Peripheral sensory reception

The three primary peripheral sensory inputs contributing to postural control are the bilateral receptors of the somatosensory, visual, and vestibular systems.^4,10 Somatosensory receptors located in the joints, ligaments, muscles, and skin provide information about muscle length, stretch, tension, and contraction; pain, temperature, and pressure; and joint position. The feet, ankles, knees, hips, back, neck, and eye muscles all furnish useful information for balance maintenance. Somatosensation is the dominant sense for upright postural control and is responsible for triggering automatic postural responses (APRs). Somatosensory loss significantly impairs balance. Loss of peripheral somatosensation occurs in clients with loss or disease of or injury to the peripheral sensory receptors or afferent sensory nerves. Examples include clients with diabetic neuropathy, peripheral vascular disease, spinal cord injury, and amputation.

Visual receptors in the eyes perform dual tasks. Central (or foveal) vision allows environmental orientation, contributing to the perception of verticality and object motion, as well as identification of the hazards and opportunities presented by the environment.¹⁰ For example, a kayaker may see rocks in a stream as a hazard to be avoided, whereas a hiker who wants to cross the stream may see the same rocks as a welcome opportunity. Peripheral (or ambient) vision detects the motion of the self in relation to the environment, including head movements and postural sway. Peripheral vision is largely subconscious, whereas central visual inputs tend to receive more conscious recognition.¹⁰ Both are normally used for postural control. Vision is critical for feed-forward, or anticipatory, postural control in changing environments. This includes planning for functional movements such as reaching and grasping, and especially for successful navigation during gait. Vision loss also impairs balance. Loss of peripheral visual inputs occurs in clients with disease of or injury to the eyes or afferent cranial nerves. Examples include clients with cataracts, macular degeneration, glaucoma, or diabetic retinopathy.

The vestibular system provides the central nervous system (CNS) with information about the position and motion of the head. The position of the head in relation to gravity is detected through the otolith system. Horizontal and vertical accelerations, as in riding in a car or an elevator, are also detected by the otoliths.¹⁴ Movements of the head are detected through the semicircular canals. Head movement stimulates both sets of semicircular canals, so that the vestibular nerve on one side becomes inhibited while the other becomes excited. The vestibular system provides sensory redundancy in the information obtained from each separate vestibular apparatus. If the peripheral vestibular system is damaged on one side, the information can be captured by the intact canals on the opposite side. The vestibular system is critical for balance because it uniquely identifies self-motion as different from motion in the environment. Box 22-1 describes the sensory components of the vestibular system. Vestibular loss also impairs balance. Loss of peripheral vestibular inputs occurs in clients with disease of or injury to the peripheral sensory receptors or afferent cranial nerves. Examples include clients with head injury involving temporal bone damage, acoustic neuroma, benign positional vertigo (BPV), or Meniere disease. For a comprehensive review of the vestibular system and vestibular disorders, please see the vestibular section of this chapter beginning on page 689.

Orientation to the wider environment, primarily from vision, allows feed-forward, or anticipatory, postural adjustments. Prior experience and high attentional capacity improve anticipatory postural adjustments significantly. Detection of head movement by the vestibular and cervical somatosensory systems and of body sway by somatosensory and peripheral visual systems provides feedback for APRs. Note that the better anticipatory abilities become, the fewer balance errors occur. Fewer balance errors mean fewer losses of balance and a reduced need to produce APRs.

Disease of or damage to any of the peripheral sensory receptors or afferent pathways impairs or removes the detection capabilities of the system, rendering sensory information unavailable for use in postural control. Many patients with neurological diagnoses have peripheral sensory impairments.

Central sensory perception

The brain processes all the environmentally available sensory information gathered by the peripheral receptors in varying degrees. This processing is usually referred to as multisensory integration or sensory organization.^4,10 Central sensory structures function first to compare available inputs between two sides and among three sensory systems. The somatosensory system alone is unable to distinguish surface tilts from body tilts. Also, the visual system by itself cannot discriminate movement of the environment from movement of the body.¹⁴ The vestibular system by itself cannot tell if head movement through space is produced by neck motion or trunk and hip motion. Therefore the brain needs information from all three senses to correctly distinguish self-motion from motion in the environment.

How are sensory inputs from separate senses combined to form perceptions of position and motion? For example, consider the movement of turning your head to one side to look over your shoulder. When the head turns to one side, firing will increase in one vestibular organ and decrease proportionately in the other. This is known as push-pull function, and the information from each side is considered to “match.” With the same example, if the eyes are open while the head moves, the rate of the visual flow will be equal and the direction of the visual flow will be opposite to the rate and direction of information from the vestibular inputs. The muscles on one side of the neck will shorten and on the other side will stretch. The inputs from these three systems are congruent. If both sides and all three systems provide compatible inputs, the process of sensory organization is simplified.

When changes in the environment occur, the relative availability, accuracy, and usefulness of information from the three sensory systems may also change. Sensory organization also includes an adaptive process, called multisensory reweighting, that permits the CNS to prioritize the sources of sensory information when environmental conditions change.^15,16 Available, accurate, and useful information is “upweighted,” whereas unavailable, inaccurate, or less-useful information is “downweighted.” For example, in dark environments, vision would be downweighted and somatosensory and vestibular information would be upweighted. This adaptive process is imperfect, however, and balance is not as well controlled when any sense must be downweighted as it is when all three senses are available and accurate. Individuals with peripheral sensory loss or central sensory processing deficits may have difficulty reweighting quickly and fully. This impairs their ability to adapt to, and remain stable in, changing environments.¹⁷

Sensory conflict can arise when information between sides or between systems is not synchronous. Sensory organization processing then becomes more complex because the brain must then recognize any discrepancies and select the correct inputs on which to base motor responses. The vestibular system may be used as an internal reference to determine accuracy of the other two senses when they conflict. For example, a driver stopped at a red light suddenly hits the brake when an adjacent vehicle begins to roll. Movement of the other car detected by the peripheral visual system is momentarily misperceived as self-motion. In this situation, the vestibular and somatosensory systems do not detect motion, but the forward visual flow is interpreted as backward motion. Because the brain failed to suppress the (mismatched) visual inputs, the braking response was generated.

When the brain recognizes that the information coming from one sensory input is inaccurate or unavailable, as is the case when somatosensory information is diminished poststroke, it must depend more on the remaining senses (in this case, vision and vestibular system) to determine position and motion in space. The brain then compares and uses information from senses it considers accurate for balance. An individual with the problem just described may compensate for the loss of somatosensory function by becoming visually dependent for balance during movement. If vision subsequently also becomes disrupted as this client ages, his ability to orient in space will be further compromised. This will impair balance and increase risk of falls.

Activities or environments that create sensory conflict or demand sensory resolution become more difficult to manage when the vestibular system is deficient or underused. These situations, such as going down stairs, riding escalators or elevators, walking on uneven ground, and making quick turns, are often avoided. When sensory conflicts cannot be resolved rapidly, dizziness or motion sickness occurs.

Intrinsic central sensory processing impairments also can produce sensory conflict. An adult hemiplegic patient with pusher syndrome illustrates an inability to integrate visual, vestibular, and somatosensory inputs for midline orientation. Within a single system, discrepancies between the sides are also problematic. Unequal firing from opposite sides of the vestibular system, as in unilateral vestibular hypofunction, produces a mismatch that is subsequently interpreted as head rotation when head movement does not occur. This spinning sensation is known as vertigo.¹⁴ Vertigo is resolved if the brain is able to adapt to the mismatch. For further information on vertigo, refer to the section on the vestibular system.

Finally, the central processing mechanisms combine any available and accurate inputs to answer the questions “Where am I?” and “How am I moving?” This includes both an internal relation of the body segments to one another (e.g., head in relation to trunk, trunk in relation to feet) and an external relation of the body to the outside world (e.g., feet in relation to surface, arm in relation to handrail). CNS disease or trauma involving the parietal lobe may impair these processing mechanisms so that even available, accurate sensory inputs are not recognized or incorporated into determinations of position and movement.^18,19 Impairments of central sensory processing may occur after stroke, head trauma, tumors, or aneurysms; with disease processes such as MS; and with aging.

Central motor planning and control

Whereas sensory processing allows the interaction of the individual and the environment, motor planning underlies the interaction of the individual and the task. Aside from reflexive activity such as breathing and blinking, most motor actions are voluntary and occur because some goal is to be achieved. That is not to say that reflexes occur separately from volitional movements; for example, the vestibuloocular reflex is active concurrently with visual tracking activity, but most actions occur because of some purposeful intent.¹⁴ These task intentions precede motor actions.^10,20 Wrist and hand movements vary depending on what is to be grasped (a cup versus a doorknob); foot placement and trunk position vary depending on what is to be lifted (a heavy suitcase versus a laundry basket). The initiation of volitional motor actions depends on intention, attention, and motivation.^10,21

Once an objective (“Where do I want to be?” “What do I want to do?”) has been chosen, the next step in motor planning is to determine how to best accomplish the goal given the many options that are potentially available. For example, when the task demands fine skills or accuracy, the dominant hand is preferred; when the task involves lifting a large or heavy object, both hands are preferred. In addition to which limbs, joints, and muscles will be used, motor planning also adjusts the timing, sequencing, and force modulation. This can be demonstrated in various reaching tasks. Reaching to remove a hot item from the oven will occur slowly, whereas reaching to put an arm through a sleeve will occur more quickly. Optimal motor plans are developed with knowledge of self (abilities and limitations), knowledge of task (characteristics of successful performance), and knowledge of the environment (risks and opportunities).²¹

The motor plan must be transmitted to the peripheral motor system to be enacted. A copy of the intended movement plan is sent to the cerebellum during the transmission. When the movement begins, incoming sensory inputs (feedback) about the actual movements and performance outcome are compared with the intended movements and performance outcome. Movement errors (the difference between the intended and the actual movement) and performance errors (desired goal not achieved) are detected, and plans for correction are then formed and transmitted. This process of error detection and error correction is the foundation of motor learning.

Clients with CNS disorders often have central motor planning and control system problems. After a stroke, clients may have hypertonus and poor reciprocal inhibition; clients with head trauma may have difficulty initiating or ceasing movements; clients with Parkinson disease exhibit bradykinesia; and those with cerebellar ataxia display modulation problems.²²

Peripheral motor execution

Movement is accomplished through the bilateral joints and muscles. Normal range of motion (ROM), strength, and endurance of the feet, ankles, knees, hips, back, neck, and eyes must be present for the execution of the full range of normal balance movements. Decreased ankle dorsiflexion ROM, for example, restricts the forward limits of stability. Strength deficits are a primary cause of movement abnormalities in both central and peripheral nervous system disorders. In addition, weakness may be the result of force modulation deficits or disuse.¹¹ Balance is directly affected by loss of strength. For example, weakness of the hip extensors and abductors will impede successful use of a hip strategy for upright trunk control. Initially adequate toe clearance may diminish with fatigue. Many clients with neurological issues also have stiffness and contractures as a result of persistent weakness or hypertonus. Restrictions in ROM also limit balance abilities.

The ability to achieve static postural alignment, although necessary for normal balance, is not sufficient to allow volitional functions. Adequate strength (to control body weight and any additional loads) through normal postural sway ranges is needed to permit dynamic balance activities such as reaching, leaning, and lifting. Postural control demands are increased during gait because the forces of momentum and the interaction between recruitment, timing, and velocity also must be regulated.²³ Traditionally considered orthopedic problems, deficits in strength, ROM, posture, and endurance have a great impact on balance abilities. Attention must be given to these musculoskeletal system problems in examination of and intervention for clients with neurological diagnoses.

Influence of other systems

Balance abilities are also influenced by other systems. Attention, cognition and judgment, and memory are critical for optimal balance function and are often impaired in hemiplegic and head-injured clients as well as those who have progressive neurological disorders. Attentional deficits reduce awareness of environmental hazards and opportunities, interfering with anticipatory postural control.¹² When balance is threatened, an inability to allocate attention to the necessary task of balance versus a secondary, less necessary task increases the risk of falls. Cognitive problems such as distractibility, poor judgment, and slowed processing also increase the risk of falls. Memory loss may preclude recall of safety measures. Depression, emotional lability, agitation, or denial of impairments also can increase the risks for loss of balance. In addition to having a direct impact on balance abilities themselves, these cognitive and behavioral problems impede motor learning processes, which are crucial for the relearning of balance skills.

Constant cyclic nature

The systems model of postural control previously presented illustrates the constant cycle that simultaneously occurs at many levels. Attention and intention allow feed-forward processing for active sensory search of the environment and motor planning, both of which are needed for anticipatory postural control. Movements are initiated and executed with resultant sensory experiences and error detection, or feedback. Successful movements are repeated and refined; unsuccessful ones are modified. The nature of this cycle presents the clinician with opportunities for intervention after the appropriate examination of sensory, motor, and cognitive functions. Through feedback and practice, balance abilities can improve.²⁴

Automatic postural responses

At the next level, automatic postural responses operate to keep the COG over the base of support. They are a set of functionally organized, long-loop responses that act to keep the body in a state of equilibrium.^3,4 Functionally organized means that the responses, although stereotypical, are matched to the perturbing stimulus in direction and amplitude. If the stimulus is a push to the right, the response is a shift to the left, toward midline. The larger the stimulus, the greater the response. Automatic postural responses always occur in response to an unexpected stimulus and are typically triggered by somatosensory inputs. Because they occur rapidly, in less than 250 ms, they are not under immediate volitional control.

Four automatic postural responses have been described. Ankle strategy describes postural sway control from the ankles and feet. The head and hips travel in the same direction at the same time, with the body moving as a unit over the feet (Figure 22-2, A). Muscle contractile patterns are from distal to proximal (i.e., gastrocnemius, hamstrings, paraspinals). This strategy is used when sway is small, slow, and near midline. It occurs when the surface is broad and stable enough to allow pressure against it to produce forces that can counteract sway to stabilize the body. Ankle strategy is typically used to control anterior-posterior sway, because most of the degrees of freedom at the ankle are in this direction.

Hip strategy involves postural sway control from the pelvis and trunk. The head and hips travel in opposite directions, with body segment movements counteracting one another (Figure 22-2, B). Muscle contractile patterns are from proximal to distal (i.e., abdominals, quadriceps, tibialis anterior). This strategy is observed when sway is large, fast, and nearing the limit of stability or if the surface is too narrow or unstable to permit effective counterpressure of the feet against the surface. Hip strategy is used to control both anterior-posterior and medial-lateral sway. Hip strategy in the medial-lateral direction involves weight shifts from foot to foot; any client with difficulty weight-shifting quickly and accurately will have difficulty with medial-lateral hip strategy.

Suspensory strategy involves a lowering of the COG toward the base of support by bilateral lower-extremity flexion or a slight squatting motion (Figure 22-2, C). By shortening the distance between the COG and the base of support, the task of controlling the COG is made easier. This strategy is often used when a combination of stability and mobility is required, as in windsurfing.

Stepping and reaching strategies involve steps with the feet or reaches with the arms in an attempt to reestablish a new base of support with the active limb(s) when the COG has exceeded the original base of support (Figure 22-2, D). A successful stepping strategy is the best way to avoid a fall after a slip or trip.

Misconceptions about these APR strategies are common. First, these strategies do not function in daily life as separately as they are described in the early research literature. In quiet standing, for example, frequency analysis of unperturbed postural sway in healthy adults reveals that both ankle and hip strategies occur in combination, simultaneously.²⁶ In perturbation studies, mixed use of strategies is often seen unless the perturbation is clearly below or above certain-sized thresholds. Second, these strategies occur in response to disturbances from all directions, not just in pure anterior-posterior or medial-lateral directions.²⁷ Third, although these strategies are stereotypical in humans, great individual variation in strategy selection and performance comes from other influential factors. For example, many people use stepping strategy for most perturbations unless specifically instructed not to step or unless the conditions do not permit a step. An anxious person may reach or step much sooner than a relaxed person with similar physical deficits. Last, all these strategies do not occur in sequence with every balance disturbance.^28,29 In other words, individuals normally do not try ankle strategy and wait until it fails before trying hip strategy, then wait until it fails before trying stepping strategy (although early learning may involve such exploration). Because these responses must occur extremely rapidly to prevent balance loss, such a sequential approach would be inefficient and ineffective. Instead, the normal response is the emergence of the single strategy best suited to the particular perturbation, the limitations of the individual, and the conditions in the environment.

Abnormal use of automatic postural responses is often observed in individuals with neurological disorders. Clients with vestibular deficits typically rely on ankle strategy, which permits the head to remain aligned with the body and sustains congruence between vestibular and somatosensory inputs. Use of hip strategy may be modified or limited because when the head is moving in the opposite direction as the COG, vestibular and somatosensory inputs are not congruent. Activities that require use of hip strategy, such as standing in tandem or on one leg, can be a problem for clients with bilateral vestibular loss or an uncompensated vestibular lesion. However, some cases involve excessive use of hip strategy on a level surface (when an ankle strategy would suffice).³⁰ This may reflect abnormal integration of the somatosensory and vestibular information. If peripheral somatosensation is impaired, as in diabetic neuropathy, or central sensory weighting of somatosensory inputs is inadequate, hip strategy may dominate.

Clients with somatosensory loss, distal lower extremity weakness or hypertonus, restricted ankle ROM, and/or reduced limits of stability typically rely on hip strategy. This occurs because the client cannot feel the surface or the feet well enough to modulate foot pressure against the surface, because the person cannot generate sufficient force against the surface with the ankle muscles, or because restricted ankle ROM prevents COG sway. The use of hip strategy is normal when the COG is at or near the limits of stability and a step is either not possible or not desired.

When the hip or ankle strategy is not efficient enough to control the movement of the center of pressure, or if conditions and instructions permit a stepping response, stepping strategy may be preferred. Individuals who are fearful of falling often perceive even slight body sway as threatening instability. They may use stepping and reaching strategies exclusively whether or not these “rescue” strategies are actually necessary.

Anticipatory postural adjustments

Anticipatory postural adjustments are similar to automatic postural responses, but they occur before the actual disturbance.²⁰ If a balance disturbance is predicted, the body will respond in advance by developing a “postural set” to counteract the coming forces. For example, if an individual lifts an empty suitcase thinking it is full and heavy, the anticipatory forces generated before the lift (to counter the anticipated weight) will cause excessive movement and brief instability. Failure to produce these anticipatory adjustments increases the risk of sudden balance loss, creating the need to use rapid, reactive automatic postural responses to prevent a fall. For clients with deficits in reaction time or automatic postural responses, superior use of anticipatory postural control can help the client avoid the unexpected perturbations that make automatic postural responses necessary.

In balance laboratories, anticipatory postural adjustments are studied using electromyography so that muscle activity before observable movement can be measured. In the clinic, problems with anticipatory adjustments may be observed when the client fails to counteract a predicted disturbance, such as “don’t let me push you backward,” or fails to integrate postural control tasks during other activities, such as the inability to step smoothly over an anticipated obstacle during gait or inability to maintain sitting balance when both arms are intentionally lifted overhead.

Volitional postural movements

Volitional postural movements are under conscious control. Weight shifts to allow an individual to reach the telephone or put the dishes in the dishwasher, for example, are self-initiated disturbances of the COG to accomplish a goal. Volitional postural movements can range from simple weight shifts to complex balance skills of skaters and gymnasts. They can occur after a stimulus or be self-initiated. Volitional postural movements can occur quickly or slowly, depending on the goal at hand. The more complex or unfamiliar the task, the slower the response time. Use of a variety of movements that might successfully achieve a goal is possible. Volitional postural movements are strongly modified by prior experience and instruction. Automatic and anticipatory postural responses allow the continuous unconscious control of balance, whereas volitional postural movements permit conscious activity. This level of postural motor control is the most frequently tested and treated in clinical practice, but it is by no means sufficient by itself to produce normal balance.