terns appear and are mastered in that order. POSTURAl/BALANCE CONTROE. AND AGING Postural sway is a natural phenomenon and generally not significant in y0ung healthy adults. With increasing age, however, sway becomes more obvious and of greater con— cern (Baloh, Jacobson, Enrietto, Corona, 8C Honrubia, 1998; Ring, Nayak, 56 Isaacs, 1989; Sheldon, 1963). There is evidence that increased postural sway is associated with increased risk of falling and fractures in the elderly. Baloh and colleagues (1998) have shown that older adults who have poor balance sway significantly more than those with good balance. Fernie, Gryfe, Holliday, and Llewellyn (1982) also report data that point to postural sway as an important indicator of increased risk of falling among institutionalized elderly and Lord and colleagues (1994) provide similar data on community—dwelling elderly. Although postural sway increases with age, the most dramatic increases in amount of sway occur after age 60 (Sheldon, 1963). Baloh and colleagues (1998) also report that the sway velocity of older individuals is higher than that of younger adults. ' WHEN BALANCE FAEES: FANS AMONG THE ELDERLY Falling is a health hazard for the older adult. One in three older adults fall each year and falls have become the sixth leading cause of death among the elderly population. Ten percent of falls result in injuries serious enough to require hospitalization, Falls are also a significant factor in 40% of admissions to nursing homes and to premature institutionalization. Age~specific data indicate that the occurrence of injuries and deaths caused by falls increases with age, with the greatest increases among the oldest age groups (Kannus et al., 1999). Falls and their consequences inevitably lead to in- creased health care costs (e.g., costs of hospitalization, nursing home care, and other health care services). Health care costs have also been shown to increase monotonically with the frequency and severity of falls (Lord et al., 1994, Rizzo et al., 1998). Understanding who is at risk for falling and identifying risk factors for falls are important issues for scientists whose focus is on the elderly. Discriminating character— istics of fallers include problems with vision (decreased visual acuity, contrast sensitiv- ity), reduced proprioception (decreased toe position sense, decreased sharp~dull sensa— tions, and poor vibration sense), lower extremity disability (foot problems, reduced lower limb strength, arthritis, and reduced ankle strength), impaired cognitive func‘ tion, and polypharmacy and gait/balance abnormalities. Tinetti, Speechley, and Ginter (1988) concluded that most of the factors that predispose individuals for falls are asso- ciated with impaired neurological and musculoskeletal functions integral to stability. upport, the arms (the base of sup— :porated into the dy and also con, walks, he or she op, and skip pat- in young healthy d of greater con- iayak, 8c Isaacs, ray is associated olleagues (1998) antly more than 982) also report ed risk of falling ride similar data th age, the most 963). Baloh and LlS is higher than BREW its fall each year erly population. italization. Falls nd to premature of injuries and mong the oldest :21ny lead to in- iome care, and mm to increase 94, Rizzo et al., tors for falls are rating character- :ontrast sensitiv- harp—dull sensa~ )blCmS, reduced cognitive func— hley, and Ginter or falls are asso~ gral to stability. Balance and Postural Control 219 5.50ka and Labiner (1992) examined the relationship between lower extremity sen— sory and motor function and first falls in community—dwelling older adults and re— , I ed that if two or three risk factors Were present, the rate of falling increased 3.9 ' 'mes. Increased numbers of risk factors are also associated with increased probability frecurrent falls. Graafmans and colleagues (1996) examined fall risk factors and find that if five risk factors were present, there was an 84% chance that individuals would experience additional falls. Tinetti and Williams (1998) also reported a strong relationship between incidence of falls and declines in activities of daily living (ADL) functioning over a 1—3 -year interval; greater declines in ADL functions were associated .With increased numbers of falls. Repetitive fallers also experienced a decline in social functioning at both 1- and 3-year follow-ups. Risk factors for falls may be different for men and women. Campbell and col— leagues (1990) reported that decreased levels of physical activity, having had a stroke, " developing arthritis in the knees, impaired gait, and increased body sway were associ— ated with increased risk for falls in men. In contrast, for women, total number of medi~ I cations, postural hypotension, and muscle weakness were factors significantly associ— ' ated with increased risk of falling. Generally, women are more likely than men to fall in the home, men are more likely to fall outside the home often during participation in physical activity (Campbell et al., 1990). O ,1 H Sensory Organization of Posture/Balance and Aging Most available evidence suggests that the increase in sway and accompanying decline _ ' in balance among elderly individuals is largely a result of multiple deficits in a number 5 . of physiological systems (e.g., Sinclair SC Nayak, 1990). More specifically, sensory sys- .tems responsible for providing information to the postural control system for mainte— nance of balance deteriorate with age, as do central processes that organize, analyze, and integrate sensory information integral to balance. Changes that occur in the sen— sory organization of postural control with age often can and do have a dramatic effect on maintenance of balance. Because important changes occur with age in visual, ves- tibular, and proprioceptive systems and integration of information from all of these systems is critical to maintenance of balance, it seems logical that deficits in any one sensory system can lead to postural instability (Berger, Whitney, Redfern, (SC Furrnan, 1999). Vision clearly is important in maintaining effective postural control throughout life. After age 65, however, individuals tend to rely more on vision for regulating balm ance than is true at younger ages (Lord 8C Ward, 1994). Elderly individuals with poor functional balance also tend to show an overreliance on visual cues for maintaining stability (Sundermier et al., 1996). Declines in vision with advancing age can and often do lead to impaired postural stability and increased risk for falls. For example, Lord and Menz (2000) reported that postural sway (during standing on a compliant sur- face) was related to visual functions such as contrast sensitivity and stereopsis. Simoneau, Leihowitz, Ulbrecht, Tyrrell, and Cavanah (1992) also found that reduced visual acuity was associated with greater postural sway in the elderly and Lord, Clark, and Webster (1991) reported that increased postural sway in older adults was related to both poor visual acuity and contrast sensitivity. Because these functions typically decline with age, these data point to an important connection between impaired vision and increased incidence of falls among older adults. 220 CLINICAL DISORDERS Proprioceptive Function Proprioceptive and somatosensory functions undergo marked declines with age (Kaplan, Nixon, Reitz, Rindfleish, 85 Tucker, 1985; Skinner, Barrack, 85 Cook, 198 ). Still, it is clear that proprioceptive input is an important source of sensory input in maintenance of stability at all ages (Lord 86 Ward, 1994; Lord et al., 1991). Colledge and colleagues (1994) assessed sway in four age groups (20—40, 40760, 60—70, and 70+ years) using posturography and found that at all ages individuals were more de— pendent on proprioceptive input than vision in maintaining balance. Similarly, Judgea King, Whippie, Ciive, and Wolfson (1996) examined the relative importance of visual (amount of sway with eyes closed) and proprioceptive inputs {amount of sway on a compliant surface) to stability; they reported that in the elderly the effect of reduced proprioceptive input on balance was four times greater than the effect of reduced 5 vision. The decline in proprioceptive function with age and the importance of proprioceptive input in maintenance of stability at all ages puts the older individual at increased risk for loss of balance and potentially for falling. Vestibular While it is clear that vestibular information plays an integral role in maintenance of balance, the relationship between vestibular function and the maintenance of stability as a function of age is less clear. Impairment of vestibular function is often present in elderly individuals and data indicate that the elderly have greater difficulty than young adults do in maintaining balance under conditions in which vestibular information is the primary source of sensory input available for detecting changes in stability (Peterka 55 Benolken, 1995). Under these conditions older individuals are clearly less stable and more likely to fall. Stiil other evidence indicates that variations in vestibular function seen in older adults are not significantly related to increased sway (Lord et al., 1991). Intersensory Integration Intersensory functions also appear to deciine with age; the older individual becomes more reliant on the availability of accurate and redundant sensory inputs (e.g., visual, proprioceptive, and vestibular) for balance control. Peterka and Black (1990b) as— sessed postural sway in individuals 7 to 81 years old under a number of different sen— sory conditions. Results indicated that when vision was absent or conflicting, older adults fell more frequently than did younger individuals. When both visual and proprioceptive inputs were diminished or conflicting, sway and the likelihood of falls increased significantly more in older than in younger adults. This was especially true for individuals 55 years and older. Judge and colleagues (1996) report similar results and showed that risk of loss of balance increased some fivefold when both visual and proprioceptive inputs were diminished and sevenfold when there was conflicting visual and diminished proprioceptive inputs. In addition, repeated exposure to reduced or conflicting sensory inputs did not improve the capacity of older individuals to adapt to such conditions. Borger and colleagues (1999) examined the effect of the “moving room” phenom- enon on postural control of healthy older adults. Compared to younger individuals, lines with age SC Cook, 1984). ensory input in 1991). Colledge 50, 60—70, and were more de- imilarly, Judge, rtance of visual .t of sway on a 'fect of reduced Fect of reduced importance of er individual at naintenance of nce of stability iften present in dry than young information is ability (Peterka ' less stable and ibular function et al., 1991). ridual becomes ltS (e.g., visual, 1k (1990b) as— f different sen— uflicting, older )th visual and :lihood of falls especially true similar results voth visual and nflicting visual to reduced or .als to adapt to 30m” phenom! er individuals, Balance and Postural Control 221 .older adults were more dramatically affected (e.g., showed significantly greater in— .-creases in sway) when there was conflicting visual information from the “moving _ mom.” In addition, when diminished proprioceptive inputs were coupled with errone~ ous visual cues from the moving room, age differences in sway and falls were even _' more dramatic. Together, all the foregoing data suggest the following: (1) that to ' maintain balance effectively, older adults require clear and redundant sensory input ; from both visual and proprioceptive systems; and (2) that the capacity of the older per~ son to regulate balance when vestibular input is the primary source of sensory infor~ mation available is at best problematic. I '_ Motor (Ioordinatiou and Balance Contro in the Elderly To effectively maintain balance, an individual must detect perturbations to balance ' 3 and then organize an appropriate response to correct for loss of balance. important changes take place in the organization and execution of postural responses with age. ' Postural Synergies and Aging - Postural responses leg, synergies) undergo a number of changes with age (Nardone, I Siliotto, Grasso, 86 Schieppati, 1995; Woollacott 8C Shuinway—Cook, 1990; Wool- lacott et al., 1986). First, the speed with which postural synergies are activated is slower and more variable in the elderly than in young adults. This is particularly true _ of the distal muscles of the lower leg and is most evident in the tibialis muscle and more dramatic in individuals 5 5 + years (Peterka SC Black, 19903; Whipple, Wolfson, Derby, Singh, 83: Tobin, 1993). The slowing of the tibialis response is believed to con— tribute significantly to diminished ability to regulate backward sway. Weeks (1994) also reported that the elderly are less consistent in temporal scaling of postural syner« gies. Overall, older adults require more time to detect postural disturbances and to ini- tiate responses to instability produced by that disturbance. The typical sequencing of activation of muscles is also disrupted more frequently in older individuals. For example, older adults frequently employ a hip strategy (muse cles are activated in a proximodistal sequence) in responding to perturbations to bal- ance when the ankie strategy (distoproximal activation of muscles) is more efficient (Manchester, Woollacott, Zederbauer-Hylton, 8C Marin, 1989; Stelrnach, Phillips, DiFabio, SC Teasdale, 1989; Woollacott 8C Shumway—Cook, 1990). Sundermeir and colleagues (1996) compared postural control of older adults with and without balance problems and reported that older adults with poorer balance tended to employ the hip strategy more often than those with better balance. Other evidence indicates that some 25—40% of older individuals display temporal reversals of activation of muscles in— volved in postural responses (e.g., the quadriceps muscle is activated prior to the tibialis) (Peterka 86 Black, 1990a; Woollacott et al., 1986). Together with the data on the slowing of postural responses, these findings point to a widespread breakdown in the timing of muscle activity in elderly individuals. There is also evidence that the variability of the relative amplitude of muscle activity involved in postural synergies increases with age (Daubney 8C Cuihmam, 1999; Quoniam, Hay, Roll, SC Harlay, 19955 Woollacott 8C Shumway~Cook, 1990; Woollacott et al., 1986). In general, older adults tend to over— or underestimate the 222 CLiNlCAL DISORDERS amplitude and velocity of the postural response needed to correct for perturbations to balance than do younger adults. Thus, the available data indicate that older adults are less able to accurately scale muscle activity to the nature of the postural disturbance. This makes them more vulnerable to falling and the potential negative health conse— quences. Older adults also frequently contract agonist and antagonist muscles simulta- , neously to reduce sway and maintain stability (e.g., Woollacott SC Shumvvay-Cook, 1990). That is, they exhibit co-contraction of postural muscles, a condition that results in stiffening of joints. It has been suggested that this co~contraction of postural muscles is a kind of protective mechanism and may be a Way that older adults compensate for their inability to fine tune postural responses (Woollacott, lnglin, 5C Manchester 1988). Central Integrative Processes, Balance, and Aging Three levels of control have been hypothesized to contribute to the regulation of bai- ance; the stretch reflex, long-latency (e.g., long loop) postural synergies, and higher levels of integration involved in synthesizing vestibular, visual, and somatosensory in— formation (Steimach, Teasdale, DiFabio, 8:: Phillips, 1989). Generally, data indicate that stretch reflex responses are maintained with age, While higher-level, central inte~ grative control of posture declines dramatically (Colledge et al., 1994; Quoniam et al., 1995; Stelmach, Phillips, et al., 1989). This decline in central integrative processing is best illustrated in the change observed with age in the deiicate timing of postural and voluntary responses. ‘ Older individuals generally exhibit poor timing of postural responses and volun- tary movement (e.g., Stelmach, Teasdale, et al., 1989). In young individuals, the pos~ rural framework needed to support voluntary action is set quickly and in advance of the onset of the voluntary response {e.g., appropriate stability is established prior to reaching for an object on a high shelf). This relationship is often disrupted and is slower and less consistent in elderly individuals. This can and often does result in loss of balance, accidental falls, and other injuries. GAE? AND MOBEHTV Normal gait depends on adequate functioning and integration among a number of physiological systems (e.g., nervous, muscular, skeletal, circulatory, and respiratory). Injury to or disease in one or more of these systems frequently leads to impairment of gait, reduced mobility, and decreased independence (Imms 5C Edholm, 1981). Distur- bances of gait are prevalent in the elderly (11131115 56 Edholm 1981). Older individuals perform more poorly than do young adults on clinical measures of gait and mobility; specific gait parameters including velocity, cadence, stride length, and singleustance du— ration all ShOW agewrelated changes (Himann, Cunningham, Rechnitzer, 84'. Paterson, 1988; Lord, Lloyd, SC Li, 1996; Okuzumi et al., 1995), Himann and colleagues (1988) described the relationship between age and self- selected speeds of walking. Persons younger than 62 showed a 1—2% decline per de— cade in normal walking speed. After 63 years of age, there was a 12% (females) to )r perturbations to :.at older adults are stural disturbance. itive health conse- : muscles simulta- : Shumway-Cook, idition that results )f postural muscles lts compensate for n, 85 Manchester regulation of bal- ergies, and higher somatosensory in- ally, data indicate level, central inte- 4; Quoniam et al., ative processing is ig of postural and sonses and volun- iividuals, the pos— and in advance of tablished prior to disrupted and is does result in loss orig a number of and respiratory). to impairment of m, 1981). Distur— Older individuals gait and mobility; lsingle-stance du- tzer, 85 Paterson, 'een age and self- % decline per de~ 12% (females) to Balance and Postural Control 223 16% (males) decrease in walking speed per decade. In general, older females walked at significantly slower speeds, had shorter strides, and took more steps than older males. There is also evidence that older persons who perform poorly on clinical tests of {gait and mobility tend to be at increased risk of falling (Lord et al., 1996). Gait characfl .teristics such as decreased arm swing, increased trunk sway, slow walking speed, un- equal or asymmetrical stepping, and broad-based gait are also more common in indi— viduals who have fallen than in those who have not (Lord et al., 1996). Lord and _ colleagues (1996) reported that individuals who‘had had multiple falls had reduced .. _ and more variable cadence and increased stance duration than nonfallers. PHYSCAE. ACTWETY, BALANiIlE, AND GAE? EN THE ELDEREX 'It has been suggested that the decline that occurs in balance and gait in the elderly is ' not an inevitable consequence of aging and is therefore treatable (Wolfson et al., I '1992). Results of research focused on the effect of physical activity on balance and gait ' have shown that physical activity is beneficial to individuais who are undergoing de- I. ; clines in balance and gait and that regular participation in physical activity can dimin- " 'ish the risk of falls in the elderly (Brown Sc Holloszy, 1991, 1993). For example, indi— viduals 61—70 years old who participated in an exercise program 1 hour a day, 5 days I '_ a week for 3 months showed significant improvament in strength, range of motion, standing balance, and ADL. All these are important factors in maintaining stability 3 and preventing falls. There were no changes in gait parameters (Brown 8C Holloszy, 1991) In a follow-up study, Brown and Holloszy (1993) examined the effects of a mod— I erate intensity endurance training program on the same elderly individuals who had _ completed the previous 3—month exercise program. Participants trained for 4-5 minutes a day, 4 days a week for 1 year. The training consisted of walking, brisk walking, up hill treadmill walking, stationary cycling, and jogging. The frequency, duration, and intensity of training were increased based on improvement in individual V02max values. Both gait and balance performance parameters showed significant improvement (single-limb stance, step, and stride length, etc.) The authors suggest that fast walking and jogging challenged both balance and gait and thus resulted in improvements in both. Shumway-Cook, Gruber, Baldwin, and Liao (1997) examined the effects of a multidimensional exercise program on balance, mobility, and risk for falls in communitydwelling older adults who had a history of falling. An individualized exer— cise program was designed to focus on specific impairments and functional disabilities of individual participants. Data indicated that those persons invoived in the individu~ alized exercise program showed improvement in both balance and mobility and had a significant reduction in fall risk. The authors point out that adherence to the program was a critical factor in whether or not improvements were realized. lndividuais who fully adhered to the exercise protocol had the greatest improvements in balance and mobility while those with low adherence showed less change. Other data {e.g., Rubenstein et al., 2000) indicate that low-to—moderate—intensity exercise (3 days/week for 12 weeks) can produce significant improvement in balance, gait, and hip and ankle ...
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