Regaining Postural Stability and Balance

Johna K. Register-Mihalik, PhD, LAT, ATC
Kevin M. Guskiewicz, PhD, ATC, FNATA, FACSM

Despite often being classified at the end of the continuum of goals associated with therapeutic exercise,50 maintenance of balance is a vital component in the rehabilitation of brain and joint injuries that should not be overlooked. Traditionally, orthopedic rehabilitation has placed the emphasis on isolated joint mechanics, such as improving ROM and flexibility, and increasing muscle strength and endurance, rather than on afferent information obtained by the joint(s) to be processed by the postural control system. Additionally, rehabilitation following traumatic brain injury/concussions has only recently been at the forefront of management options.⁷⁷

Research in the area of proprioception and kinesthesia has emphasized the need to train the joint’s neural system.^51–55 Joint position sense, proprioception, and kinesthesia are vital to all athletic performance requiring balance. Current rehabilitation protocols should therefore focus on a combination of open and closed kinetic chain exercises. The necessity for a combination of open and closed kinetic chain exercises can be seen during gait (walking or running), as the foot and ankle prepare for heel strike (open chain) and prepare to control the body’s COG during midstance and toe off (closed chain). Concerning concussion, recent evidence suggests that balance training and vestibular specific training in individuals with these deficits can improve outcomes.⁷⁷ As such, these types of activities should be considered in the management of concussion.

This chapter focuses on the postural control system, various balance training techniques, and technologic advancements that are enabling athletic trainers to assess and treat balance deficits in physically active people.

POSTURAL CONTROL SYSTEM

To design effective rehabilitation programs, the athletic trainer must first have an understanding of the postural control system and its various components. The postural control system uses complex processes involving both sensory and motor components. Maintenance of postural equilibrium includes sensory detection of body motions, integration of sensorimotor information within the central nervous system (CNS), and execution of appropriate musculoskeletal responses. Most daily activities, such as walking, climbing stairs, reaching, or throwing a ball, require static foot placement with controlled balance shifts, especially if a favorable outcome is to be attained. So, balance should be considered both a dynamic and static process. The successful accomplishment of static and dynamic balance is based on the interaction between body and environment.⁴⁹ Figure 7-1 shows the complexity of this dynamic process.

From a clinical perspective, separating the sensory and motor processes of balance means that a person may have impaired balance for one or a combination of 2 reasons: the position of the COG relative to the base of support is not accurately sensed for either peripheral or central reasons, and/or the automatic movements required to bring the COG to a balanced position are not timely or effectively coordinated.⁶⁷ The position of the body in relation to gravity and its surroundings is sensed by combining visual, vestibular, and somatosensory inputs. Balance movements also involve motions of the ankle, knee, and hip joints, which are controlled by the coordinated actions along the kinetic chain (Figure 7-2). These processes are all vital for producing both normal, everyday movements such as gait as well as fluid, sport-related movements.

CONTROL OF BALANCE

The human body is a very tall structure balanced on a relatively small base, and its COG is quite high, being just above the pelvis. Many factors enter into the task of controlling balance within the base of support. Balance control involves a complex network of neural connections and centers that are related by peripheral and central feedback mechanisms.38

The postural control system operates as a feedback control circuit between the brain and the musculoskeletal system. The sources of afferent information supplied to the postural control system collectively come from visual, vestibular, and somatosensory inputs. The involvement of the CNS in maintaining upright posture can be divided into 2 components. The first component—sensory organization—involves those processes that determine the timing, direction, and amplitude of corrective postural actions based upon information obtained from the vestibular, visual, and somatosensory (proprioceptive) inputs.⁶³ Despite the availability of multiple sensory inputs, the CNS generally relies on only one sense at a time for orientation information. For healthy adults, the preferred sense for balance control comes from somatosensory information (ie, feet in contact with the support surface and detection of joint movement).^42,63 In considering orthopedic injuries, the somatosensory system is of the most importance and is the focus of this chapter.

The second component—muscle coordination—is the collection of processes that determine the temporal sequencing and distribution of contractile activity among the muscles of the legs and trunk that generate supportive reactions for maintaining balance. Research suggests that balance deficiencies in people with neurologic problems can result from inappropriate interaction among the 3 sensory inputs that provide orientation information to the postural control system. A patient may be inappropriately dependent on one sense for situations presenting intersensory conflict.^63,80

From a clinical perspective, stabilization of upright posture requires the integration of afferent information from the 3 senses, which work in combination and are all critical to the execution of coordinated postural corrections. Impairment of one component is usually compensated for by the remaining 2 components. Often, one of the systems provides faulty or inadequate information such as different surfaces and/or changes in visual acuity and/or peripheral vision. In this case, it is crucial that one of the other senses provides accurate and adequate information so that balance may be maintained. For example, when somatosensory conflict is present, such as a moving platform or a compliant foam surface, balance is significantly decreased with the eyes closed compared to eyes open.

Somatosensory inputs provide information concerning the orientation of body parts to one another and to the support surface.^25,68 Vision measures the orientation of the eyes and head in relation to surrounding objects, and plays an important role in the maintenance of balance. On a stable surface, closing the eyes should cause only minimal increases in postural sway in healthy patients. However, if somatosensory input is disrupted because of ligamentous injury, closing the eyes will increase sway significantly.^{15,20,42,43,67} The vestibular apparatus supplies information that measures gravitational, linear, and angular accelerations of the head in relation to inertial space. It does not, however, provide orientation information in relation to external objects and, therefore, plays only a minor role in the maintenance of balance when the visual and somatosensory systems are providing accurate information.⁶⁷

SOMATOSENSATION AS IT RELATES TO BALANCE

The terms somatosensation, proprioception, kinesthesia, and balance are often used to describe similar phenomena. Somatosensation is a more global term used to describe the proprioceptive mechanisms related to postural control. Consequently, somatosensation is best defined as a specialized variation of the sensory modality of touch that encompasses the sensation of joint movement (kinesthesia) and joint position (joint position sense).^51,55 As previously discussed, balance refers to the ability to maintain the body’s COG within the base of support provided by the feet. Somatosensation and balance work closely, as the postural control system uses sensory information related to movement and posture from peripheral sensory receptors (eg, muscle spindles, Golgi tendon organs, joint afferents, cutaneous receptors). So the question remains: how does proprioception influence postural equilibrium and balance?

Somatosensory input is received from mechanoreceptors; however, it is unclear whether the tactile senses, muscle spindles, or Golgi tendon organs are most responsible for controlling balance. Nashner62 concluded after using electromyography responses following platform perturbations that other pathways had to be involved in the responses they recorded because the latencies were longer than those normally associated with a classic myotatic reflex. The stretch-related reflex is the earliest mechanism for increasing the activation level of muscles about a joint following an externally imposed rotation of the joint. Rotation of the ankles is the most probable stimulus of the myotatic reflex that occurs in many persons. It appears to be the first useful phase of activity in the leg muscles after a change in erect posture.⁶² The myotatic reflex can be seen when perturbations of gait or posture automatically evoke functionally directed responses in the leg muscles to compensate for imbalance or increased postural sway.^17,62 Muscle spindles sense a stretching of the agonist, thus sending information along its afferent fibers to the spinal cord. There, the information is transferred to alpha and gamma motor neurons that carry information back to the muscle fibers and muscle spindle, respectively, and contract the muscle to prevent or control additional postural sway.¹⁷

Postural sway was assessed on a platform moving into a “toes-up” and “toes-down” position, and a stretch reflex was found in the triceps surae after a sudden ramp displacement into the “toes-up” position.¹⁶ A medium latency response (103 to 118 ms) was observed in the stretched muscle, followed by a delayed response of the antagonistic anterior tibialis muscle (108 to 124 ms). The investigators also blocked afferent proprioceptive information in an attempt to study the role of proprioceptive information from the legs for the maintenance of upright posture. These results suggested that proprioceptive information from pressure and/or joint receptors of the foot (ischemia applied at ankle) plays an important role in postural stabilization during low frequencies of movement but is of minor importance for the compensation of rapid displacements. The experiment also included a “visual” component, as patients were tested with eyes closed followed by eyes open. Results suggest that when patients were tested with eyes open, visual information compensated for the loss of proprioceptive input.

Another study¹⁷ used compensatory electromyography responses during impulsive disturbance of the limbs during stance on a treadmill to describe the myotatic reflex. Results revealed that, during backward movement of the treadmill, ankle dorsiflexion caused the COG to be shifted anteriorly, thus evoking a stretch reflex in the gastrocnemius muscle, followed by weak anterior tibialis activation. In another trial, the movement was reversed (plantar flexion), thus shifting the COG posteriorly and evoking a stretch reflex of the anterior tibialis muscle. Both of these studies suggest that stretch reflex responses help to control the body’s COG, and that the vestibular system is unlikely to be directly involved in the generation of the necessary responses.

Elimination of all sensory information from the feet and ankles revealed that proprioceptors in the leg muscles (gastrocnemius and tibialis anterior) were capable of providing sufficient sensory information for stable standing.²⁴ Researchers speculated that group I or group II muscle spindle afferents and group Ib afferents from Golgi tendon organs were the probable sources of this proprioceptive information. The study demonstrated that normal patients can stand in a stable manner when receptors in the leg muscles are the only source of information about postural sway.

Other researchers^6,43 have examined the role of somatosensory information by altering or limiting somatosensory input through the use of platform sway referencing or foam platforms. These studies reported that patients still responded with well-coordinated movements, but the movements were often either ineffective or inefficient for the environmental context in which they were used.

THE EFFECTS OF VIBRATION ON BALANCE

The ability to detect postural sway and evoke corrective muscle activation strategies in an effort to maintain balance is inherently linked to somatosensory function. Sensation of joint position and motion (eg, ankle plantar flexion/dorsiflexion) is utilized for recognition of postural sway in combination with visual and vestibular inputs. Damage to joint structures (eg, ligament) impedes postural control,18 thus, using vibration to target somatosensory contributors to postural sway may enhance balance.

Whole-body vibration is delivered via a stationary platform that cyclically accelerates the body upward (Figure 7-3). Improvements in balance with vibration have been reported in a range of clinical populations including anterior cruciate ligament (ACL) reconstruction,¹⁰ functional ankle instability,¹¹ multiple sclerosis,⁷⁸ and Parkinson’s disease.⁸⁶ Furthermore, repeated exposure to vibration improves balance and reduces the risk of falling in elderly adults.⁸⁹ It should be noted, however, that each of these studies evaluated either static (ie, stance on a fixed base of support) or semi-dynamic (ie, stance on a moveable base of support such as a wobble board) balance. Adelman et al¹ reported that a single exposure to vibration did not improve dynamic postural control (eg, stabilization following landing from a jump) in individuals with chronic ankle instability. It is unclear from these results if improvements in dynamic balance would be observed with repeated exposure to vibration as part of a rehabilitation protocol, or if the stimulus provided by vibration is insufficient to improve dynamic balance. While vibration appears to improve static and semi-dynamic balance, future research is necessary to determine if these effects transfer to more dynamic, functional tasks.

BALANCE AS IT RELATES TO THE CLOSED KINETIC CHAIN

Balance is the process of maintaining the COG within the body’s base of support. Again, the human body is a very tall structure balanced on a relatively small base, and its COG is quite high, being just above the pelvis. Many factors enter into the task of controlling balance within this designated area. One component often overlooked is the role balance plays within the kinetic chain. Ongoing debates as to how the kinetic chain should be defined and whether open or closed kinetic chain exercises are best have caused many therapists to lose sight of what is most important. An understanding of the postural control system as well as the theory of the kinetic (segmental) chain about the lower extremity helps conceptualize the role of the chain in maintaining balance. Within the kinetic chain, each moving segment transmits forces to every other segment along the chain, and its motions are influenced by forces transmitted from other segments (see Chapter 12).¹³

The act of maintaining equilibrium or balance is associated with the closed kinetic chain, as the distal segment (foot) is fixed beneath the base of support. The coordination of automatic postural movements during the act of balancing is not determined solely by the muscles acting directly about the joint. Leg and trunk muscles exert indirect forces on neighboring joints through the inertial interaction forces among body segments.^64,65 A combination of one or more strategies (ankle, knee, hip) are used to coordinate movement of the COG back to a stable or balanced position when a person’s balance is disrupted by an external perturbation. Injury to any one of the joints or corresponding muscles along the kinetic chain can result in a loss of appropriate feedback for maintaining balance.

Three principal joint systems (ankles, knees, and hips) are located between the base of support and the COG. This allows for a wide variety of postures that can be assumed while the COG is still positioned above the base of support. As described by Nashner,67 motions about a given joint are controlled by the combined actions of at least one pair of muscles working in opposition. When forces exerted by pairs of opposing muscle about a joint (eg, anterior tibialis and gastrocnemius/soleus) are combined, the effect is to resist rotation of the joint relative to a resting position. The degree to which the joint resists rotation is called joint stiffness. The resting position and the stiffness of the joint are each altered independently by changing the activation levels of one or both muscle groups.^44,67 Joint resting position and joint stiffness are by themselves an inadequate basis for controlling postural movements, and it is theorized that 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.⁶⁷

When a person’s balance is disrupted by an external perturbation, movement strategies involving joints of the lower extremity coordinate movement of the COG back to a balanced position. Three strategies (ankle, hip, stepping) have been identified along a continuum.⁴² In general, the relative effectiveness of ankle, hip, and stepping strategies in repositioning the COG over the base of support depends on the configuration of the base of support, the COG alignment in relation to the LOS, and the speed of the postural movement.^42,43

The ankle strategy shifts the COG while maintaining the placement of the feet by rotating the body as a rigid mass about the ankle joints. This is achieved by contracting either the gastrocnemius or anterior tibialis muscles to generate torque about the ankle joints. Anterior sway of the body is counteracted by gastrocnemius activity, which pulls the body posteriorly. Conversely, posterior sway of the body is counteracted by contraction of the tibialis anterior. Thus, the importance of these muscles should not be underestimated when designing the rehabilitation program. The ankle strategy is most effective in executing relatively slow COG movements when the base of support is firm and the COG is well within the LOS perimeter. The ankle strategy is also believed to be effective in maintaining a static posture with the COG off set from the center. The thigh and lower trunk muscles contract, thereby resisting the destabilization of these proximal joints as a result of the indirect effects of the ankle muscles on the proximal joints (Table 7-1).

Under normal sensory conditions, activation of ankle musculature is most often selected to maintain equilibrium. However, there are subtle differences associated with loss of somatosensation and with vestibular dysfunction in terms of postural control strategies. Persons with somatosensory loss appear to rely on their hip musculature to retain their COG while experiencing forward or backward perturbation or with different support surface lengths.²⁵

If the ankle strategy is not capable of controlling excessive sway, the hip strategy is available to help control motion of the COG through the initiation of large and rapid motions at the hip joints with antiphase rotation of the ankles. It is most effective when the COG is located near the LOS perimeter and when the LOS boundaries are contracted by a narrowed base of support. Finally, when the COG is displaced beyond the LOS, a step or stumble (stepping strategy) is the only strategy that can be used to prevent a fall.^65,67

It is proposed that LOS and COG alignment are altered in individuals exhibiting a musculoskeletal abnormality such as an ankle or knee sprain. For example, weakness of ligaments following acute or chronic sprain about these joints is likely to reduce ROM, thereby shrinking the LOS and placing the person at greater risk for a fall with a relatively smaller sway envelope.65 Pintsaar et al⁷⁴ suggested that impaired function was related to a change from ankle synergy toward hip synergy for postural adjustments among patients with functional ankle instability. More recently, a systematic review suggested that individuals with chronic ankle instability do not utilize somatosensory information to the extent that uninjured controls do and that they rely more heavily on visual information.⁸² These findings, which were consistent with previous results reported by Tropp et al,⁸⁵ suggest that sensory proprioceptive function for the injured patients was affected. Importantly, these combined results suggest that not only do strategies differ, but the use of various sensory inputs may be reweighted following injury.

ASSESSMENT OF BALANCE

Several methods of balance assessment have been proposed for clinical use. Historically, many of the techniques have been criticized for offering only subjective (“qualitative”) measurement information regarding balance rather than an objective (“quantitative”) measure.⁷⁰

Subjective Assessment

Prior to the mid 1980s, there were very few methods for systematic and controlled assessment of balance. The assessment of static balance in athletes has traditionally been performed through the use of the standing Romberg test. This test is performed standing with feet together, arms at the side, and eyes closed. Normally, a person can stand motionless in this position, but the tendency to sway or fall to one side is considered a positive Romberg sign, indicating a loss of proprioception.⁹ The Romberg test has, however, been criticized for its lack of sensitivity and objectivity. It is considered to be a rather qualitative assessment of static balance because a considerable amount of stress is required to make the patient sway enough for an observer to characterize the sway.⁴⁷

The use of a quantifiable clinical test battery such as the Balance Error Scoring System (BESS) is recommended instead of the standard Romberg test.³⁶ Three different stances (double, single, and tandem) are completed twice: once while on a firm surface and once while on a piece of medium density foam (balance pad by Airex is recommended) for a total of 6 trials (Figure 7-4). Patients are asked to assume the required stance by placing their hands on the iliac crests, and upon eye closure, the 20-second test begins. During the single-leg stances, patients are asked to maintain the contralateral limb in 20 to 30 degrees of hip flexion and 40 to 50 degrees of knee flexion. Additionally, the patient is asked to stand quietly and as motionless as possible in the stance position, keeping his or her hands on the iliac crests and eyes closed. The single-limb stance tests are performed on the nondominant foot. This same foot is placed toward the rear on the tandem stances. Patients are told that, upon losing their balance, they are to make any necessary adjustments and return to the testing position as quickly as possible. Performance is scored by adding 1 error point for each error listed in Table 7-2. Trials are considered to be incomplete if the patient is unable to sustain the stance position for longer than 5 seconds during the entire 20-second testing period. These trials are assigned a standard maximum error score of 10. No trial can have a score of greater than 10. Balance test results during injury recovery are best used when compared to baseline measurements, and clinicians working with athletes or patients on a regular basis should attempt to obtain baseline measurements when possible. However, normative data are available in some populations.³⁹

Semi-dynamic and dynamic balance assessment can be performed through functional-reach tests; timed agility tests such as the figure 8 test,19,23 carioca, or hop test⁴⁵; Bass Test for Dynamic Balance; timed “T-Band kicks”; and timed balance beam walking with the eyes open or closed. The objective in most of these tests is to decrease the size of the base of support in an attempt to determine a patient’s ability to control upright posture while moving. Many of these tests have been criticized for failing to quantify balance adequately as they merely report the time that a particular posture is maintained, angular displacement, or the distance covered after walking.^7,25,51,67 At any rate, they can often provide the athletic trainer with valuable information about a patient’s function and/or return to play capability.

Objective Assessment

Advancements in technology have provided the medical community with commercially available balance systems for quantitatively assessing and training static and dynamic balance (Table 7-3). These systems provide an easy, practical, and cost-effective method of quantitatively assessing and training functional balance through analysis of postural stability. Thus, the potential exists to assess injured patients and (a) identify possible abnormalities that might be associated with injury; (b) isolate various systems that are affected; (c) develop recovery curves based on quantitative measures for determining readiness to return to activity; and (d) train the injured patient.

Most manufacturers use computer-interfaced force plate technology consisting of a flat, rigid surface supported on 3 or more points by independent force-measuring devices. As the patient stands on the force plate surface, the position of the center of vertical forces exerted on the force plate over time is calculated (Figure 7-5). The center of vertical force movements provide an indirect measure of postural sway activity.⁶⁶ The Kistler and, more recently, Bertec force plates, are used for much of the work in the area of postural stability and balance.^{7,21,31,58,61} NeuroCom International, Inc. has also developed systems with expanded diagnostic and training capabilities that make interpretation of results easier for athletic trainers. Athletic trainers must be aware that the manufacturers often use conflicting terminology to describe various balance parameters, and should consult frequently with the manufacturer to ensure that there is a clear understanding of the measure being taken. These inconsistencies have created confusion in the literature because what some manufacturers classify as dynamic balance, others claim as really static balance. Our classification system (see the following section “Balance Training”) will hopefully clear up some of the confusion and allow for a more consistent labeling of the numerous balance-related exercises.

Force platforms ideally evaluate 3 aspects of postural control: steadiness, symmetry, and dynamic stability. Steadiness is the ability to keep the body as motionless as possible. This is a measure of postural sway. Symmetry is the ability to distribute weight evenly between the 2 feet in an upright stance. This is a measure of center of pressure (COP), center of balance (COB), or center of force (COF), depending on which testing system you are using. Although inconsistent with our classification system, dynamic stability is often labeled as the ability to transfer the vertical projection of the COG around a stationary supporting base.31 This is often referred to as a measure of one’s perception of his or her “safe” LOS, as one’s goal is to lean or reach as far as possible without losing one’s balance. Some manufacturers measure dynamic stability by assessing a person’s postural response to external perturbations from a moving platform in 1 of 4 directions: tilting toes up, tilting toes down, shifting medial-lateral, and shifting anterior-posterior. Platform perturbation on some systems is unpredictable and determined by the positioning and sway movement of the patient. In such cases, a person’s reaction response can be determined. Other systems have a more predictable sinusoidal waveform that remains constant regardless of patient positioning.

Many of these force platform systems measure the vertical ground reaction force and provide a means of computing the COP. The COP represents the center of the distribution of the total force applied to the supporting surface. The COP is calculated from horizontal moment and vertical force data generated by triaxial force platforms. The center of vertical force, on NeuroCom’s EquiTest, is the center of the vertical force exerted by the feet against the support surface. In any case (COP, COB, COF), the total force applied to the force platform fluctuates because it includes both body weight and the inertial effects of the slightest movement of the body that occur even when one attempts to stand motionless. The movement of these force-based reference points is theorized to vary according to the movement of the body’s COG and the distribution of muscle forces required to control posture. Ideally, healthy athletes should maintain their COP very near the anterior-posterior and medial-lateral midlines.

Once the COP or COF is calculated, several other balance parameters can be attained. Deviation from this point in any direction represents a person’s postural sway. Postural sway can be measured in various ways depending on which system is being used. Mean displacement, length of sway path, length of sway area, amplitude, frequency, and direction with respect to the COP can be calculated on most systems. An equilibrium score, comparing the angular difference between the calculated maximum anterior to posterior COG displacements to a theoretical maximum displacement, is unique to NeuroCom’s EquiTest.

Force plate technology allows for quantitative analysis and understanding of a patient’s postural instability. These systems are fully integrated with hardware or software systems for quickly and quantitatively assessing and rehabilitating balance disorders. Most manufacturers allow for both static and dynamic balance assessment in either double- or single-leg stances, with eyes open or eyes closed. NeuroCom’s EquiTest System is equipped with a moving visual surround (wall) that allows for the most sophisticated technology available for isolating and assessing sensory modality interaction (Figure 7-6).

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Maintaining Cardiorespiratory Fitness During Rehabilitation Restoring Range of Motion and Improving Flexibility Regaining Muscular Strength, Endurance, and Power Rehabilitation of Lower Leg Injuries Rehabilitation of Injuries to the Spine Rehabilitation of Ankle and Foot Injuries

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