balance

1. NEUROPHYSIOLOGY
OF BALANCE
By-Dr.Ranjeet Singha,PT(MPT in Neurology)
HAAD Licensed
Associate Professor,
College of Physiotherapy and Medical Sciences,
Guwahati,Assam.

2. TOPICS:
 Balance
 Role of sensory systems-
vision,proprioceptors,vestibular
 Role of Musculoskeletal system
 Biomechanics in balance
 Contextual factors in balance
 Role of nervous system
 Strategies-ankle, hip,stepping

3.  Balance refers to an individuals ability to
maintain their line of gravity within their Base
of support (BOS). It can also be described
as the ability to maintain equilibrium, where
equilibrium can be defined as any condition
in which all acting forces are cancelled by
each other resulting in a stable balanced
system.

4. VARIATION IN TERMINOLOGIES
 In literature the balance term has been used
synonymously with:
 Postural Control
 Postural Stability
 Equilibrium

5.  Balance, or postural stability, is a generic
term used to describe the dynamic process
by which the body’s position is maintained in
equilibrium.
 Equilibrium means that the body is either at
rest (static equilibrium) or in steady-state
motion (dynamic equilibrium).

6.  Balance is greatest when the body’s center
of mass (COM) or center of gravity (COG) is
maintained over its base of support (BOS).

7. BALANCE CONTROL
 Balance is a complex motor control task
involving the detection and integration of
sensory information to assess theposition and
motion of the body in space and the execution
of appropriate musculoskeletal responses to
control body position within the context of the
environment and task.
 Thus, balance control requires the interaction of
the nervous and musculoskeletal systems and
contextual effects

8.  ■ The nervous system provides the (1) sensory
processing for perception of body orientation in
space provided mainly by the visual, vestibular,
and somatosensory systems; (2) sensorimotor
integration essential for linking sensation to
motor responses and for adaptive and
anticipatory (i.e., centrally programmed postural
adjustments that precede voluntary movements)
aspects of postural control; and (3) motor
strategies for planning, programming, and
executing balance responses

9. BALANCE SYSTEMS
The following systems provides input regarding
the body's equilibrium and thus maintains
balance.
 Somatosensory / Proprioceptive System
 Vestibular System
 Visual System

10.  The Central Nervous System receives feedback
about the body orientation from these three
main sensory systems and integrates this
sensory feedback and subsequently generates
a corrective, stabilizing torque by selectively
activating muscles. In normal condition, healthy
subjects rely 70% on somatosensory
information and 20% Vestibular & 10% on Vision
on firm surface but change to 60% vestibular
information, 30% Vision & 10% somatosensory
on unstable surface.

11. SENSORY SYSTEMS AND BALANCE CONTROL
 Perception of one’s body position and
movement in space require a combination of
information from peripheral receptors in
multiple sensory systems, including the
visual, somatosensory(proprioceptive, joint,
and cutaneous receptors), and vestibular
systems.

12. VISUAL SYSTEM
The visual system provides information
regarding (1) the position of the head relative
to the environment; (2) the orientation of the
head to maintain level gaze; and (3) the
direction and speed of head movements,
because as your head moves, surrounding
objects move in the opposite direction.

13.  Visual stimuli can be used to improve a
person’s stability when proprioceptive or
vestibular inputs are unreliable by fixating the
gaze on an object. Conversely, visual inputs
sometimes provide inaccurate information for
balance control, such as when a person is
stationary and a large object, such as a
nearby bus, starts moving, causing the
person to have an illusion of movement.

14.  For non-impaired individuals, under normal
conditions the contribution of visual system to
postural control is partially redundant as the
visual information has longer time delays as
long as 150-200 ms.Friedrich et al.observed
that adults with visual disorders were able to
adapt peripheral, vestibular, somatosensory
perception and cerebellar processing to
compensate for their visual information deficit
and to provide good postural control.

15.  In addition, Peterka found that adults with
bilateral vestibular deficits can enhance their
visual and proprioceptive information even
more than healthy adults in order to reach
effective postural stability. The influence of
moving visual fields on postural stability
depends on the characteristics of the visual
environment, and of the support surface,
including the size of the base of support, its
rigidity or compliance

16. SOMATOSENSORY SYSTEM
The somatosensory system provides
information about the position and motion of the
body and body parts relative to each other and
the support surface. Muscle proprioceptors,
including muscle spindles and Golgi tendon
organs (sensitive to muscle length and tension),
joint receptors (sensitive to joint position,
movement, and stress), and skin
mechanoreceptors (sensitive to vibration, light
touch, deep pressure, skin stretch),are the
dominant sensory inputs for maintaining balance
when the support surface is firm, flat, and fixed

17. However, when standing on a surface that is
moving (e.g., on a boat) or on a surface that
is not horizontal (e.g., on a ramp), inputs
about body position with respect to the
surface are not appropriate for maintaining
balance; therefore, a person must rely on
other sensory inputs for stability in these
conditions.

18. Information from joint receptors does not
contribute greatly to conscious joint position
sense. It has been demonstrated that local
anesthetization of joint tissues and total joint
replacement does not impair joint position
awareness .

19.  Muscle spindle receptors appear to be
mostly responsible for providing joint position
sense, whereas the primary role of joint
receptors is to assist the gamma motor
system in regulating muscle tone and
stiffness to provide anticipatory postural
adjustments and to counteract unexpected
postural disturbances.

20.  Proprioceptive information from spino-
cerebellar pathways, processed
unconsciously in the cerebellum, are
required to control postural
balance. Proprioceptive information has the
shortest time delays, with monosynaptic
pathways that can process information as
quickly as 40–50 ms and hence the major
contributor for postural control in normal
conditions.

21. VESTIBULAR SYSTEM
The vestibular system provides information
about the position and movement of the head
with respect to gravity and inertial forces.
Receptors in the semicircular canals (SCCs)
detect angular acceleration of the head,
whereas the receptors in the otoliths (utricle
and saccule) detect linear acceleration and
head position with respect to gravity.

22.  The SCCs are particularly sensitive to fast
head movements, such as those made
during walking or during episodes of
imbalance (slips, trips, stumbles), whereas
the otoliths respond to slow head
movements, such as during postural sway.

23. By itself, the vestibular system can give no
information about the position of the body.
For example, it cannot distinguish a simple
head nod (head movement on a stable trunk)
from a forward bend (head movement in
conjunction with a moving trunk).

24.  Consequently, additional information,
particularly from mechanoreceptors in the
neck, must be provided for the central
nervous system (CNS) to have a true picture
of the orientation of the head relative to the
body.

25. The vestibular system uses motor pathways
originating from the vestibular nuclei for
postural control and coordination of eye and
head movements.

26.  The vestibular system generates compensatory
responses to head motion via:
 Postural responses (Vestibulo-Spinal Reflex) - keep the
body upright and prevent falls when the body is
unexpectedly knocked off balance.
 Ocular-motor responses (Vestibulo-Ocular Reflex) -
allows the eyes to remain steadily focused while the
head is in motion.
 (Vestibulo-Colic Reflex) - help keep the head and neck
centred, steady, and upright on the shoulders.
 To achieve this the vestibular system measures head
rotation and head acceleration through semicircular
canals and otolith organs (utricle and saccule).

27. SENSORY ORGANIZATION FOR BALANCE
CONTROL
 Vestibular, visual, and somatosensory inputs
are normally combined seamlessly to
produce our sense of orientation and
movement. Incoming sensory information is
integrated and processed in the cerebellum,
basal ganglia, and supplementary motor
area. Somatosensory information has the
fastest processing time for rapid responses,
followed by visual and vestibular inputs.

28.  When sensory inputs from one system are
inaccurate owing to environmental conditions or
injuries that decrease the information-
processing rate, the CNS must suppress the
inaccurate input and select and combine the
appropriate sensory inputs from the other two
systems.
 This adaptive process is called sensory
organization. Most individuals can compensate
well if one of the three systems is impaired;
therefore, this concept is the basis for many
treatment programs.

29. ROLE OF MUSCULOSKELETAL SYSTEM
 Muscle
 Skeletal framework integrity
 Joints integrity

30.  ■ Musculoskeletal contributions include
postural alignment, musculoskeletal flexibility
such as joint range of motion (ROM), joint
integrity, muscle performance (i.e., muscle
strength, power, and endurance)

31. BIOMECHANICS IN BALANCE
 COG
 BOS
 Limit of stability
 Alignment(LOG position affected)
 Type of muscle contraction/work(Isometric
,concentric or eccentric)

32. CENTER OF MASS
 The COM is a point that corresponds to
the
center of the total body mass and is the point
at which the body is in perfect equilibrium. It
is determined by finding the weighted
average of the COM of each body segment

33. CENTER OF GRAVITY
The COG refers to the vertical projection of
the center of mass to the ground. In the
anatomical position,the COG of most adult
humans is located slightly anterior to the
second sacral vertebra or approximately 55%
of a person’s height

34. BASE OF SUPPORT
The BOS is defined as the perimeter of the
contact area between the body and its support
surface; foot placement alters the BOS and
changes a person’s postural stability. A wide
stance, such as is seen with many elderly
individuals, increases stability, whereas a
narrow BOS, such as tandem stance or walking,
reduces it. So long as a person maintains the
COG within the limits of the BOS, referred to as
the limits of stability, he or she does not fall.

35. LIMITS OF STABILITY
“Limits of stability” refers to the sway
boundaries in which an individual can
maintain equilibrium without changing his or
her BOS . These boundaries are constantly
changing depending on the task, the
individual’s biomechanics, and aspects of the
environment.

36. CONTEXTUAL FACTORS IN BALANCE
Contextual effects that interact with the two
systems are the environment whether it is
closed (predictable with no distractions) or open
(unpredictable and with distractions), the
support surface (i.e., firm versus slippery, stable
versus unstable, type of shoes), the amount of
lighting, effects of gravity and inertial forces on
the body, and task characteristics (i.e., well-
learned versus new, predictable versus
unpredictable, single versus multiple tasks).

37.  Even if all elements of the neurological and
musculoskeletal systems are operating
effectively, a person may fall if contextual
effects force the balance control demands to
be so high that the person’s internal
mechanisms are overwhelmed.

38. ROLE OF NERVOUS SYSTEM
 Cerebellum
 Higher center

39. STRATEGIES FOR BALANCE
 Ankle
 Hip
 Stepping

40. TYPES OF BALANCE CONTROL
 Functional tasks require different types of
balance control, including (1) static balance
control to maintain a stable antigravity position
while at rest, such as when standing and sitting;
(2) dynamic balance control to stabilize the body
when the support surface is moving or when the
body is moving on a stable surface, such as sit-
to-stand transfers or walking; and (3) automatic
postural reactions to maintain balance in
response to unexpected external perturbations,
such as standing on a bus that suddenly
accelerates forward.

41.  Feedforward (open loop motor control) is utilized for
movements that occur too fast to rely on sensory
feedback (e.g., reactive responses) or for anticipatory
aspects of postural control.
■ Anticipatory control involves activation of postural muscles
in advance of performing skilled movements, such as
activation of posterior leg and back extensor muscles
prior to a person pulling on a handle when standing30 or
planning how to navigate to avoid obstacles in the
environment.
■ Closed loop control is utilized for precision movements
that require sensory feedback (e.g., maintaining balance
while sitting on a ball or standing on a balance beam).

42. MOTOR STRATEGIES FOR BALANCE CONTROL
 To maintain balance, the body must continually
adjust its position in space to keep the COM of
an individual over the BOS or to bring the COM
back to that position after a perturbation.
 Horak and Nashner described three primary
movement strategies used by healthy adults to
recover balance in response to sudden
perturbations of the supporting surface (i.e.,
brief anterior or posterior platform
displacements) called ankle, hip, and stepping
strategies .

43. Results of research examining the patterns of muscle
activity underlying these movement strategies suggest
that preprogrammed muscle synergies comprise the
fundamental movement unit used to restore balance.
A synergy is a functional coupling of groups of muscles,
so they must act together as a unit; this organization
greatly simplifies the control demands of the CNS. The
CNS uses three movement systems to regain balance
after the body is perturbed: reflex, automatic, and
voluntary systems.
■ “Stretch” reflexes mediated by the spinal cord comprise
the first response to external perturbations. They have the
shortest latencies (70 ms), are independent of task
demands, and produce stereotyped muscle contractions
in response to sensory inputs.

44. ■ Voluntary responses have the longest latencies (>150
ms),are dependent on task parameters, and produce highly
variable motor outputs (e.g., reach for a nearby stable
support surface or walk away from a destabilizing condition).
■ Automatic postural reactions have intermediate latencies
(80 to 120 ms) and are the first responses that effectively
prevent falls. They produce quick, relatively
invarianmovements among individuals (similar to reflexes),
but they require coordination of responses among body
regions and are modifiable depending on the demands of
the task (similar to voluntary responses).
 The reflex, automatic, and voluntary movement systems
interact to ensure that the response matches the postural
challenge.

45.  References:
 Kisner
 Physiopedia
 Articles from Google site
 Youtube