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5
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Practical Biomechanics and
Physiology of Balance
Lewis M. Nashner
INTRODUCTION
Balance is a complex process involving the coordinated
activities of multiple sensory, motor, and biomechanical components. The position of the body in relation
to gravity and the surrounds is sensed by combining
visual, vestibular, and somatosensory inputs. Balance
movements involve motions of the ankle, knee, and hip
joints, which are controlled by the coordinated actions
of ankle, thigh, and lower trunk muscles. The aim of
this chapter is to describe the principal biomechanical,
sensory, and motor components of balance and their
interactions within a systematic model of balance control. Chapter 15 describes a method for assessing the
sensory and motor components of balance based on
this model and provides illustrative patient examples
of how the information is clinically applied.
BIOMECHANICS OF BALANCE
Definition of Balance
To balance with the feet in place, the position of the
body’s center of gravity (COG) must be maintained
vertically over the base of support (Gurfinkel & Osevets, 1972; Koozekanani, Stockwell, McGhee, & Firoozmand, 1980; Nashner, 1981). When this condition is
met, a person can both resist the destabilizing influ-
ence of gravity and actively move the COG. When a
person moves slowly, the in-place limits of stability are
exceeded whenever the COG position has been displaced beyond the perimeter of the base of support.
At this point a rapid step or stumble to reestablish the
base of support beneath the COG, or additional external support, is required to prevent a fall. When a person is moving more rapidly, the definition of balance is
more complex, because momentum of the body motion
must also be taken into consideration. For example, if
the COG is displaced only slightly forward but moving
forward rapidly, the person has lost balance because
the forward momentum is sufficient to carry the COG
beyond the forward stability limit.
The state of a person’s balance is most simply
described in terms of angular displacement of the
COG from the gravitational vertical. Center of gravity sway is then defined as the angle formed by the
intersection of a first line from the center of the base
of support through the COG and a second line extending vertically from the center of support, as shown in
Figure 5–1. This definition of balance pertains whether
the person moves about the ankles, the hips, or both
joints, as well as taking height into account (Nashner
& McCollum, 1985). Thus, a given sway angle indicates
a comparable state of balance for all body movement
patterns and heights.
This definition of balance is further discussed in
the section on limits of stability. More recent studies
have expanded the definition of balance to incorporate
multiple joints of the body (Maurer & Peterka, 2005).
87
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Figure 5–1. Center of gravity (COG) sway angle in relation to the “limits of stability” cone.
is moving about the ankles; the figure on the right is
moving about the hips. The COG sway angles of the two
figures are approximately the same, although the joint
motions are much larger using the hip strategy. Filled
diamonds show the body C
book of Balance Function Testing, Jacobson et al., 1997.
The figure on the left
OG positions. From Hand-
Base of Support
The base of support for standing on a flat, firm surface
is defined as the area contained within the perimeter of
contact between the surface and the two feet. The base
of support area is nearly square when the feet are placed
comfortably apart while the person is quietly standing.
Similarly, a diagonal stance produces a parallelogramshaped base of support extending forward on one side
and backward on the other, while a tandem stance position creates a long but very narrow base of support.
When the support surface area is smaller than the
feet, or when surface irregularities limit the contact
between the feet and the surface, the base of support
is reduced. Standing sideways on a narrow beam, for
example, provides a base with a normal width but very
short length. Thus, the person’s limits of stability are
effectively reduced in the anterior-posterior (AP) but
not the lateral dimensions.
Limits of Stability
The limits of stability (LOS) is a two-dimensional quantity defining the maximum possible COG sway angle
as a function of sway direction from the center position
(Koozekanani et al., 1980; McCollum & Leen, 1989). The
LOS depends on the placement of the feet and characteristics of the base of support. In normal adults standing on a flat, firm surface with feet spaced comfortably
apart, the LOS perimeter can best be described as an
ellipse, as shown in Figure 5–1. The AP dimension
of this ellipse is approximately 12.5° from the backwardmost to the forwardmost points on the perimeter
(Nashner & McCollum, 1985). While height of the COG
above the surface and foot length affect the AP LOS,
these two features co-vary, resulting in approximately
the same AP limits for people of various heights (Duncan, Weiner, Chandler, & Studenski, 1990).
The lateral LOS depends on the person’s height
relative to the spacing between the feet. When the
feet of a 70-inch tall person are placed 4 inches apart,
for example, the lateral dimension of the LOS ellipse
is approximately 16° from the left to the rightmost
points on the perimeter. For taller individuals, a wider
spacing between the feet is required to produce a 16°
ellipse, whereas shorter people can place their feet
closer together.
The biomechanical properties that determine the
LOS are similar while standing in place, walking, and
sitting without trunk support, as shown in Figure 5–2.
During in-place standing, the COG moves randomly
within an LOS perimeter determined by the base of
support and the placement of the feet. During walking, the COG progresses forward through the LOS in a
smooth, rhythmic movement (Nashner, 1986; Nashner
& Forssberg, 1986). At heel strike, LOS is established
with the COG positioned slightly behind the posterior
perimeter but with sufficient forward momentum to
carry it to the perimeter. As the step progresses, the
COG moves forward within the LOS. As the COG
approaches and then exceeds the anterior perimeter
of the LOS, the next step establishes a new LOS and
the rhythmic process is repeated. When sitting without
trunk support, the height of the COG above the support
surface is considerably less and the base area is larger.
Therefore, the LOS perimeter is larger in terms of COG
sway angles when one is seated than in quiet standing.
Limits of Sway
It is impossible to maintain the COG motionless,
because in-place standing is an inherently unstable
task requiring intermittent corrections to overcome the
destabilizing influence of gravity (Begbie, 1967; Diener,
Dichgans, Bacher, & Gompf, 1984; Scott & Dzendolet,
1972). Thus, a person attempting to maintain balance
spontaneously sways back and forth and from side to
side. The limits of sway is a two-dimensional quantity
defining the maximum spontaneous COG sway angle
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Figure 5–2. Limits of stability boundaries during standing, walking and sitting. From Handbook of Balance Func-
tion Testing, Jacobson et al., 1997.
as a function of the sway direction, as shown in Figure 5–3. A person’s limits of spontaneous sway vary
with the sensory conditions and the configuration of
the base of support. But, unless the person loses balance, the limits of sway remain within the LOS.
Center of Gravity Alignment
A point at the center of the area contained within the
limits of sway perimeter defines the COG alignment, as
shown in Figure 5–3. This definition of COG alignment
is based on the assumption that a person is attempting
to maintain a COG position which is at the center of
the limits of sway perimeter. When a normal person is
asked to stand erect, the most stable position is to align
the COG above the center of the base of support, since
this position maximizes the limits of sway amplitude.
The limits of sway and COG alignment affect a
person’s balance. When the COG is aligned over the
center of the base of support, the limits of sway can
be as large as the LOS before balance is lost. A person
whose COG alignment is offset from the center of support must reduce the limits of sway in the direction of
the offset to maintain stability.
low, gravity is the only significant destabilizing force
that must be overcome, and the COG can be moved
within the full range of the LOS. For the average adult,
COG movements within the full range of the LOS are
possible when a sway oscillation cycle (front to back
or side to side and then back again) takes two to three
seconds or longer. By contrast, when sway oscillation
cycles take less than one second, momentum is an
additional destabilizing force and the LOS perimeter is
reduced to as little as 3°.
Understanding the impact of COG sway frequency is important in assessing a person’s balance.
As higher frequencies reduce the effective LOS, a person using fast sway movements is closer to exceeding
the LOS than an individual swaying slowly through a
comparable arc. This principle is further explained by
a theoretical multidimensional model of postural sway
which demonstrates that 92% of the variance in sway is
explained by two variables, one related to sway amplitude and the other to sway velocity (Maurer & Peterka,
2005; Pavol, 2005).
SENSING THE POSITION OF BALANCE
Limits of Stability and Sway Frequency
In addition to the base of support, the actual LOS is
influenced by COG sway velocity, which is directly
related to sway frequency (Nashner, Shupert, Horak,
& Black, 1989). When the frequency of COG sway is
Sensory and Motor
Components of Balance
To execute the constant corrections required to resist
the destabilizing effect of gravity and the perturbing
effects of purposeful motor actions while standing
and walking, the balance system must determine the
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Figure 5–3. Relations between the LOS, the sway envelope, and the COG alignment.
The middle figure shows the COG alignment within the LOS. The right figure shows the
C
OG aligned forward relative to the center. From Handbook of Balance Function Testing,
Jacobson et al., 1997.
position of the COG relative to gravity and the base
of support and then execute intermittent coordinated
movements to correct COG deviations. Although the
neural processes for determining COG position and
moving the COG are highly integrated, the two are
separated here for simplifying the systematic model of
balance control. From a clinical perspective, separating
the sensory and motor processes of balance means that
a patient may have impaired balance for one or a combination of two reasons: (1) the position of the COG
relative to the base of support is not accurately sensed,
and (2) the automatic movements required to bring the
COG to a balanced position are untimely, poorly coordinated, or inappropriate.
Visual, Vestibular, and
Somatosensory Inputs
Sensing the position of the COG relative to gravity and
the base of support requires a combination of visual,
vestibular, and somatosensory (tactile, deep pressure, joint receptor, and muscle proprioceptor) inputs.
Utilization of the three balance senses is reviewed in
Table 5–1. Three senses are required because no single
sense directly measures COG position. Somatosensory
inputs provide information on the orientation of the
body parts relative to one another, the support surface,
and gravity. The vestibular system measures gravitational, linear, and angular accelerations of the head in
relation to inertial space. Vision measures the orientation
of the eyes and head in relation to surrounding objects.
Under some task conditions, one or more of the
senses may provide information that is misleading or
inaccurate for purposes of balance control. For example, when a person stands next to a large bus that
suddenly begins to move forward, momentary disorientation or unsteadiness may result. A fraction of a second is required for the brain to determine whether the
resulting visual stimulus indicates backward sway of
the person or forward movement of the bus. If a downwardly tilted support surface is encountered, the brain
must determine whether the surface is tilted downward
or the surface is level and the body is tilted back. During sensory conflict situations, the brain must quickly
select the sensory inputs providing accurate orientation information and ignore the misleading ones.
The process of changing the combination of
sensory inputs to balance is usually termed sensory
“adaptation” or “reweighting.” Using signal processing techniques, investigators have demonstrated that
under normal sensory conditions when all senses are
accurate and there is more than enough useful information, subjects vary in their weighting of the three senses
(Peterka & Loughlin, 2004). These same techniques
have been used to show that, as the sensory conditions
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table 5–1. Utilization of the Senses for Balance
Sense Reference Conditions Favoring Use Conditions Disrupting Use
Somatosensory Support surface Fixed support surface Irregular or moving support
Visual Surrounding objects Fixed visible surrounds and
irregular or moving support
Vestibular Gravity and inertial space Irregular or moving support
and moving surrounds or
darkness
change, subjects reweigh inputs, reducing the influence
of misleading inputs in favor of those providing the
more useful information (Asslander & Peterka, 2014;
Logan, Kiemel, & Jeka, 2014).
positions and the direction of gaze. Precise head, eye,
and gaze control are critical in the execution of many
complex motor activities, such as running and either
kicking or catching a moving ball.
The weighting of the vestibular input, however,
Somatosensory Input
is increased when both the somatosensory and visual
inputs are misleading or unavailable (Allum, HonSomatosensory input derived from the contact forces
and motions between the feet and the support surface
is the dominant sensory input to balance under normal
(fixed) support surface conditions (Aggashyan, Gurfinkel, Mamsakhlisov, & Elner, 1973; Diener & Dichgans,
1988; Diener, Dichgans, & Guschlbauer, 1986; Dietz,
Horstmann, & Berger, 1989; Gurfinkel, Lipshits, et al.,
1974). When a person stands on a firm, level surface,
the extent of COG sway is very small relative to the
LOS. Closing the eyes to eliminate vision causes little
if any functionally significant increase in COG sway.
Even a well-compensated patient with a bilateral vestibular loss sways well within the LOS with the eyes
closed (Black & Nashner, 1984, 1985; Nashner, Black,
& Wall, 1982; Shupert, Black, & Horak, 1988). When
the support surface is sway referenced to disrupt useful somatosensory inputs, subjects reduce the weight
given to somatosensory inputs and increase weighting
of visual and vestibular inputs (Logan, Kiemel, & Jeka,
2014; Peterka & Laughlin, 2004).
egger, & Pfaltz, 1989; Peterka & Loughlin, 2004). The
patient with a profound bilateral vestibular loss, for
example, is unsteady standing in darkness on a com-
pliant or irregular surface. Because vestibular input is
seldom if ever misleading (except in cases of disease
or other disorder and unusual motion environments),
vestibular information is critical for balance when
conflicting visual and/or somatosensory informa-
tion requires a person to identify and quickly ignore
a misleading input (Black & Nashner, 1984, 1985). This
is why patients with peripheral vestibular deficits fre-
quently complain of dizziness and/or unsteadiness
during exposure to conflicting visual and support sur-
face stimuli.
Exposure to zero gravity or to a simulation of
zero gravity is believed to cause changes in the way
the brain interprets orientation input from the vestibular system. The utricular otoliths normally sense both
the linear acceleration of the head and the tilt angle
of the head with respect to gravity. Under zero-gravity
conditions, the brain must adapt to an absence of the
Vestibular Input
tilt angle component of the otolith input. The adaptive
changes in interpretation of the vestibular input fol-
When functionally useful somatosensory and visual
inputs are available, vestibular inputs play a minor role
in controlling COG position (Bles, deJong, & deWit,
1984; Nashner et al., 1982; Shupert et al., 1988). This
is because the somatosensory and visual inputs are
more sensitive to body sway than the vestibular system (Nashner et al., 1989). The primary role of vestibular input under these conditions is most probably to
allow independent and precise control of head and eye
lowing zero-gravity exposure may be viewed as a temporary, environmentally induced pathologic condition
(Paloski, Reschke, Doxey, & Harm, 1992; Young, Oman,
Watt, Money, & Lichtenberg, 1984). These maladaptive
changes are most potent immediately after return to
normal terrestrial conditions and are most pronounced
when the returning astronauts are exposed to conflicting visual conditions (Parker, Reschke, Arrott, Hormick, & Lichtenberg, 1985).
Moving surrounds, darkness
Unusual motion
environments
92 BAlAnCE FunCtion AssEssmEnt And mAnAgEmEnt
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Visual Input
Among the three balance senses, vision is without
doubt the richest and most varied in providing orientation and motion information. A comprehensive
summary of visual orientation perception has been
described by Gibson (1979). Understanding the nature
of visual perception is important for posture control,
because the advent of modern computer animated virtual reality techniques allows assessment and training
of the full range of visual influences on balance. Some
of the key elements of visual motion perception that
influence balance include visual field flow and changes
in object size, both of which are influenced by the distance of objects from the observer, parallax effects influenced by the relative distances between near and far
objects, and the spatial frequencies and contrast differences among objects in the surround. Recent virtual
reality research demonstrates that individual elements
of visual orientation perception differ in their influences on balance (Keshner & Kenyan, 2009; Streepey,
Kenyan, & Keshner, 2007). For example, flow within
the peripheral visual field has a stronger influence on
balance control than the changing sizes of objects in the
central visual field.
Vision plays a significant role in balance, especially
when the support surface is unstable (Begbie, 1967; Diener et al., 1986; Lee & Lishman, 1975; Paulus, Straube,
& Brandt, 1984, 1987). For example, when toes-up and
toes-down tilting of the surface in direct relation to the
AP sway disrupts somatosensory input useful for balance, COG sway is significantly less with eyes open
than with eyes closed (Black & Nashner, 1984, 1985;
Nashner, Black, & Wall, 1982). The stabilizing effect of
vision is also illustrated by comparing eyes-open and
eyes-closed sway while a person stands on a compliant foam rubber pad. Vision also influences COG alignment. When a person is exposed to a constant linear or
rotational movement of the visual field, for example, the
alignment of the COG over the base of support shifts in
the direction of the visual field motion (Brandt, Paulus,
& Straube, 1986; Lestienne, Soechting, & Berthoz, 1977).
The reader may have experienced the effect of
vision on balance, for example, at the seashore when
a wave causes a large area of the surrounding water
to move in or out at constant velocity. If a person is
attending to the moving water, there is a tendency to
sway and sometimes even stumble in the direction of
the moving water. Alterations in body alignment also
occur when subjects are exposed to a room with tilted
walls. Similar visual illusions are sometimes used in
carnival fun-houses to throw participants off balance.
MOTOR CONTROL OF BALANCE
Anatomy and Physiology of Movement
During erect standing with the arms at the sides or
folded at the waist, the COG is located in the area of the
lower abdomen, with the exact position at any given
moment dependent on the relative positions of the
ankle, knee, and hip joints (McCollum & Leen, 1989).
Because there are three principal joint systems — ankles,
knees, and hips — between the base of support and the
COG during standing, a wide variety of different postures can be assumed with the COG over the center of
the base of support, as illustrated in Figure 5–4. For
similar reasons, a wide variety of active ankle, knee,
and hip movement patterns can be used to produce
similar shifts in COG position. Examples of this diversity of postures and balance movement patterns can
be observed in individuals performing highly trained
dance or martial arts routines.
A detailed description of the large number of
muscles controlling ankle, knee, and hip joint motions
is beyond the scope of this chapter. Instead, this section focuses on the key muscle groups involved in
balance and on the general physiologic principles governing coordination of these muscles during postural
movements.
Motions about a given joint are controlled by
the combined actions of at least one pair of muscles
working in opposition. All leg and lower trunk joints
have multiple pairs of opposing muscles. Furthermore,
many leg muscles act about two neighboring joints. At
the ankle joint, the gastrocnemius and tibialis anterior
are the major extensor (plantar flexor) and flexor (dorsiflexor) muscles, respectively. The quadriceps is the
major knee extensor, while the hamstrings and gastrocnemius are both knee flexors. The hamstrings and
lower back muscles are hip extensors, while hip flexion
is controlled by quadriceps and abdominals.
An isolated muscle acts like a spring, tending to
resist attempts to stretch the muscle beyond a resting
length (Hill, 1953). The degree of the muscle’s resistance to stretch is called muscle stiffness. Both the rest
length of the muscle and the muscle stiffness vary
depending on how strongly the muscle is being activated. An inactive muscle has an extended rest length
and offers little resistance to stretching. The rest length
of a highly active muscle is shorter, and the muscle
vigorously resists stretching. When muscles acting in
opposition about a joint co-contract, stiffness increases
without associated joint motion.
5. PRACTICAL BIOMECHANICS AND PHYSIOLOGY OF BALANCE 93
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Figure 5–4. Examples of the variety of different postures during
which the body C
Handbook of Balance Function Testing, Jacobson et al., 1997.
OG is centered over the base of support. From
Stretch Reflex Movement
When the forces exerted by pairs of opposing muscles
about a joint are combined, the effect is to resist joint
rotation like a compliant spring. How compliant the
joint is in resisting rotation is called joint stiffness. The
resting position and the stiffness of the joint are each
altered independently by changing the background
activation levels of one or both muscles. The myotatic
stretch reflex is the earliest mechanism for increasing
the activation level of the muscles of a joint following an
externally imposed rotation of the joint. This response
component is initiated by inputs from muscle spindles,
tiny stretch sensitive receptors embedded within the
muscle. Output fibers from the muscle spindles enter
the spinal cord and, by way of single synapses within
the cord, activate muscle fibers within the same muscle
originating the spindle inputs (Houk, 1979). Myotatic
stretch reflexes are thought to improve the nonlinear
stiffness properties of muscle (Houk, 1979).
Recent advances in computer modeling of balance control suggest that leg joint stiffness is an important component of balance control and that, like the
reweighting of senses, the system actively also modifies leg joint stiffnesses depending on the conditions of
the task (Suzuki, Nomura, Casadio, & Morasso, 2012).
For example, forces generated by ankle joint stiffness in
response to rotations of the support surface are destabilizing and therefore inappropriate for balance control
(Nashner, 1976).
Automatic and Volitional
Movement Systems
The influences of joint stiffness, automatic movement
system, and volitional movement system on standing
balance are reviewed in Table 5–2. As described previously, the active regulation of joint stiffness provides
assistance but is insufficient to maintain postural stability (Casadio & Morasso, 2005; Gurfinkel, Lipshits,
& Popov, 1974).
Automatic postural movements are the earliest
actively generated force responses helping to maintain stability when a standing individual’s balance is
perturbed (Nashner, 1976, 1977; Nashner, Woollacott,
& Tuma, 1979). Automatic postural movements resemble both reflexive and voluntary movements. Like
reflexes, they are triggered by external stimuli, occur
at fixed latencies, and are relatively stereotyped. Like
voluntary postural movements, automatic responses
involve the coordinated actions of many leg and trunk
muscles, and the amplitudes and patterns of automatic
responses adapt to the task conditions. Although the
pathways mediating automatic postural movements
have not been fully elucidated, the 90 to 100 msec latencies of electromyographic (EMG) responses are sufficient to include significant brainstem and subcortical
involvement (Evarts & Tajii, 1974; Marsden, Merton, &
Morton, 1973; Melville Jones & Watt, 1971).
Voluntary postural movements can occur either
in the presence or absence of external stimuli and the
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