Journal of Gerontology 1975, Vol. 30, No. 6,661-667
Aging and Postural Sway in Women1 Betty Risteen Hasselkus, MS, and Georgia M. Shambes, PhD2
in the living T HEhumancenterbodyofshiftsgravity continuously during
upright stance. As Hellebrandt (1938) stated, standing is actually " . . . movement upon a stationary base." The extent of this postural sway might be interpreted as a measure of the body's ability to balance. In the young adult, the mean area of the functional supportive base that is utilized by the sway is less than 1 % and is well within the margins of safety (Hellebrandt, Mueller, Summers, Houtz, Heap, & Eubank, 1950). Balance and equilibrium appear to be more easily threatened in the elderly individual than in the young adult. It has been suggested that central nervous system control of postural muscle tone and balance diminishes with aging and postural responses become less efficient (Hasselkus, 1974; Sheldon, 1960). This may be partially the result of changes in muscle tone and reflex patterns which have been demonstrated in the aged (Paulson & Gottlieb, 1968; Sheldon, 1960). Other investigators have cited structural changes in the neuromuscular and central nervous systems which accompany aging (Critchley, 1931; Frolkis, 1968; Gutmann, Hanzlikova, & Jakoubek, 1968; Jen•The authors express their appreciation to the following individuals for their assistance in the preparation of this paper: Ms Shirley Hunsaker, Project Specialist, and Mr. T. P. Stewart, Specialist in Photography, Laboratory of .Neurophysiology, Univ. of Wisconsin, Madison. 'Depts. of Physical Education and Neurophysiology, Univ. of Wisconsin, Madison 53706.
nekins, Tomlinson, & Walton, 1971; McComas, Upton, & Sica, 1973; Monagle & Brody, 1974; Retzlaff & Fontaine, 1965; Serratrice, Roux, & Aquaron, 1968; Swash & Fox, 1972). Logically, one might expect to find a demonstrable increase in postural sway in older adults as a reflection of increasing testability. Hellebrandt and Braun (1939) measured postural sway in a group of 109 subjects ranging in age from 3 to 86 years. The authors concluded that the mean location of the center of gravity projection near the center of the supportive base was consistent at all ages, but the magnitude of the sway about the center of the base tended to be larger in the very young and very old. Postural stability was greatest in the young adult and middle-aged subjects. Differences between the age groups were not tested for statistical significance. The effect on postural sway of a standing position other than upright stance was not considered. No other literature was found regarding the effects of age on postural sway. The purpose of this study was to investigate the effects of aging on postural control in two stance positions — upright stance and forward lean stance. The mean areas of postural sway were statistically compared across two age groups, 20-30 years old and 70-80 years old. Center of gravity data were analyzed in order to answer the following questions: (1) Is the mean area of postural sway of the older adults larger 661
Downloaded from http://geronj.oxfordjournals.org/ at University of Western Ontario on December 2, 2014
The effects of aging on postural sway in upright and forward lean stance were investigated. Postural sway was measured on a center of gravity apparatus using two age groups of female subjects 20 to 30 years old and 70 to 80 years old. The older adults demonstrated significantly larger sway areas than the young adults in both stance positions. The patterning of the center of gravity projections on the base of support tended to be similar under all conditions except in the young adults/upright position where the antero-postero excursion was larger than the medial-lateral. The mean locations of the center of gravity projections were often posterior and to the left of the geometric center of the base of support. The distance between the two points was least in the older adults/forward lean position, i.e., the experimental unit which demonstrated the largest area of sway.
662
HASSELKUS AND SHAMBES
For this study, center of gravity projections were plotted using a Hewlett-Packard 9862 Calculator Plotter in conjunction with a Hewlett-Packard 9810A Programmable Calculator. Each trial yielded 18 projections at a rate of one per second. The area of sway was determined from these plotted projections by using Hewlett-Packard 9864A Digitizer, the discrete points mode, on the outermost center of gravity projections of each trial (Fig. 1). The area of the functional base of support (Morton, 1935) was similarly obtained. The METHODOLOGY continuous sampling mode was used to digitize Subjects for this study were female volun- directly from tracings on the base of support or teers who had no known neurological or severe "footprint" paper (Fig. 1). physiological disorders. The actual age range of Both the area of postural sway and the functhe 10 subjects in each group was 21-30 years tional base of support were computed in square and 73-80 years. inches. The sway area was then divided by the Data collection. — The 20 subjects were each area of the base of support, thus expressing the given six 18-sec. trials on the center of gravity area of postural sway as a percentage. apparatus — three trials in the upright standing The maximal excursions of the center of position and three trials in the forward lean gravity and the mean location of the center of position. The order of the stance tasks was ran- gravity for each trial were calculated from the domly assigned. The upright stance was a print-out of the "Hewlett-Packard Biplane natural erect standing position and the forward Center of Gravity and Plot Program." The lean stance was a standing posture with the mean center of gravity over trials for each trunk flexed anteriorly at the hip joints to 45°. stance position was then plotted manually on Data from a pilot study indicated that variation the "footprint" paper. The geometric center of within each individual between trials was so the functional base of support was also large that at least three trials were necessary to manually plotted directly on the "footprint" obtain an acceptable intraclass reliability coef- paper. The distance between each mean center ficient. of gravity and the geometric center of the base In order to standardize foot placement, a could then be digitized using a line length 1 1 / 2 " X 1 W X 13" wood block was placed be- program (Fig. 2). tween the subject's feet prior to beginning the trials. The subject positioned both feet against Analysis and treatment of the data. — The the sides of the block, with the anterior tip of data were analyzed using a 2 X 2 analysis of digit I even with a line drawn across the block. variance fixed effects model. The dependent than that of the young adults in both stance positions? (2) Is the increase in postural sway area across stance tasks proportionately larger for the older adults than for the young adults? Other parameters of postural sway were dealt with descriptively, in particular the mean location of the center of gravity projection as it related to the geometric center of the base of support and the maximal excursions of the center of gravity locations in the antero-postero and medial-lateral planes.
KJ. Fig. 1. Eighteen plotted center of gravity projections located between the feet (one trial) with outlines delineating the area of sway and the area of the functional base of support.
Downloaded from http://geronj.oxfordjournals.org/ at University of Western Ontario on December 2, 2014
Data reduction. — The apparatus, procedures, and calculations used to determine center of gravity locations have been described previously by Waterland and Shambes (1970). The center of gravity board was in the shape of a right triangle. A metal screw was embedded in each corner of the triangular platform and the tip of each screw rested upon a dial scale. The subject's total weight was transmitted through the screws and was equal to the sum of the three scale readings. Synchronized biplane photographs of the scales, taken at specific intervals, enabled the investigators to determine the vertical projections of the center of gravity on the base of support over time. Scale readings were obtained from the negatives by using a Model P-40 Recordak.
AGING AND POSTURAL SWAY IN WOMEN
663
Table 1. Analysis of Variance of Subject Age and Stance Position. SS
MS
F
Between-subject age
1
0.4041
0.4041
14.48a
Between-stance position
1
0.1210
0.1210
4.34b
Subject age x stance position
1
0.0624
0.0624
2.24
36
1.0033
0.0279
39
1.5908
Error
Total • =Mean location of center of gravity, three trials upright • = Mean location of center of gravity, three trials forward lean • = Geometric center of functional base of support
Fig. 2. Mean locations of center of gravity (upright position and forward lean position) and geometric center of functional base of support.
0.50 0.40 c a> O
•F,,36.01 = 7 . 4 4 'F,,36.O5 = 4.13
The distance of the mean location of the center of gravity from the geometric center of the functional base of support was descriptively analyzed for both age groups in both stance positions. Maximal excursions of the center of gravity projections in the antero-postero plane and the medial-lateral plane were also compared. RESULTS
0.30 -
0.20 0.10 -
Upright Position
Forward Lean Position
Fig. 3. Mean percentage of postural sway.
variable was the mean area of sway/base of support ratio. Independent variables were age and stance tasks. Age groups were compared to determine if the area of sway in the older adult group was significantly larger than that of the younger group. The interaction between age and stance tasks, i.e., a proportionately larger increase in sway area across stance tasks in the older group, was also tested for significance. Using a method described by Kirk (1968), power for the first test was calculated to be .97 and power for the interaction test was greater than .99 (alpha level .05).
The results of this study were based on 2160 center of gravity readings (108 readings per subject) and 20 footprint drawings. The data included mean areas of postural sway, maximal excursions of the center of gravity projections in the antero-postero and lateral planes, and the mean center of gravity locations as they related to the geometric center of the functional base of support. Inferential results. — Data from the analysis of variance concerning the main effects and the interaction of age and stance tasks are presented in Table 1. The mean area of postural sway of subjects in the young adult group was 0.23% of the base of support compared to 0.43970 in the older adult group. The difference in postural sway between age groups was statistically significant at the /K.01 level. Thus the area of sway was significantly larger in the older adults than in the young adults in both stance positions. The main effect of stance position was also significant. The increase in postural sway across stance tasks was greater in the older adults than in the younger group, although this difference was not statistically significant. Fig. 3 depicts the interaction in graph form.
Downloaded from http://geronj.oxfordjournals.org/ at University of Western Ontario on December 2, 2014
DF
Source
664
HASSELKUS AND SHAMBES
was exactly the same in both stance positions
(0.63 in.), and that of the older adults/upright stance was slightly larger than this (0.67 in.). In the young adults/upright position, the amount of excursion of the center of gravity in the antero-postero plane (0.52 in.) was considerably larger than that in the lateral plane (0.31 in.). In the young adults/forward lean and in the older adults in both positions, the amount of excursion in both planes was more alike. The mean excursions in both planes were largest in the older adults in the forward lean position. DISCUSSION
The fact that the postural sway area of the older adults in this study was significantly larger than that of the young adults in both stance positions lends support to the theory that the postural control of the human neuromuscular system declines with aging. The measurement extremes demonstrated by the older adults in the forward lean position suggest that this combination of variables yields a postural response which differs from that found under the other conditions. Upright stance findings. — The significant difference in the postural sway area of the subjects at the two age levels supports the findings of Hellebrandt and Braun (1939) in their early study on the influence of sex and age on postural sway. However, Hellebrandt et al. (1950) stated that the normal adult utilizes 0.66% of his functional base of support in postural sway during upright stance. Mean percentages in this study were much lower with the young adults demonstrating 0.21% in the upright position and the older adults averaging 0.33%. The difference between these findings
and Hellebrandt's may be due to greater accuracy in the measurement instrument and in the methods of data reduction used in this experiment. Hellebrandt found the area of sway by outlining only the four maximal points of sway — the most anterior, posterior, and lateral. By using the Hewlett-Packard 9864A Digitizer, it was possible to more precisely delineate the sway area by including all peripheral points in the boundaries. The area of the functional base of support was also obtained by digitizing along the actual lateral borders of the feet rather than relying on the trapezoidal figure. In addition, Hellebrandt's subjects were both males and females (1950), and Hellebrandt and Braun (1939) had previously demonstrated that males utilize a greater percentage of their base of support in sway. Hellebrandt (1938) reported that the excursion of the center of gravity during stance was greater in the antero-postero plane than in the medial-lateral plane. This predicted pattern was obtained in the young adult group in the upright position but was not clearly demonstrated in the older adults. Hellebrandt's subjects were young college freshmen, very close in age to the young adult subjects in this study. Perhaps her findings are specific to this age grouping. The position of the mean center of gravity tended to be posterior and to the left of the geometric center of the base in both age groups. This was in agreement with Hellebrandt's later studies (Hellebrandt & Fries, 1942; Hellebrandt, Fries, Larsen, & Kelso, 1944). The mean distance between the mean center of gravity and the geometric center of the base in the young subjects (0.63 in.) was only slightly different from that of the older subjects (0.67 in.) in the upright position. An evaluation of individual mean scores revealed no clear-cut pattern of relationship between the extent of this distance and the amount of sway that was demonstrated. The large mtarindividual variation which existed in both the young and old was unexpected. Hellebrandt (1938) and Hellebrandt and Fries (1942) had concluded that individuals are consistent from trial to trial in the magnitude and pattern of their sway. The findings of this study tend to contradict these early assertions. In this experimental setting, with 18sec. trials and one center of gravity projection per second, most subjects were very in-
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Descriptive results. — The mean position of the center of gravity — across the three trials in each stance position — was plotted for each subject to determine its spatial relationship to the geometric center of the base of support. Of the 20 plotted means in each stance position 18 were to the left of the geometric center of the base and 13 were posterior to the center of the base. The actual distance in inches between the center of gravity and the geometric center of the base was also measured for each subject. The mean distance between the two centers was smallest in the older adults in the forward lean position (0.49 in.), the same experimental unit which demonstrated the highest percentage of sway. The mean distance of the young adults
AGING AND POSTURAL SWAY IN WOMEN consistent in their amount of sway across trials. Hellebrandt (1938) suggested that inconsistency is due to cortical interference, a source of error which is difficult to control. It is obvious that the testing situation itself increases the subject's awareness of standing balance, a motor act which would normally be executed on a more subconscious level.
both tasks was 0.0459 compared to 0.0166 for the young adults. Although the F-test for interaction between subject age and stance task was not significant, the possibility that such an interaction does occur is strongly suggested and warrants further study, perhaps with larger sample populations. Measurement of the actual distance between the mean center of gravity and the geometric center of the base yielded unexpected results. Historically, the implication of this parameter has been that stability increases as the distance between the two points decreases. In this study, the distance was the least under the conditions of greatest /^stability, i.e., the older adults in the forward lean position. In other words, the subjects who demonstrated the greatest area of sway were paradoxically closest to the geometric center of their base. This casts some doubt on previous interpretations of the relationship between this variable and postural sway. Hellebrandt's (1938) conclusions that the excursion of the center of gravity in the anteropostero plane is larger than that in the mediallateral plane were not supported with either age group in the forward lean stance. In the older adults in the forward lean position, the mediallateral excursion was even slightly larger than that in the antero-postero plane. Hellebrandt's findings were supported only under the most stable conditions — the young adults in the upright position. The presence of a trial effect in the older adults in the forward lean position is interesting. It was probably at least partly due to greater cortical involvement in this forward lean task than in upright stance. The decrease in sway area across trials is akin to a "practice" effect in a skill. Not every older subject demonstrated this trend, but the over-all effect was one of greatest postural stability during the third trial. Additional observations. — This small sample of seemingly very healthy and active elderly people exhibited an unexpected preponderance of foot abnormalities. The bony structures seem to almost "collapse" after a lifetime of support and ambulation. One wonders what effect this alone might have on stance stability. The foot position which was imposed in this study may have been more nearly normal for the young adults than for the older adults. Hellebrandt and Braun (1939) found that their older subjects tended to demonstrate a wider
Downloaded from http://geronj.oxfordjournals.org/ at University of Western Ontario on December 2, 2014
Forward lean findings. — One of the few generalizations which emerges from much of the literature on aging is that the effects of age are most obvious in situations of stress (Birren & Botwinick, 1955; Sheldon, 1960; Shock, 1962; Weg, 1973). The effort required by the aging human body to maintain its homeostasis probably increases as the cells of its body systems decline in number and/or functional efficiency (Hayflick, 1968; Shock, 1962; VonHahn, 1973). The effects of such a phenomenon are magnified when the departure from homeostasis is in any way extraordinary. It follows logically that the older adults in this study, when placed in the more threatening and thus stressful forward lean position, would cope less effectively with that situation than the younger adults and would consequently demonstrate an ever greater difference in postural sway than was evident in the more stable upright position. The range of mean scores within each age group over both stance positions (young, 0.07% to 0.52%; old, 0.13% to 0.99%) showed a considerable overlap in sway performance. However, the overlap between age groups in the forward lean position (37% of the total range of scores for that position) was markedly less than the 57% overlap demonstrated in the upright position. This, together with the actual difference in over-all mean scores as plotted in Fig. 3, suggested a greater effect of age on the postural sway in the forward lean stance than in the upright stance. The reasons this interaction was not statistically significant are not clear but perhaps the assumptions of normalcy of population distribution and/or homogeneity of variances were violated. VonHahn (1973) and Weg (1973) both claim that individual variability in all biological parameters increases progressively with age. If this is true, then with increasing age, people become less and less alike and consequently it becomes more and more difficult to obtain a representative sample for experimental research. Indeed, in this study, the interindividual variance of the older adults over
665
666
HASSELKUS AND SHAMBES
base of support than the younger subjects, perhaps in an unconscious effort to retain stability. One older adult in this study frankly stated, "I don't usually stand with my feet this close together." There is need in future studies to measure accurately the normal base of support in elderly individuals, to compare this to young adults, and to determine the area of postural sway under those conditions.
to the theory that the aging process involves a decline in the central control of posture. These findings have practical implications for the elderly with regard to environmental design, accident prevention, homemaking, self-care activities and safe participation in daily community life. Thus, many needs of the aged can be met more adequately with a basic understanding of the effects of aging on postural control.
SUMMARY AND CONCLUSIONS REFERENCES Birren, J. E., & Botwinick, J. Speed of response as a function of perceptual difficulty and age. Journal of Gerontology, 1955, /0, 433-436. Critchley, M. The neurology of old age. Lancet, 1931, /, 1119-1127. Frolkis, V. V. Regulatory processes in the mechanism of aging. Experimental Gerontology, 1968,3, 113-123. Gutmann, E., Hanzlikova, V., & Jakoubek, B. Changes in the neuromuscular system during old age. Experimental Gerontology, 1968,3, 141-146. Hasselkus, B. R. Aging and the human nervous system. American Journal of Occupational Therapy, 1974, 28, 16-21. Hayflick, L. Human cells and aging. Scientific American, 1968,275,32-37. Hellebrandt, F. A. Standing as a geotropic reflex — the mechanism of the asynchronous rotation of motor units. American Journal of Physiology, 1938,121, 471474. Hellebrandt, F. A., & Braun, G. L. The influence of sex and age on the postural sway of man. American Journal of Physical Anthropology, 1939,24, 347-360. Hellebrandt, F. A., & Fries, E. C. The constancy of oscillographic stance patterns. Physiotherapy Review, 1942,22, 17-23. Hellebrandt, F. A., Fries, E. C , Larsen, E. M., & Kelso, L. The influence of the army pack on postural stability and stance mechanics. American Journal of Physiology, 1944,140, 645-655. Hellebrandt, F. A., Mueller, E. E., Summers, I. M., Houtz, S. J., Heap, M. F., & Eubank, R. N. Influence of lower extremity amputation on stance mechanics. Journal of American Medical Association, 1950, 142, 1353-1356. Jennekins, F. G., Tomlinson, B. E., & Walton, J. N. Histochemical aspects of five limb muscles in old age. Journal of Neurological Science, 1911,14, 259-276. Kirk, R. E., Experimental design: Procedures for the behavioral sciences. Wadsworth, Belmont, CA, 1968. McComas, A. J., Upton, A., & Sica, R. Motoneurone disease and aging. Lancet, 1973, 7844, 1477-1480. Monagle, R. D., & Brody, H. Effects of age upon the main nucleus of the inferior olive in the human. Journal of Comparative Neurology, 1974, 755, 61-66. Morton, D. J. The human foot: Its evolution, physiology and functional disorders. Columbia Univ. Press, New York, 1935. Paulson, G., & Gottlieb, G. Developmental reflexes: The reappearance of foetal and neonatal reflexes in aged patients. Brain, 1968,91, 37-52. Retzlaff, E., & Fontaine, J. Functional and structural changes in motor neurons with age. In A. T. Welford &
Downloaded from http://geronj.oxfordjournals.org/ at University of Western Ontario on December 2, 2014
The purpose of this study was to investigate the effects of aging on postural control in varying stance positions. Postural sway was measured in two age groups of female subjects — 20-30 years old and 70-80 years old. The 10 subjects in each group were each given six 18sec. trials on the center of gravity platform — three trials in an upright position and three trials in a forward lean position. Synchronized bi-plane photographs were taken of the scales at 1-sec. intervals yielding 18 center of gravity projections per trial. With the aid of a HewlettPackard Programmable Calculator, Digitizer and Plotter, it was possible to obtain the area of sway per trial, maximal excursions of the sway points in the antero-postero and lateral planes, the location of the mean center of gravity per trial and its spatial relationship to the geometric center of the base. Within the limitations of this study, the following conclusions seem justified: (1) The difference in postural sway between age groups in both stance positions was statistically significant at the /K. 01 level with the older adults demonstrating larger sway areas. (2) The excursions of the center of gravity projections tended to be similar in the anteropostero and lateral planes under all conditions except the young adult/upright position where the antero-postero excursions were considerably larger. (3) The mean center of gravity position was most often, though not exclusively, located posterior and to the left of the geometric center of the base of support. (4) The distance measured between the mean center of gravity location and the geometric center of the base of support was least in the older adults/forward lean position — the same experimental unit which demonstrated the largest area of postural sway. The significance of the difference between sway areas in the two age groups lends support
AGING AND POSTURAL SWAY IN WOMEN J. E. Birren (Eds.), Behavior, aging and the nervous system. Charles C. Thomas, Springfield, IL, 1965. Serratrice, G., Roux, H., & Aquaron, R. Proximal muscle weakness in elderly subjects. Journal of Neurological Science, 1968, 7, 275-299. Sheldon, J. H. On the natural history of falls in old age. British MedicalJournal, 1960,-/, 1685-1690. Shock, N. W. The physiology of aging. Scientific American, 1962,206, 100-110. Swash, M., & Fox, K. The effect of age on human skeletal muscle (studies of the morphology and innervation of
667
muscle spindles). Journal of Neurological Science, 1972,16, 417-432. VonHahn, H. P. Primary causes of ageing: a brief review of some modern theories and concepts. Mechanisms of Ageing and Development, 1973,2, 245-250. Waterland, J. C , & Shambes, G. M. Biplane center of gravity procedures. Perceptual Motor Skills, 1970, 30, 511-514. Weg, R. B. The changing physiology of aging. American Journal of Occupational Therapy, 1973,27, 213-217.
Downloaded from http://geronj.oxfordjournals.org/ at University of Western Ontario on December 2, 2014
Aging and Postural Sway in Women1 Betty Risteen Hasselkus, MS, and Georgia M. Shambes, PhD2
in the living T HEhumancenterbodyofshiftsgravity continuously during
upright stance. As Hellebrandt (1938) stated, standing is actually " . . . movement upon a stationary base." The extent of this postural sway might be interpreted as a measure of the body's ability to balance. In the young adult, the mean area of the functional supportive base that is utilized by the sway is less than 1 % and is well within the margins of safety (Hellebrandt, Mueller, Summers, Houtz, Heap, & Eubank, 1950). Balance and equilibrium appear to be more easily threatened in the elderly individual than in the young adult. It has been suggested that central nervous system control of postural muscle tone and balance diminishes with aging and postural responses become less efficient (Hasselkus, 1974; Sheldon, 1960). This may be partially the result of changes in muscle tone and reflex patterns which have been demonstrated in the aged (Paulson & Gottlieb, 1968; Sheldon, 1960). Other investigators have cited structural changes in the neuromuscular and central nervous systems which accompany aging (Critchley, 1931; Frolkis, 1968; Gutmann, Hanzlikova, & Jakoubek, 1968; Jen•The authors express their appreciation to the following individuals for their assistance in the preparation of this paper: Ms Shirley Hunsaker, Project Specialist, and Mr. T. P. Stewart, Specialist in Photography, Laboratory of .Neurophysiology, Univ. of Wisconsin, Madison. 'Depts. of Physical Education and Neurophysiology, Univ. of Wisconsin, Madison 53706.
nekins, Tomlinson, & Walton, 1971; McComas, Upton, & Sica, 1973; Monagle & Brody, 1974; Retzlaff & Fontaine, 1965; Serratrice, Roux, & Aquaron, 1968; Swash & Fox, 1972). Logically, one might expect to find a demonstrable increase in postural sway in older adults as a reflection of increasing testability. Hellebrandt and Braun (1939) measured postural sway in a group of 109 subjects ranging in age from 3 to 86 years. The authors concluded that the mean location of the center of gravity projection near the center of the supportive base was consistent at all ages, but the magnitude of the sway about the center of the base tended to be larger in the very young and very old. Postural stability was greatest in the young adult and middle-aged subjects. Differences between the age groups were not tested for statistical significance. The effect on postural sway of a standing position other than upright stance was not considered. No other literature was found regarding the effects of age on postural sway. The purpose of this study was to investigate the effects of aging on postural control in two stance positions — upright stance and forward lean stance. The mean areas of postural sway were statistically compared across two age groups, 20-30 years old and 70-80 years old. Center of gravity data were analyzed in order to answer the following questions: (1) Is the mean area of postural sway of the older adults larger 661
Downloaded from http://geronj.oxfordjournals.org/ at University of Western Ontario on December 2, 2014
The effects of aging on postural sway in upright and forward lean stance were investigated. Postural sway was measured on a center of gravity apparatus using two age groups of female subjects 20 to 30 years old and 70 to 80 years old. The older adults demonstrated significantly larger sway areas than the young adults in both stance positions. The patterning of the center of gravity projections on the base of support tended to be similar under all conditions except in the young adults/upright position where the antero-postero excursion was larger than the medial-lateral. The mean locations of the center of gravity projections were often posterior and to the left of the geometric center of the base of support. The distance between the two points was least in the older adults/forward lean position, i.e., the experimental unit which demonstrated the largest area of sway.
662
HASSELKUS AND SHAMBES
For this study, center of gravity projections were plotted using a Hewlett-Packard 9862 Calculator Plotter in conjunction with a Hewlett-Packard 9810A Programmable Calculator. Each trial yielded 18 projections at a rate of one per second. The area of sway was determined from these plotted projections by using Hewlett-Packard 9864A Digitizer, the discrete points mode, on the outermost center of gravity projections of each trial (Fig. 1). The area of the functional base of support (Morton, 1935) was similarly obtained. The METHODOLOGY continuous sampling mode was used to digitize Subjects for this study were female volun- directly from tracings on the base of support or teers who had no known neurological or severe "footprint" paper (Fig. 1). physiological disorders. The actual age range of Both the area of postural sway and the functhe 10 subjects in each group was 21-30 years tional base of support were computed in square and 73-80 years. inches. The sway area was then divided by the Data collection. — The 20 subjects were each area of the base of support, thus expressing the given six 18-sec. trials on the center of gravity area of postural sway as a percentage. apparatus — three trials in the upright standing The maximal excursions of the center of position and three trials in the forward lean gravity and the mean location of the center of position. The order of the stance tasks was ran- gravity for each trial were calculated from the domly assigned. The upright stance was a print-out of the "Hewlett-Packard Biplane natural erect standing position and the forward Center of Gravity and Plot Program." The lean stance was a standing posture with the mean center of gravity over trials for each trunk flexed anteriorly at the hip joints to 45°. stance position was then plotted manually on Data from a pilot study indicated that variation the "footprint" paper. The geometric center of within each individual between trials was so the functional base of support was also large that at least three trials were necessary to manually plotted directly on the "footprint" obtain an acceptable intraclass reliability coef- paper. The distance between each mean center ficient. of gravity and the geometric center of the base In order to standardize foot placement, a could then be digitized using a line length 1 1 / 2 " X 1 W X 13" wood block was placed be- program (Fig. 2). tween the subject's feet prior to beginning the trials. The subject positioned both feet against Analysis and treatment of the data. — The the sides of the block, with the anterior tip of data were analyzed using a 2 X 2 analysis of digit I even with a line drawn across the block. variance fixed effects model. The dependent than that of the young adults in both stance positions? (2) Is the increase in postural sway area across stance tasks proportionately larger for the older adults than for the young adults? Other parameters of postural sway were dealt with descriptively, in particular the mean location of the center of gravity projection as it related to the geometric center of the base of support and the maximal excursions of the center of gravity locations in the antero-postero and medial-lateral planes.
KJ. Fig. 1. Eighteen plotted center of gravity projections located between the feet (one trial) with outlines delineating the area of sway and the area of the functional base of support.
Downloaded from http://geronj.oxfordjournals.org/ at University of Western Ontario on December 2, 2014
Data reduction. — The apparatus, procedures, and calculations used to determine center of gravity locations have been described previously by Waterland and Shambes (1970). The center of gravity board was in the shape of a right triangle. A metal screw was embedded in each corner of the triangular platform and the tip of each screw rested upon a dial scale. The subject's total weight was transmitted through the screws and was equal to the sum of the three scale readings. Synchronized biplane photographs of the scales, taken at specific intervals, enabled the investigators to determine the vertical projections of the center of gravity on the base of support over time. Scale readings were obtained from the negatives by using a Model P-40 Recordak.
AGING AND POSTURAL SWAY IN WOMEN
663
Table 1. Analysis of Variance of Subject Age and Stance Position. SS
MS
F
Between-subject age
1
0.4041
0.4041
14.48a
Between-stance position
1
0.1210
0.1210
4.34b
Subject age x stance position
1
0.0624
0.0624
2.24
36
1.0033
0.0279
39
1.5908
Error
Total • =Mean location of center of gravity, three trials upright • = Mean location of center of gravity, three trials forward lean • = Geometric center of functional base of support
Fig. 2. Mean locations of center of gravity (upright position and forward lean position) and geometric center of functional base of support.
0.50 0.40 c a> O
•F,,36.01 = 7 . 4 4 'F,,36.O5 = 4.13
The distance of the mean location of the center of gravity from the geometric center of the functional base of support was descriptively analyzed for both age groups in both stance positions. Maximal excursions of the center of gravity projections in the antero-postero plane and the medial-lateral plane were also compared. RESULTS
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0.20 0.10 -
Upright Position
Forward Lean Position
Fig. 3. Mean percentage of postural sway.
variable was the mean area of sway/base of support ratio. Independent variables were age and stance tasks. Age groups were compared to determine if the area of sway in the older adult group was significantly larger than that of the younger group. The interaction between age and stance tasks, i.e., a proportionately larger increase in sway area across stance tasks in the older group, was also tested for significance. Using a method described by Kirk (1968), power for the first test was calculated to be .97 and power for the interaction test was greater than .99 (alpha level .05).
The results of this study were based on 2160 center of gravity readings (108 readings per subject) and 20 footprint drawings. The data included mean areas of postural sway, maximal excursions of the center of gravity projections in the antero-postero and lateral planes, and the mean center of gravity locations as they related to the geometric center of the functional base of support. Inferential results. — Data from the analysis of variance concerning the main effects and the interaction of age and stance tasks are presented in Table 1. The mean area of postural sway of subjects in the young adult group was 0.23% of the base of support compared to 0.43970 in the older adult group. The difference in postural sway between age groups was statistically significant at the /K.01 level. Thus the area of sway was significantly larger in the older adults than in the young adults in both stance positions. The main effect of stance position was also significant. The increase in postural sway across stance tasks was greater in the older adults than in the younger group, although this difference was not statistically significant. Fig. 3 depicts the interaction in graph form.
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was exactly the same in both stance positions
(0.63 in.), and that of the older adults/upright stance was slightly larger than this (0.67 in.). In the young adults/upright position, the amount of excursion of the center of gravity in the antero-postero plane (0.52 in.) was considerably larger than that in the lateral plane (0.31 in.). In the young adults/forward lean and in the older adults in both positions, the amount of excursion in both planes was more alike. The mean excursions in both planes were largest in the older adults in the forward lean position. DISCUSSION
The fact that the postural sway area of the older adults in this study was significantly larger than that of the young adults in both stance positions lends support to the theory that the postural control of the human neuromuscular system declines with aging. The measurement extremes demonstrated by the older adults in the forward lean position suggest that this combination of variables yields a postural response which differs from that found under the other conditions. Upright stance findings. — The significant difference in the postural sway area of the subjects at the two age levels supports the findings of Hellebrandt and Braun (1939) in their early study on the influence of sex and age on postural sway. However, Hellebrandt et al. (1950) stated that the normal adult utilizes 0.66% of his functional base of support in postural sway during upright stance. Mean percentages in this study were much lower with the young adults demonstrating 0.21% in the upright position and the older adults averaging 0.33%. The difference between these findings
and Hellebrandt's may be due to greater accuracy in the measurement instrument and in the methods of data reduction used in this experiment. Hellebrandt found the area of sway by outlining only the four maximal points of sway — the most anterior, posterior, and lateral. By using the Hewlett-Packard 9864A Digitizer, it was possible to more precisely delineate the sway area by including all peripheral points in the boundaries. The area of the functional base of support was also obtained by digitizing along the actual lateral borders of the feet rather than relying on the trapezoidal figure. In addition, Hellebrandt's subjects were both males and females (1950), and Hellebrandt and Braun (1939) had previously demonstrated that males utilize a greater percentage of their base of support in sway. Hellebrandt (1938) reported that the excursion of the center of gravity during stance was greater in the antero-postero plane than in the medial-lateral plane. This predicted pattern was obtained in the young adult group in the upright position but was not clearly demonstrated in the older adults. Hellebrandt's subjects were young college freshmen, very close in age to the young adult subjects in this study. Perhaps her findings are specific to this age grouping. The position of the mean center of gravity tended to be posterior and to the left of the geometric center of the base in both age groups. This was in agreement with Hellebrandt's later studies (Hellebrandt & Fries, 1942; Hellebrandt, Fries, Larsen, & Kelso, 1944). The mean distance between the mean center of gravity and the geometric center of the base in the young subjects (0.63 in.) was only slightly different from that of the older subjects (0.67 in.) in the upright position. An evaluation of individual mean scores revealed no clear-cut pattern of relationship between the extent of this distance and the amount of sway that was demonstrated. The large mtarindividual variation which existed in both the young and old was unexpected. Hellebrandt (1938) and Hellebrandt and Fries (1942) had concluded that individuals are consistent from trial to trial in the magnitude and pattern of their sway. The findings of this study tend to contradict these early assertions. In this experimental setting, with 18sec. trials and one center of gravity projection per second, most subjects were very in-
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Descriptive results. — The mean position of the center of gravity — across the three trials in each stance position — was plotted for each subject to determine its spatial relationship to the geometric center of the base of support. Of the 20 plotted means in each stance position 18 were to the left of the geometric center of the base and 13 were posterior to the center of the base. The actual distance in inches between the center of gravity and the geometric center of the base was also measured for each subject. The mean distance between the two centers was smallest in the older adults in the forward lean position (0.49 in.), the same experimental unit which demonstrated the highest percentage of sway. The mean distance of the young adults
AGING AND POSTURAL SWAY IN WOMEN consistent in their amount of sway across trials. Hellebrandt (1938) suggested that inconsistency is due to cortical interference, a source of error which is difficult to control. It is obvious that the testing situation itself increases the subject's awareness of standing balance, a motor act which would normally be executed on a more subconscious level.
both tasks was 0.0459 compared to 0.0166 for the young adults. Although the F-test for interaction between subject age and stance task was not significant, the possibility that such an interaction does occur is strongly suggested and warrants further study, perhaps with larger sample populations. Measurement of the actual distance between the mean center of gravity and the geometric center of the base yielded unexpected results. Historically, the implication of this parameter has been that stability increases as the distance between the two points decreases. In this study, the distance was the least under the conditions of greatest /^stability, i.e., the older adults in the forward lean position. In other words, the subjects who demonstrated the greatest area of sway were paradoxically closest to the geometric center of their base. This casts some doubt on previous interpretations of the relationship between this variable and postural sway. Hellebrandt's (1938) conclusions that the excursion of the center of gravity in the anteropostero plane is larger than that in the mediallateral plane were not supported with either age group in the forward lean stance. In the older adults in the forward lean position, the mediallateral excursion was even slightly larger than that in the antero-postero plane. Hellebrandt's findings were supported only under the most stable conditions — the young adults in the upright position. The presence of a trial effect in the older adults in the forward lean position is interesting. It was probably at least partly due to greater cortical involvement in this forward lean task than in upright stance. The decrease in sway area across trials is akin to a "practice" effect in a skill. Not every older subject demonstrated this trend, but the over-all effect was one of greatest postural stability during the third trial. Additional observations. — This small sample of seemingly very healthy and active elderly people exhibited an unexpected preponderance of foot abnormalities. The bony structures seem to almost "collapse" after a lifetime of support and ambulation. One wonders what effect this alone might have on stance stability. The foot position which was imposed in this study may have been more nearly normal for the young adults than for the older adults. Hellebrandt and Braun (1939) found that their older subjects tended to demonstrate a wider
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Forward lean findings. — One of the few generalizations which emerges from much of the literature on aging is that the effects of age are most obvious in situations of stress (Birren & Botwinick, 1955; Sheldon, 1960; Shock, 1962; Weg, 1973). The effort required by the aging human body to maintain its homeostasis probably increases as the cells of its body systems decline in number and/or functional efficiency (Hayflick, 1968; Shock, 1962; VonHahn, 1973). The effects of such a phenomenon are magnified when the departure from homeostasis is in any way extraordinary. It follows logically that the older adults in this study, when placed in the more threatening and thus stressful forward lean position, would cope less effectively with that situation than the younger adults and would consequently demonstrate an ever greater difference in postural sway than was evident in the more stable upright position. The range of mean scores within each age group over both stance positions (young, 0.07% to 0.52%; old, 0.13% to 0.99%) showed a considerable overlap in sway performance. However, the overlap between age groups in the forward lean position (37% of the total range of scores for that position) was markedly less than the 57% overlap demonstrated in the upright position. This, together with the actual difference in over-all mean scores as plotted in Fig. 3, suggested a greater effect of age on the postural sway in the forward lean stance than in the upright stance. The reasons this interaction was not statistically significant are not clear but perhaps the assumptions of normalcy of population distribution and/or homogeneity of variances were violated. VonHahn (1973) and Weg (1973) both claim that individual variability in all biological parameters increases progressively with age. If this is true, then with increasing age, people become less and less alike and consequently it becomes more and more difficult to obtain a representative sample for experimental research. Indeed, in this study, the interindividual variance of the older adults over
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base of support than the younger subjects, perhaps in an unconscious effort to retain stability. One older adult in this study frankly stated, "I don't usually stand with my feet this close together." There is need in future studies to measure accurately the normal base of support in elderly individuals, to compare this to young adults, and to determine the area of postural sway under those conditions.
to the theory that the aging process involves a decline in the central control of posture. These findings have practical implications for the elderly with regard to environmental design, accident prevention, homemaking, self-care activities and safe participation in daily community life. Thus, many needs of the aged can be met more adequately with a basic understanding of the effects of aging on postural control.
SUMMARY AND CONCLUSIONS REFERENCES Birren, J. E., & Botwinick, J. Speed of response as a function of perceptual difficulty and age. Journal of Gerontology, 1955, /0, 433-436. Critchley, M. The neurology of old age. Lancet, 1931, /, 1119-1127. Frolkis, V. V. Regulatory processes in the mechanism of aging. Experimental Gerontology, 1968,3, 113-123. Gutmann, E., Hanzlikova, V., & Jakoubek, B. Changes in the neuromuscular system during old age. Experimental Gerontology, 1968,3, 141-146. Hasselkus, B. R. Aging and the human nervous system. American Journal of Occupational Therapy, 1974, 28, 16-21. Hayflick, L. Human cells and aging. Scientific American, 1968,275,32-37. Hellebrandt, F. A. Standing as a geotropic reflex — the mechanism of the asynchronous rotation of motor units. American Journal of Physiology, 1938,121, 471474. Hellebrandt, F. A., & Braun, G. L. The influence of sex and age on the postural sway of man. American Journal of Physical Anthropology, 1939,24, 347-360. Hellebrandt, F. A., & Fries, E. C. The constancy of oscillographic stance patterns. Physiotherapy Review, 1942,22, 17-23. Hellebrandt, F. A., Fries, E. C , Larsen, E. M., & Kelso, L. The influence of the army pack on postural stability and stance mechanics. American Journal of Physiology, 1944,140, 645-655. Hellebrandt, F. A., Mueller, E. E., Summers, I. M., Houtz, S. J., Heap, M. F., & Eubank, R. N. Influence of lower extremity amputation on stance mechanics. Journal of American Medical Association, 1950, 142, 1353-1356. Jennekins, F. G., Tomlinson, B. E., & Walton, J. N. Histochemical aspects of five limb muscles in old age. Journal of Neurological Science, 1911,14, 259-276. Kirk, R. E., Experimental design: Procedures for the behavioral sciences. Wadsworth, Belmont, CA, 1968. McComas, A. J., Upton, A., & Sica, R. Motoneurone disease and aging. Lancet, 1973, 7844, 1477-1480. Monagle, R. D., & Brody, H. Effects of age upon the main nucleus of the inferior olive in the human. Journal of Comparative Neurology, 1974, 755, 61-66. Morton, D. J. The human foot: Its evolution, physiology and functional disorders. Columbia Univ. Press, New York, 1935. Paulson, G., & Gottlieb, G. Developmental reflexes: The reappearance of foetal and neonatal reflexes in aged patients. Brain, 1968,91, 37-52. Retzlaff, E., & Fontaine, J. Functional and structural changes in motor neurons with age. In A. T. Welford &
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The purpose of this study was to investigate the effects of aging on postural control in varying stance positions. Postural sway was measured in two age groups of female subjects — 20-30 years old and 70-80 years old. The 10 subjects in each group were each given six 18sec. trials on the center of gravity platform — three trials in an upright position and three trials in a forward lean position. Synchronized bi-plane photographs were taken of the scales at 1-sec. intervals yielding 18 center of gravity projections per trial. With the aid of a HewlettPackard Programmable Calculator, Digitizer and Plotter, it was possible to obtain the area of sway per trial, maximal excursions of the sway points in the antero-postero and lateral planes, the location of the mean center of gravity per trial and its spatial relationship to the geometric center of the base. Within the limitations of this study, the following conclusions seem justified: (1) The difference in postural sway between age groups in both stance positions was statistically significant at the /K. 01 level with the older adults demonstrating larger sway areas. (2) The excursions of the center of gravity projections tended to be similar in the anteropostero and lateral planes under all conditions except the young adult/upright position where the antero-postero excursions were considerably larger. (3) The mean center of gravity position was most often, though not exclusively, located posterior and to the left of the geometric center of the base of support. (4) The distance measured between the mean center of gravity location and the geometric center of the base of support was least in the older adults/forward lean position — the same experimental unit which demonstrated the largest area of postural sway. The significance of the difference between sway areas in the two age groups lends support
AGING AND POSTURAL SWAY IN WOMEN J. E. Birren (Eds.), Behavior, aging and the nervous system. Charles C. Thomas, Springfield, IL, 1965. Serratrice, G., Roux, H., & Aquaron, R. Proximal muscle weakness in elderly subjects. Journal of Neurological Science, 1968, 7, 275-299. Sheldon, J. H. On the natural history of falls in old age. British MedicalJournal, 1960,-/, 1685-1690. Shock, N. W. The physiology of aging. Scientific American, 1962,206, 100-110. Swash, M., & Fox, K. The effect of age on human skeletal muscle (studies of the morphology and innervation of
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muscle spindles). Journal of Neurological Science, 1972,16, 417-432. VonHahn, H. P. Primary causes of ageing: a brief review of some modern theories and concepts. Mechanisms of Ageing and Development, 1973,2, 245-250. Waterland, J. C , & Shambes, G. M. Biplane center of gravity procedures. Perceptual Motor Skills, 1970, 30, 511-514. Weg, R. B. The changing physiology of aging. American Journal of Occupational Therapy, 1973,27, 213-217.
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