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The influence of sex and age on the postural sway of man.

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THE INFLUENCE O F SEX AND AGE ON T H E
POSTURAL SWAY O F MAN*
FRANCES A. HELLEBRANDT AND G E N E V I E V E L. BRAUN
Department of Pliysiology, University of Wismnsin, Madison
ONE FIQWE
Standing is a complex neuromuscular act, dependent upon
the integrity of the myotatic reflex (Sherrington, '24) and
much affected by the influence upon the final common path of
impulses descending over extra-pyramidal tracts (Magnus,
'26). The ever-present collapsing stresses of gravity must
be constantly equilibrated by muscular contraction. This
involves the harmonious functioning of a large number of
muscles acting upon a multijointed structure of a size and
proportions which give to the biped stance considerable
mechanical instability. It is not surprising therefore that
the steadiness of standing has been used as a criterion of
both motor power (Bolton, '03) and neuromuscular control
(Miles, '22).
In 1893 Romberg directed attention to the diagnostic significance of the inordinate augmentation of postural sway evoked
in subjects with posterior column disease by simple closure
of the eyes and narrowing of the base of support. Efforts
had already been made to obtain quantitative measures of the
insecurity of the vertical stance (Mitchell and Lewis, 1886).
At the instigation of Wier Mitchell, Hinsdale (1887) developed one of the earliest precision devices. This, the atauiograph of Dana (1892) and the ataxiometer of Miles ( '22)
evaluate static equilibrium by modifications of a method generally credited to Vierordt, that of measuring postural instability by recording head sway. The method assumes that
the multijointed body oscillates in toto over the ankle joint.
* Supported in part
I)? a grant from the Wisconsin Alumni Research Foundation.
34i
AM!2RIPAN JOURV.4L O F PHYSICAL ANTH&OPOU)OY, VOL. XXIV, NO.
JANUARY-XARCH. 1939
3
348
F. A. HELLEBRANDT AND G. L. BRAUN
I n its application to heterogenous groups a correction for
variations in height must be applied.
Recently Moss ('31) devised a type of apparatus in the
form of an unstable platform which moves in a vertical direction upon a central pivotal point as the weight of the subject
shifts from foot to foot and heel to toe. Methods developed
in our laboratory (Kelso and Hellebrandt, '37) have made
it possible to obtain accurate simultaneous determinations of
the shifts in the center of gravity occurring in the two cardinal
vertical orientation planes of the body during natural comfortable standing and to project these into the base of support
a s a function of time. The object of the study here reported
was to determine the influence of sex and age on static equilibrium by this method which evaluates postural stability in
terms of one of the most fundamental mechanical factors by
which it is conditioned.
METHODS
By equating moments the maximal shifts in the center of
gravity existing simultaneously in the coronal and sagittal
planes were estimated from kymographically recorded changes
in balance weight which occurred when a subject stood on
the uppermost of a series of platforms constructed to affect
concurrently two scales placed at right angles to each other.
The scales were equipped with levers for the graphic registration of changes in spring tension. A secondary coil was
suspended from each lever so as to hang freely in the air-gap
of a slotted core transformer with a uniform flux density.
The current induced was led to integrating watt-hour meters.
From these could be obtained the average position of the
lever in any unit of time, hence the average scale load, and by
equating moments, the mean location of the perpetually shifting center of weight. The points of maximal sway in the two
vertical orientation planes and the mean position of the
center of weight during the total period of standing were
projected into the base of support which was delimited in the
following way. The soles of the feet were moistened with
STATIC EQUILIBRIUM
349
potassium permanganate and the subjpct stood on absorbent
paper in a fixed position on the platform of the center of
gravity apparatus. This left a durable footprint a known
distance from the supporting knife edges. The heels were
uniformly separated and the toes turned out so that the
breadth of the base of support a t the level of the fifth metatarso-phalangeal joints equalled the anteroposterior diameter
a s measured from the heel to the great toe. The guide
standardizing such a foot posture was removed from the plntform before making the center of gravity determinations.
The subjects then stood for 3 minutes, maintaining a natural
and comfortable stance with the gaze fixed on a designated
area giving a constant visual stimulus.
The subjects numbered 111. They ranged in age from 3
to 86. Being drawn largely from the university community
they represent a group superior in nutrition, fitness and bearing. This is particularly true of the young adult and middle
aged women, most of whom were professional students or
teachers of physical education. I n the age group comparisons, two sisters in the fourth decade were discarded because,
although manifesting no neurological disease, they demonstrated degrees of postural instability several hundred per
cent in excess of the mean value for the remaining. Their
inclusion distorted the group findings. Both assumed an
extremely lax type of standing which, for most subjects, was
impossible to simulate. This left a group of 109, 43 males
and 66 females. They were divided into five categories : those
of the first decade representing childhood; the second decade,
the period of adolescence ; the third decade, young adult life ;
the fourth and fifth decades, maturity. The remainder, most
of whom were well over 50 were classified as aged. After
such condensation the groups were too small to warrant a
detailed statistical analysis of the data.
350
F. A. HELLEBRANDT AND G . L. B R A U B
RESULTS AND THEIR INTERPRETATION
1. T h e proximity of the center of gravity projection to the
geometric cen.ter of the base of support in standing. The area
of functional support in the foot position selected forms a
trapezoid which may be bounded roughly by tangents to the
heel and lateral borders of each foot and a line through the
imprint left by the heads of the first metatarsals, since according to Morton ('35) the toes do not participate in normal
standing. The geometric center of the trapezoid representing
the functional base was introduced into each footprint. The
separation between this and the plumb falling from the experimentally determined center of weight was evaluated in
terms of eccentricity ratios. The perpendicular distance from
the gravity line to the transverse median was divided by
one-half the length of the anteroposterior median. Similarly,
the perpendicular distance from the projection point to the
anteroposterior median was divided by one-half the length
of the lateral median. The eccentricity was trifling in extent
but significantly consistent in its direction. In 85% of the
subjects the projection of the center of weight fell slightly in
front of the geometric center of the base. The mean eccentricity ratio in the anteroposterior plane was f0.2309. I n
only 15% of the subjects did the experimentally determined
center fall behind the mathematically derived one, and the
mean ratio was -0.0541. Thus we see that not only is there
a tendency for the average center of weight to be held in
front of the geometric center of the base, but the eccentricitv
in this direction is 76% greater than the deviation backward.
Basler ('29) reported that when his subject assumed a
free standing position with the feet parallel, the weight l h e
cut the base of support about 4 cm. to the right of the midline,
within the area covered by the right foot. Reynolds and
Hooton ('36) also noted a lack of symmetry in the stance of
their subjects. I n about one-third the weight line coincided
with the middle of the base. I n one-third it fell to the right
and in the remaining to the left. The asymmetry was unrelated to inequality in leg length. With a method slightly
STATIC EQUILIBRIUM
351
more accurate than that of Reynolds and Hooten but identical
with it in principle we found the gravity line equidistant between the heels in only 10.98% of 327 college girls standing
in their best posture and 13.25% of eighty-seven professional
physical education students assuming a comfortable stance
(Hellebrandt et al., ,'38). The weight fell preponderantly to
the left in the natural stance but when a deliberate effort was
made to assume a good posture, the center of gravity drifted
to the right. The data of all of these investigations (Basler,
Reynolds and Hooten, and Hellebrandt) came from single
instantaneous observations of the center of gravity. The findings of the present study have considerably more significance
because each observation represents the mean position of the
center of weight during 3 minutes of standing. The location
of the center of gravity of the normal subject in the vertical
posture oscillates rhythmically and the extent and range of
movement may sometimes be extreme (Hellebrandt, '38).
I n 72% of the subjects of the present study the mean eccentricity was to the left, averaging -0.1051. I n the remaining 28% it was to the right, the mean value being +0.0838.
The center of weight deviates more frequently to the left,
confirming our original observation on natural standing, and
exceeds by 20% the eccentricity to the right. In 63% the
eccentricity was to the left and in front of the geometric
center of the base; in 22% to the right and in front; in 9%
behind and to the left; and in 6% behind and to the right.
The peculiarities in the eccentricity of the center of weight
in its relation to the geometric center of the base were borne
out in both sexes. There were no significance age differences.
Hinsdale (1887) believed that his subjects swayed to the
heavier and stronger side, forward and to the right. Our
subjects are falling away from the heavier side, if the right
is more developed than the left as is generally accepted.
They appear to over compensate for their right-sided dominance. The tendency to overshoot in the forward direction
is more easily explained. The center of gravity falls considerably in front of the transverse axis of the ankle joint
332
F. A. HELLEBRANDT AND Q. L. BRAUN
thus creating a rotatory stress which tends constantly to
project the body forward. It must, however, be emphasized
that the mean eccentricity of the center of gravity projection
in both of the cardinal vertical orientation planes is in general
very slight.
Haycraft (Schafer, ’00)has reviewed the early, conflicting
evidence which variously placed the plumb passing through
the center of gravity in front of, above or behind the joints
of the weight bearing limbs. Steindler (’35) fixes the gravity
line “about 4 cm. in front of the center of the ankle joint.”
We observed it 4.92 cm. anterior to the malleolus in 327 college
freshmen standing in their best posture, and 5.65 cm. in 87
professional physical education students maintaining a
natural comfortable stance (Hellebrandt et al., ’38). The difference was more apparent than real because of group dissimilarity in average foot length. We have already called
attention to the fallacy of relating the gravity line to the
ankle joint without heed to the diameters of the base. Our
present findings indicate that the perpendicular dropped from
the average location of the center of weight falls remarkably
close to the geometric center of the base. This is in keeping
with the dictates of stable construction and is apparently a
fundamental relation not easily altered. We have demonstrated that it remains undisturbed by the heel height of the
shoes worn by women unless their pitch is extreme (Hellebrandt et al., ’37). Compensation is automatically achieved
by knee flexion and lordosis. With each new base there is a
new posture, and an accurate preservation of equal stability in
all diameters of the base, the center of weight projection always occupying a very nearly middle position within the area
of support.
2. T h e magnitude of the postural instability. Postural sway
was slight when measured in terms of shifts of the center of
gravity. The maximum displacement of the center of weight
in the two vertical orientation planes was projected into the
base. When the area enclosed by these four points indicating
greatest sway forward, back, to the right and left, was estimated as a percentage of the total area of support, it amounted
STATIC EQUILIBRIUM
353
on the average to 1.28%, ranging from 0.15 to 6.13 in the
steadiest and the most unstable respectively. I n absolute
units, the area of maximum sway ranged from 0.5 to 13.7 sq.cm.,
averaging 3.73. Small as is the area of underpropping in
relation to the disproportionate height of the center of gravity
of the human body in the vertical posture, it includes a relatively enormous margin of safety. The involuntary shifts in
the center of weight during standing occupy a very small core
surrounding the geometric c h t e r of the base. The area of
maximum sway in the two unstable subjects who were discarded from the group findings measured 19.9 and 26.9 sq.cm.
Even these exaggerated degrees of instability encroached
respectively upon only 5.41 and 7.35% of the base.
The group studied was anything but homogeneous. The
subjects varied in age from 3 to 86 and differed widely in
height, weight and configuration. Yet the eccentricity of the
mean location of the center of gravity projection was singularly minute, and the base exceeded that demanded because
of involuntary postural swap by a notably generous margin.
I n spite of mechanical factors in build conducive to static
insecurity, the normal subject maintains the biped stance with
an inordinate degree of symmetry and stability. These observations suggest, on a priori reasoning, that this is an
inherent phenomenon under reflex control.
The effective stimulus for the postural contraction of the
antigravity muscles is stretch (Liddell and Sherrington, '24).
The receptors involved are the proprioceptive end-organs of
the muscles primarily concerned. If the myotatic reflexes are
impaired, as they are in posterior column disease, standing
is almost impossible, particularly when the eyes are closed.
Tonus is much affected by visual, auditory, labyrinthine, touch
and pressure sensations.
3. Age differences in postural sway. Balance, as demonstrated by a mean centering of the weight over the middle of
the base, seems from our data to be uniformly good at all
ages. The magnitude of the oscillation of the center of gravity
about the geometric center of the base is, however, less constant. Figure 1illustrates the tendency for the young and the
354
F. A. HELLEBRANDT A N D G. L. BRAWN
9
li
Fig. 1 Graphs showing the relation of postural stability as measured by shifts
in the center of gmvitr of the body t o age and to certain characteristics of the
base of support.
STATIC EQUILIBRIUM
355
old to be less stable than the young adult and the middle
aged subject of both sexes. Hinsdale (1887) and Hancock
(1894) had observed that children sway more than adults.
In our experiments the area enclosed by the maximal shifts
of the center of gravity in the cardinal vertical orientation
planes and its relative encroachment upon the limits of static
security fall and then rise when graphed against age. The
differences however, may not be statistically significant. Many
of the subjects in the very stable young adult group were
professional students of physical education possessed of a
better than average neuromuscular development. Nevertheless, morphological evidence is in agreement with the expectation of maximal postural stability in the third decade.
Many years ago Flechsig suggested that the development of
the nervous system could be traced by ascertaining periods
at which various fiber tracts acquire myelin sheaths. I n
general the acquisition of myelin occurs in the same order as
that in which nerves develop. Medullation is demonstrable
early in fiber systems which are phylogenetically old and in
those becoming functional first. The posterior columns, spinocerebellar and cortico-spinal pathways are among the earliest
fiber tracts to undergo myelination (Coghill, '29 ; Langworthy, '26, '28).
Myelin first appears in the human in the column of Burdach
at about 14 weeks (Keene and Hewer, '31; Arnott, '37). It
is demonstrable in the spino-cerebellar tracts by the sixteenth
week. The afferent limb of the long circuit myotatic reflexes
thus originates at a time which antidates any utility. The
efferent limb, pyramidal and extra-pyramidal, matures much
later. Myelination of the pyramidal and rubro-spinal tracts
is scant at term and is not complete in the former until the
second year. Langworthy ( '33) believes that myelination
continues until the beginning of puberty. Hand in hand with
this orderly growth of the nervous system there is a similar
uniformity as to the time of appearance of motor behavior
patterns (Abramson, '37). Maturation rather than exercise
seems to play the dominant role in the initiation of basic motor
356
F. A. HELLEBRANDT AND 0. L. BRAUN
acts and the sequence of the development of fiber tracts from
which behavior patterns are formed is determined by evolutionary influences (Herrick, '30). It is impossible to say,
however, from the scattered evidence at hand, if the growing
child is at any significant neurological disadvantage in the
maintenance of a neuromuscular act as complex as the biped
stance.
I n 1936 Newman and Corbin reported an increase in the
threshold of vibratory acuity with age, the loss being distiiict
at 30 and very striking in those beyond 50. The vibratory
sense was most acute in the second and third decades. Subsequently Corbin and Gardner ('37) counted the number of
myelinated fibers in the dorsal and ventral roots af cadavers
varying in age from 1 day to 84 years. They found a degeneration of peripheral process from dying neurones in the
spinal ganglia which they believed might account for the
progressive loss of vibratory sense with age. This suggests
that there might be similar degeneration in the aged of posterior column afferents mediating proprioceptive impulses.
The crowning instability was present in the very young.
This may have causes other than the neurological one suggested. Mechanical factors in build probably play some part.
The physical proportions of the child are not conducive to
postural stability. The lower extremities are ill developed
and the center of gravity of the body as a whole has been
found by Basler ('35) to be relatively higher in the younger
age groups of girls. He found, however, no such relationship to age in boys. The age differences in the various foot
length diameters of Morton and the area enclosed between
the weight bearing imprints of the base of support are presented in figure 1. From the second decade on the base
remains more or less uniform in size.
Inadequacy in muscular development may also account in
part f o r the instability of the young. Peak muscle power is
reached relatively late and it falls off in the aged (Bolton,
'03; Lipovetz, '33). This is, however, probably of minor
importance because the equilibrium of collapsing gravital
STATIC EQUILIBRIUM
357
stresses is met very economically by the postural contraction
of the anti-gravity muscles (Simonson, ’26 ; Tepper and
Hellebrandt, ’38). The energy metabolism is little increased
by the vertical in contrast with the horizontal posture, even
when instability is great, involving the participation of a large
number of skeletal muscles. We have observed a case of
muscular dystrophy in a boy of 14 who reported gradually
increasing difficulty in walking which became seriously incapacitating during the winter because not enough strength
remained to lift the feet in the snow. The boy was unable
to get up from the sitting position without assistance and had
to be lifted onto the platform of the center of gravity apparatus. Yet he stood perfectly, with little involuntary
postural sway and the Romberg sign was negative. A similar
observation was made on a case of Addison’s Disease followed
through several crises. Although muscular weakness was
profound, standing was essentially normal when measured in
terms of the ability to retain the center of weight in the
middle of the base with a minimum of oscillation. Suggestive
as these case histories are, we have no direct evidence that the
antigravity muscles of the weight bearing limbs are sufficiently
developed during the first decade of life to meet the demands
placed upon them by the assumption of a vertical stance as
effectively as they are met in later life.
To stand quietly in a natural, comfortable posture for even
as short a time as 3 minutes is a difficult feat for the young
child. Every extraneous motion is reflected by a change in the
location of the center of gravity unless exactly counterbalanced. I n the handling of the older group the converse
problem presented itself. Few subjects know how to relax
while maintaining a vertical stance. The habit of ‘good
posture,’ when strongly developed, introdnces an element of
cerebral interference which cannot be broken down without
prolonged training. It was sometimes difficult to know when
great steadiness was an artefact of this origin in the young
adult and when instability mas a manifestation of restlessness in the young.
355
F. A. HELLEBRANDT AND 0. L. B U U N
Hancock (1894) observed that girls were steadier than boys.
The mean area of maximal sway was 1.06 sq.cm. in our total
group of sixty-six females, occupying 3.28% of a base which
averaged 433.8 sq.cm. in contrast with 1.59 sq.cm. in our group
of forty-three males. Although the mean base in this sex is
larger, 462.1 sq.cm., the area of maximal sway occupied
4.4% of the total base, codrming Rancock’s observation.
SUMMARY AND CONCLUSIONS
Postural stability was measured in 109 subjects, 43 male
and 66 female, ranging in age from 3 to 86, using as a criterion,
shifts in the center of gravity of the body as a whole projected
into the base of support as a function of time. The following
conclusions may be drawn from the evidence presented:
1. A t all ages and in both sexes the mean position of the
center of weight during 3 minutes of standing falls remarkbbly
close to the geometric center of the base.
2. The mean eccentricity of the center of gravity is slight,
falling preponderantly in front and to the left of the geometric
center of the base.
3. The area enclosed by the maximal shifts of the center
of gravity in the cardinal vertical orientation planes is very
small, averaging 3.73 sq.cm. or 1.28%of the total base.
4. The foot posture of Morton affords a base of support in
the vertical stance with a relatively enormous margin of safety.
5. The tendency is for postural stability to be greater in the
young adult than in the child or the aged, but variations in
muscular power, neurological development, factors in build
and disturbances of psychic origin give the data presented no
quantitative significance.
359
STATIC EQUILIBRIUM
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