Multiple resources required for postural stability and orientation

Multiple resources are required for postural stability and orientation (Fig. 8.3), and a basic understanding of these functional systems is necessary in order to diagnose balance disorders and design treatments that focus specifically on the affected systems. Biomechanical constraints, such as muscle strength and range of motion, play an important role in balance control, as they can limit the strategies available for use. Dynamic control is essential for challenges to balance that occur during gait, such as proactive modifications in order to avoid or accommodate obstacles. Disruptions of cognitive control may make balance difficult in dual-task situations where attention is divided, and/or make it difficult to improve postural control based on prior experience. Abnormal perception of spatial orientation relative to the environment can skew the internal model of the body. Two additional resources that are crucial for balance control are sensory strategies and movement strategies, both of which are covered in more detail in the following sections.

Sensory integration and weighting

Multiple sources of sensory information are required for postural stability and orientation (Fig. 8.3). Sensory contributions to balance control come from three major sensory systems: visual, vestibular, and

Biomechanical constraints

• Degrees of freedom

• Limits of stability

Sensory strategies

• Sensory integration

• Sensory reweighting

• Visual, vestibular, somatosensory

Sensory strategies

• Sensory integration

• Sensory reweighting

• Visual, vestibular, somatosensory

Spatial orientation

• Perception

• Verticality

• Gravity, surfaces, vision

Control of dynamics

• Gait and transfers

• Proactive Head/trunk in space

• Obstacle avoidance

Spatial orientation

• Perception

• Verticality

• Gravity, surfaces, vision

Movement strategies

• Anticipatory

• Voluntary CoM movement

Cognitive control

Figure 8.3. Resources required for balance control.

somatosensory. The visual system allows recognition of, and detection of orientation and movement of, objects in the environment. This information can be used to determine the orientation of the body with respect to the environment and to adapt movements to meet environmental conditions, for example, avoiding an obstacle in one's path during locomotion. The visual system also detects relative motion of the self with respect to the environment, such as postural sway during standing or progression during locomotion. Objects at about arm's length away allow for more effective balance control than objects at a great distance, as the nearer an object the greater angular displacement of its image on the retina with body movement. Objects greater than 2.5 m away cannot be used to effectively control sway during quiet stance (Paulus et al., 1984) and sway increases for many subjects in the absence of vision (Romberg, 1851; Chiari et al., 2001). As the visual system cannot differentiate self-motion from environmental motion, vision can induce illusions of movement when the visual scene moves slowly with respect to a stationary individual. An example of this phenomenon that many of us have experienced is the sensation of movement that is briefly felt when sitting in a parked car as the neighboring car starts to move. Forward motion of the neighboring car results in an illusion of self-motion backwards. The influence that vision has on postural control varies with changes in lighting conditions, visual acuity, and the location and size of a visual stimulus within the visual field (Leibowitz et al., 1972).

The vestibular system is specialized for the control of postural orientation and balance (see Chapter 20 of Volume II). The system can detect both linear and rotational accelerations of the head in space. The otoliths sense all of the linear accelerations acting on the head, including the constant acceleration due to gravity. As such, the otoliths are sensitive to changes in orientation (e.g. tilt) with respect to gravity and provide an important source of information for the perception of verticality. In contrast, the semicircular canals sense angular head accelerations. Pitch (sagittal) and roll (frontal) movements of the head are detected by the anterior and posterior canals, whereas yaw movements are detected by the horizontal canals. Since pitch and roll movements move the body CoM toward its limits of stability, the anterior and posterior semicircular canals are particularly important for control of postural sway. The semicircular canals are most sensitive to high-frequency head movements, such as during locomotion, in contrast to the low-frequency sensitive otoliths that are suited to control static postural alignment (Nashner, 1972; Nashner et al., 1989; Mittelstaedt, 1999).

Both the visual and the vestibular systems are located in the head, which moves independently from the body. Thus, these two systems do not provide direct information about body orientation in space. This important information comes from somatosen-sors, such as muscle spindles, Golgi tendon organs, cutaneous mechanoreceptors, pressure receptors, and joint receptors, that are distributed throughout the body. In fact, without important somatosensory information from the neck and trunk, visual and vestibular information alone cannot help the nervous system distinguish between: (1) head movements on a stable body and head movements accompanied by movement of the body CoM that may result in postural instability and (2) forward head movements when the body CoM moves backward via hip flexion and forward head movements when the body CoM moves forward via ankle dorsiflexion. In addition to relaying information about body configuration, the somatosensory system also provides information about the support surface and the forces exerted by the body against those surfaces. For example, cutaneous and deep mechanoreceptors on the soles of the feet are activated in relation to forces under the feet.

It has been clear for some time that the visual, vestibular, and somatosensory systems all contribute to balance control, as evidenced by the fact that stimulation of each of the three systems can induce body sway (Coats and Stoltz, 1969; Lestienne et al., 1977; Hlavacka and Nijiokiktjien, 1985). But information from each of these sources alone would not be sufficient to allow selection of an appropriate postural response. For example, if an image moves across the retina this could be interpreted as body movement with respect to the object being viewed or as movement of the object relative to a stationary body. Without vestibular and/or proprioceptive information to use in addition to the visual input, the retinal slip signal cannot be successfully interpreted.

The processing and integration of information from the three systems, obviously an essential component of successful balance, relies on sensory integration and sensory channel reweighting (Peterka,

2002). Sensory channel reweighting refers to the ability of the nervous system to modify the relative contributions of the three sensory systems to balance control based on the context in which balance is to be maintained. For example, movement of a visual surround has a stronger influence when the support surface is also moving, which provides evidence of an increased weighting of visual information in conditions where the support surface is unstable or compliant (Soechting and Berthoz, 1979; Peterka and Benolken, 1992). Recent studies have established that a healthy subject standing on a stable surface in a well-lit room relies primarily upon somatosensory information for postural orientation (Peterka and Benolken, 1992). However, that same person will gradually increase dependence on vestibular information (and vision, if available) in proportion to the amplitude of random surface orientations (Peterka, 2002). Light haptic touch from a single finger on a stable support can also provide a powerful sensory reference, even more powerful than the effects of vision (Jeka and Lackner, 1994). This may explain why the use of a cane as a sensory substitution device, rather than as a mechanical support, is so effective in improving balance and reducing falls in patients with sensory deficits (Jeka, 1997; Dickstein et al., 2001; Horak et al., 2002). Thus, sensory reweighting occurs not only in response to the context in which a task in being performed, but also in response to injury to one of the sensory systems. When one of the sensory systems is damaged, the nervous system can increase the weight placed on the intact sensory systems to compensate for the damage (Nashner et al., 1982; Marchand and Amblard, 1984; Horak and Hlavacka, 2001). Studies have shown that patients with loss of somatosensory or vestibular information can increase the sensitivity of their other senses for balance control and particularly benefit from substituting light touch for their missing sensory information for postural stability.

Balance strategies

Postural movement strategies take advantage of body biomechanics to allow for effective and efficient control of body CoM. Although postural strategies are rather stereotyped, they are constantly being shaped, or adapted, based on prior experience and knowledge of results. Anticipatory postural adjustments are those that occur prior to a voluntary movement. For example, prior to pulling on a handle to open a door, calf muscles are activated to stabilize the body and prevent a forward fall during the pull (Cordo and Nashner, 1982). These anticipatory postural adjustments are specific to each particular voluntary movement pattern and do not consist of simple stiffening of the body (Horak and Anderson, 1980). Anticipatory postural adjustments are dependent on predictive, feedforward control, as the postural muscles are activated prior to, or at the same time as, the muscles that are the prime movers for the voluntary movement (Crenna et al., 1987; Oddsson and Thorstensson, 1987). Thus, sensory feedback is not available to trigger anticipatory postural adjustments. However, sensory feedback control is important for postural responses to unexpected external perturbations. Patients with injuries affecting the cerebellum or parts of frontal cortex may have poor control of anticipatory postural adjustments.

Responses to external perturbations, which can be elicited by movement of the support surface or of the body, consist of a continuum of strategies that may or may not involve changes in the BoS (Horak et al., 1997). Fixed support strategies are those where the BoS is not altered (Fig. 8.4). One fixed support strategy, the ankle strategy, is generated in response to small perturbations experienced when standing

Ankle

Stepping

Ankle

Stepping

Figure 8.4. Illustration of ankle, hip, and stepping strategies used in response to backward (a-c) and lateral (d-f) perturbations (reprinted with permission from Horak and Shumway-Cook, 1990).

on a firm, wide surface (Horak and Nashner, 1986). Use of the ankle strategy in response to forward or backward perturbations is characterized by distal-to-proximal activation of muscles on the side of the body opposite the direction of sway with torque generated primarily around the ankles. Use of a similar strategy in response to lateral perturbations involves hip abduction/adduction and loading/ unloading of the limbs, allowing upright orientation of the body during motion of the body CoM (Henry et al., 1998). Another fixed support strategy, the hip strategy, is generated in response to larger perturbations or perturbations experienced on support surfaces that do not allow use of ankle torque. The hip strategy in response to forward or backward perturbations is characterized by proximal-to-distal activation of muscles, resulting in hip torque that rotates the trunk opposite the legs to more rapidly move the body CoM (Runge et al., 1999). Use of a similar strategy that involves lateral flexion of the trunk in response to lateral perturbations involves activation of trunk paraspinal muscles (Henry et al., 1998). The ankle and hip strategies both have muscle activation latencies of 75-100 ms and represent the two extremes of the continuum of fixed support strategies. Different combinations of the ankle and hip strategies can be used depending on the characteristics of the perturbation, expectations based on prior experience, and the environmental context in which the perturbation is experienced.

Change in support strategies are those that involve an alteration in the BoS (Maki and McIlroy, 1997). Stepping is one example of a change in support strategy that is used for very large or fast perturbations. A second change in support strategy is to grasp a nearby object, increasing the size of the BoS. These strategies are more complex than the hip and ankle strategies in that they involve anticipatory unloading of the stepping leg and longer latencies than the ankle and hip strategies, but they are not simply used as a last resort when fixed support strategies are unsuccessful (Maki and McIlroy, 1997). For example, elderly subjects who are prone to falls or who fear falling are more likely to use a stepping or arm reaching strategy than a fixed support strategy.

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