Balance

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Balance

What is balance?

In biomechanical terms, balance is a state whereby the projection of the centre of mass (COM) falls within the stability limits of the base of support (BOS). The stability limit being the point at which balance is lost and corrective action is required. Balance is a core component of all functional activities and as such, incorporates both the concepts of posture (arrested movement) and movement. The control of posture and movement in attaining a state of balance is often termed postural control and describes the motor action that occurs following the integration of sensory, perceptual, cognitive and motor processes. The aims of postural control are:

• Postural equilibrium/stability involving the coordination of movement strategies to:

• Maintain an upright posture against gravity and any other external forces

• Maintain the COM within the stability limits of the BOS during internally and externally initiated movement (planned/intentional and unpredictable).

• Postural orientation involving the control of body segment alignment with respect to:

• Internal references

• The environment (Horak 2006).

Sensory input and balance

The state of balance is maintained through complex postural control mechanisms which are reliant on adequate sensory input. The principal input systems to balance are the vestibular system (S2.10), the visual system (S2.10) and the somatosensory systems (S3.23) (Massion et al. 2004). Visual input provides a reference for upright vertical but is also essential in predicting forthcoming threats to balance from the environment. The somatosensory system, primarily proprioception, provides a reference for the body’s position in relation to the supporting surface and to other body parts and finally the vestibular system provides a reference for head position and movement of the head in relation to gravity.

In general this sensory input has two main functions:

Sensory feed forward which allows a preparation for movement. These anticipatory adjustments require input from both the internal and external environment and are an integral part of every movement. They are strongly linked to previous experiences.

Sensory feedback which allows ongoing regulation and appropriate muscle adjustments to be made in response to planned and unplanned displacement of the COM during movement.

Integration

Integration of this sensory information for an appropriate motor response is facilitated by various higher centres, including: the brain stem and cranial nerves (S2.10); the cerebellum (S2.12); reticular formation (S2.10) and the cerebral cortex (S2.7). The amount of cognitive processing by the cortex for postural control is usually minimal, with its contribution depending on the complexity of the task and the capability of the individual’s postural control system. For example, postural control relies on accurate sensory input, but also on the ability of the central nervous system (CNS) to attend to the relevant sensory cues and to prioritize or weight the input according to its relevance to the context and task (Horak 2006). The CNS also intervenes if a sensory conflict exists when it must weight the sources and reject the potential source of error (Karnath et al. 2000b).

Motor output and balance

During movement there is inevitably a displacement of the COM in relation to the BOS. This occurs whether the movement involves the trunk, the upper limb, the lower limb, turning the head or simply breathing. What determines whether the displacement leads to a fall is the motor response by which balance is recovered. These motor adjustments are flexible and varied and dependent on the task, the environmental context and the individual. An appropriate motor response requires a certain level of muscle strength, endurance and an available range of movement (Cholewicki et al. 1997; Ebenbichler and Oddsson 2001; Hodges and Richardson 1997) but also fine grading of agonists, antagonists and synergistic muscles, appropriate co-contraction and a high level of reciprocal innervation.

Although postural control mechanisms are varied, certain patterns of activation are described, however in an adult these are integrated into movement and may not always be evident:

Movement strategies

An individual can slow down the displacement of the COM by rapidly generating muscle torque at the ankles, hips or other joints around a fixed BOS. For anteroposterior displacements of the COM the predominant torque is generated via an ankle strategy (Fig. 32.1) although a hip strategy (Fig. 32.2) may also be present. For a mediolateral displacement, the hip strategy is dominant (Maki and McIlroy 2006). These strategies occur in a feed forward manner to maintain balance and are continuously interchangeable during movement. However, if the perturbation is large and these strategies are unsuccessful, the individual may actually change the BOS by stepping (Fig. 32.3) or using an outstretched arm. Although the latter response is often referred to as protective it can also occur during small perturbations as a normal strategy for balance.

Figure 32.1 Movement strategy: Ankle.

Figure 32.2 Movement strategy: Hip.

Figure 32.3 Movement strategy: Step backwards.

Balance reactions

Equilibrium reactions

These are subtle changes in muscle tone required to maintain equilibrium and are analogous with postural sway (Fig. 32.4). When people stand with their eyes closed, postural sway may increase by 20–70% (Lord and Menz 2000).

Figure 32.4 Equilibrium reactions.

Righting reactions

The existence of righting reactions in a mature adult is debated as it is proposed that they become integrated with equilibrium reactions by age 3 months. However, as the debate appears to be one of terminology and is as yet unresolved, this text describes the two phenomena separately.

Righting reactions occur in response to displacement of the COM beyond the stability limits in an attempt to regain equilibrium. They are a basic component in maintaining equilibrium while changing between positions and hence during movement. Righting reactions may involve the head (moving on the trunk to maintain the head in vertical), the trunk (in response to weight transfer or head/limb movement) or limb movement (acting as a counter balance to offset the displacement of COM) (Fig. 32.5).

Figure 32.5 Righting reactions in sitting.

Saving/protective

When the displacement of the COM is beyond the stability limits and righting reactions are inadequate a step or an extended upper limb may be used in order that a new base of support is established and equilibrium restored.

Why do we need to assess balance?

Balance impairments have been shown to increase the risk of falls (S3.34) resulting in high economic costs and social problems (Lamb et al. 2003; Harris et al. 2005; Belgen et al. 2006). Balance problems in neurologically impaired patients are common, being associated with the following impairments:

Motor dysfunction

Musculoskeletal

• Decreased muscle strength: In cerebrovascular accident (CVA) (Belgen et al. 2006; Tyson et al. 2006; Au-Yeung et al. 2003; Niam et al. 1999)

• Trunk instability: In CVA (Verheyden et al. 2006; Karatas et al. 2004)

• Decreased range of movement

• Altered muscle tone.

Biomechanics

• Reduced stability limits: In CVA (De Haart et al. 2004) and in Parkinson’s disease (PD) (Mancini et al. 2008)

• Reduced balance response (magnitude and velocity): In PD (Mancini et al. 2008)

• Altered movement strategies: In CVA there is a reliance on hip and stepping strategies which are less efficient in terms of stability (Maki and McIlroy 1997; Chen et al. 2000).

Sensory dysfunction

Altered sensation

• Deceased ankle proprioception: In CVA (Tyson et al. 2006) and in PD (Vaugoyeau et al. 2007)

• Vestibular system damage

• Visual deficits.

Altered sensory integration

• Difficulty dividing attention between two tasks (Stapleton et al. 2001)

• Impaired ability to use sensory weighting: Reduced speed of integration in PD (Brown et al. 2006; De Nunzio et al. 2007). In CVA there is often excessive reliance on visual input which may lead to inappropriate postural adjustments (Bonan et al. 2004)

• Delayed or inadequate anticipatory response: In CVA and PD (Horak et al. 1997)

• Altered perceived stability limits: In CVA (De Haart et al. 2004) linked to anxiety and lack of confidence

• Abnormal perception of vertical via the somatosensory systems in CVA, particularly in association with visuospatial neglect (Yelnik et al. 2002; Bonan et al. 2006).

Clinical hints and tips

It is interesting that in healthy subjects our perceived stability limit and the consequent motor response may be altered if we fear a threat to our balance (e.g. icy ground or wet floor). This is highly relevant in patients who readily lose confidence and as a consequence, they may limit themselves functionally over time.

How do I assess balance?

The state of balance is primarily a background to performing a movement and in the main is automatically controlled. Therefore it should be assessed in this context, that is, during performance of a task or function. However, several options are available to the therapist.

Functional observation

Patient

In order to assess balance in context the patient should be requested to perform a functional activity. The choice of activity will be dependent on the patient’s ability.

Therapist

1. The therapist should set an appropriate balance task. When setting a task the therapist should consider the following:

a. Create a safe situation (environment, assistance as required).

b. Incorporate various environmental contexts and various tasks as our postural control is highly specific to both.

c. Challenge their balance: This means making them unstable and could be achieved by:

• Reducing the size of the base of support

• Raising the COM

• Displacing the COM towards the stability limits by either a movement created by the therapist or the patient. If the patient is unstable requesting them to turn their head may be challenge enough

• Placing them on an unstable surface

• Alter their sensory input: Use eyes open and eyes closed.

d. Assess all aspects of balance responses:

• Posture and movement

• Balance strategies: ankle, hip and step

• Balance reactions: equilibrium/righting and saving

• Different planes of movement (transverse, sagittal and frontal planes).

2. Observe the patient performing the balance task.

Can they balance (maintain the COM within the BOS) in a specific posture? Instability during a stable posture may indicate a severe balance deficit. If there is inappropriate excessive sway in all postures with eyes open, then sufficient assistance is recommended before challenging the patient any further.

Can they balance during movement? If not, do they fall?

If they can balance, what strategies do they use to achieve balance? Are their responses appropriate for the amount of COM displacement?

Does the patient use compensatory strategies to maintain balance? Using trunk or limb muscles to fix is inefficient and may indicate underlying instability. Visual fixation is also common.

Are these compensations effective?

Is the goal completed successfully? Without a stable/balanced base the execution may be unsuccessful.

Is the task performance effortful? This may indicate extra muscle work to hide poor balance.

Do they interact with their BOS during the movement? The BOS should act as a point of reference for movement. The base providing a stable point that the patient can move around confidently.

What happens to the balance if sensory input is reduced (e.g. eyes closed)?

Special tests: Romberg’s test

Therapist

1. The patient is asked to begin in tandem stance (feet heel to toe) with eyes open.

2. This is held for 5–10 seconds.

3. The test is repeated with eyes closed.

4. The therapist should observe the amount of postural sway in both conditions.

Is there an increased postural sway exhibited when the eyes are open and closed? This is indicative of a deficit of the cerebellum.

Is there normal postural sway with eyes open which excessively deteriorates when eyes are closed? This is attributed to a somatosensory deficit.

Recording

A simple text description of the therapist’s assessment and findings is sufficient. This should include the patient’s functional ability to balance and some description related to the quality of the motor response.

Example

Patient is sitting – Pt is able to sit unsupported for a limited time (10 seconds) and maintains this posture using excessive activity around both hips. Postural sway is normal.

Patient was able to lift both hands off his lap for approx. 2 seconds, during which he became anxious. Further assessment was inappropriate.

Analysis

Postural control is hugely complex and therefore simple balance measures are unlikely to identify the specific deficits of the sensorimotor processes. Comprehensive assessment is required to evaluate the potential impairments implicated in a deficit of balance. Therefore the findings from your balance assessment must be considered in conjunction with the findings from other assessment tools (S3.19–34). Particular attention should be paid to vision (S3.27) and sensory testing (S3.23) and if a lesion of the vestibular system is suspected, referral to a specialist unit is advised. However, trunk stability, muscle tone, alignment, pain, strength, range of movement and cognition/perception may also be linked to reduced balance.

In the above example, the patient maintains balance in sitting using an ineffective compensatory strategy. His underlying balance deficit is related to hypotonia of his trunk, based on other findings. He also appears to have developed an altered perception of his stability limits as he becomes anxious without apparent need. This could limit the voluntary use of his upper limbs in the future.

Outcome measures

Research

Posturography refers to any technique used to quantify postural control in standing. These objective quantitative measures of balance can assess with greater sensitivity than observation by the therapist (De Oliveira 2008). Postural reactions can be quantified on force platforms with the sensory environment being manipulated to challenge different systems of balance. However, the output from a force platform does not tell the therapist about the quality of the postural control mechanisms.

Clinical

• Tinetti balance

• Berg balance scale

• Romberg’s test

• Sharpened Romberg’s

• Functional reach test

• Functional activities: validated in stroke (Tyson and DeSouza 2004)

• Sitting: supported sitting balance, sitting arm raise, sitting forward reach

• Standing: supported standing balance, standing arm raise, standing forward reach, static tandem standing, weight shift

• Walking: timed 5-m walk with and without an aid, tap and step.

References and Further Reading

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Bonan, IV, Colle, FM, Guichard, JP, et al. Reliance on visual information after stroke, Part I: Balance on dynamic posturography. Archives of Physical Medicine Rehabilitation. 2004; 852:268–273.

Bonan, IV, Guettard, E, Leman, MC. Subjective visual vertical perception relates to balance in acute stroke. Archives of Physical Medicine Rehabilitation. 2006; 875:642–646.

Brown, LA, Sleik, RJ, Winder, TR. Attentional demands for static postural control after stroke. Archives of Physical Medicine Rehabilitation. 2002; 8312:1732–1735.

Brown, LA, Cooper, SA, Doan, JB, et al. Parkinsonian deficits in sensory integration for postural control: temporal response to changes in visual input. Parkinsonism Related Disorders. 2006; 12:376–381.

Chen, IC, Cheng, PT, Hu, AL, et al. Balance evaluation in hemiplegic stroke patients. Chan Gung Medical. 2000; J236:339–347.

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