Postural control and balance are perfect examples of perception and action as an ecosystem. To control our posture in order to sit, stand, or assume any desired position, we must continually change our motor response patterns according to the perceptual information that specifies the environment and our bodies’ orientation in it. Several perceptual systems are involved in maintaining posture and balance. Vision tells us how our bodies are positioned relative to the environment. Kinesthetic input from our bodies’ proprioceptors tells us how our limbs and body parts are positioned relative to each other. Kinesthetic input from the vestibular system provides information about our head position and movement. Even the auditory system can contribute information about balance (Horak & MacPherson, 1995).
We must maintain posture and balance in an almost infinite number of situations. Sometimes we balance when stationary (static balance) and sometimes when moving (dynamic balance). We must also balance on a variety of body parts, not just two feet. Think of all the body parts on which gymnasts must balance in their various events. Sometimes we need to balance on surfaces other than the ground, such as a ladder. We might even have to balance without all the information we would like - for example, when we have to walk in the dark.
Given the number of perceptual systems involved in balance and the wide range of environmental and task constraints that are possible for any given balance task, the triangular model of constraints provides a good perspective on the development of balance. A developmental trend for a certain set of task and environmental constraints might differ from the trend for another set of constraints. In fact, movement scientists recognized some time ago that performance levels on various types of balancing tasks are specific to that task (Drowatzky & Zuccato, 1967). We discuss postural control and balance in infants in chapter 5. Now let’s consider the development of balance in childhood through older adulthood.
The timing of developmental trends in balance is related to the type of balance task under consideration.
Balance in Childhood
Balance performance improves on a variety of balance tasks from 3 to 19 years of age (Bachman, 1961; DeOreo & Wade, 1971; Espenschade, 1947; Espenschade, Dable, & Schoendube, 1953; Seils, 1951; Winterhalter, 1974). The exact nature of the improvement trend depends on the task. For example, on some tasks we might see a plateau in performance for several years. This could reflect the way we measure improvement on that particular task; perhaps the child is improving in a way not detected by our measurement. It is also possible that children begin to rely more on kinesthetic information and somewhat less on visual information for balance.
Children 4 to 6 years of age have been observed to regress on moving platform tests, and children 3 to 6 years have shown both adultlike and nonadultlike postural responses to a moving room (Schmuckler, 1997). They take longer to respond than younger children and vary greatly in the way they respond (i.e., in how the various muscles are activated to regain balance). This finding does not seem to be accounted for by changes accompanying physical growth (e.g., changes in limb and trunk proportion and mass), which leads to the suspicion that shifts in reliance on different perceptual systems are perhaps involved (Woollacott, Debu, & Mowatt, 1987). By the time children reach the 7- to 10-year-old range, however, they show adultlike postural responses (Nougier, Bard, Fleury, & Teasdale, 1998; Shumway-Cook & Woollacott, 1985; Woollacott, Shumway-Cook, & Williams, 1989).
As children grow, they rely more on kinesthetic information for balance and less on visual information.
Balancing during locomotion is a challenging task. When we walk or run, for example, we must maintain our stability yet propel the body forward in order to travel. To do so, we probably use two frames of reference. One is the supporting surface, and the other is gravity. Another challenge is to control the degrees of freedom of movement at the various body joints. On one hand, individuals might stabilize the head on the trunk in order to minimize the movement they must control. On the other hand, they might stabilize head position in space and use the orientation of the head and trunk to control their equilibrium.
Assaiante and Amblard (1995; Assaiante, 1998) proposed a model to explain the development of balance in locomotion over the life span. The model describes four important periods. The first covers birth to the onset of standing and is characterized by a cephalocaudal direction of muscle control. The second includes the achievement of upright stance to about 6 years of age; during this time, coordination of the lower and upper body must be mastered. The third period, from about age 7 to sometime in adolescence, is characterized by the refinement of head stabilization in balance control. The fourth and last period, which begins in adolescence and extends through adulthood, is characterized by refined control of the degrees of freedom of movement in the neck. Thus, the task of childhood is to learn how the different frames of reference complement one another during movement. This is an intriguing model that may well stimulate future research on the development of dynamic balance.
Think of your own experience in visiting an amusement park attraction or "haunted house" that used visual displays to confuse you. How did the visual display conflict with your other senses? Which of your systems were put into conflict? How did you maintain your balance (if you did)?
Balance Changes With Aging
In adulthood, individuals standing on a force platform (see the "Assessing Balance" sidebar) show a minimal amount of sway. If the platform is moved repetitively back and forth, adults use visual information to stabilize the head and upper body, and the muscle response to movement occurs in the ankles (Buchanan & Horak, 1999). When adults stand on such a platform and it is moved slightly or slowly but unexpectedly, they use an ankle strategy to regain balance. That is, they use lower leg muscles that cross the ankle joint to bring themselves upright once again. When the movement is larger or faster, a hip strategy is used; muscles crossing the hip and knee joints bring the center of gravity back over the base of support (Horak, Nashner, & Diener, 1990; Kuo & Zajac, 1993).
Older adults experience a decline in the ability to balance. Those over 60 sway more than younger adults when standing upright, especially if they are in a leaning position (Hasselkus & Shambes, 1975; Hellebrandt & Braun, 1939; Perrin, Jeandel, Perrin, & Bene, 1997; Sheldon, 1963). Age-related changes in balance are also seen with older adults on movable platform tests. In comparison with young adults, slightly more time passes before an older adult’s leg muscles respond after a perturbation in order to maintain balance, and sometimes the upper leg muscles respond first instead of the lower leg muscles, a pattern opposite that found in young adults. The strength of the muscles’ response is more variable among repetitions in older adults (Perrin et al., 1997; Woollacott, Shumway-Cook, & Nashner, 1982, 1986).
Age-related changes in balance ability could be related to a variety of changes in the body’s systems, especially in the nervous system. As mentioned previously, some older adults experience changes in the kinesthetic receptors, and these changes might be more extreme in the lower limbs than in the upper ones. Older adults might also be placed at a disadvantage due to vision changes as well as changes that occur in the vestibular receptors and nerves in adults over 75 (Bergstrom, 1973; Johnsson & Hawkins, 1972; Rosenhall & Rubin, 1975). A decrease in fast-twitch muscle fibers or a loss of strength could hamper an older adult’s quick response to changes in stability, as might arthritic conditions in the joints.
Perrin et al. (1997) recorded electromyograph activity in older adults during a backward tilt of a movable force platform. They observed some of the reflexes in the lower legs that were not involved in balance control, as well as the responses necessary for regaining balance. By comparing the time from the balance perturbation with the onset of each of these muscle responses in young and older adults, the investigators determined that nerve conduction speed in both the peripheral and central nervous systems was slower in the older adults. Thus, the declines in balance performance with aging most likely are associated with age-related changes in a variety of systems.
The difficulties that older adults experience with balance most likely reflect changes in more than one system.
Falls are a significant concern in older adults. In fact, falls are the leading cause of accidental death for people over 75 years old. A common result of falling, especially among older adults with osteoporosis, is fracture of the spine, hip (pelvis or femur), or wrist. Complications of such a fracture can result in death. Even when older adults recover, they experience heavy health care costs, a period of inactivity, and dependence on others. A fear of falling again can make them change their lifestyles or be overly cautious in subsequent activities.
Woollacott (1986) studied the reaction of older adults when a movable platform tipped forward or backward to perturb their balance unexpectedly. Half the older adults she observed lost their balance the first time, but these adults learned to keep their balance after a few more tries. Thus, older adults are more liable to fall on a slippery surface than young adults but are capable of improving their stability with practice. Campbell et al. (1997) and Campbell, Robertson, Gardner, Norton, and Buchner (1999) compared the number of falls over a 1-year period in women over 80 years of age who participated in an individualized exercise program stressing strength and balance with the number of falls in women over 80 who did not participate in an exercise program. The number of falls in the exercise group (88) was significantly lower than the number of falls in the other group (152). Prevention and rehabilitation programs, then, are useful in reducing the risk of falls in older adults, but they must be ongoing. In chapter 15, we discuss the role of aerobic exercise in maintaining the speed of cognitive processes.
Web Study Guide
Test several people on balance tasks and rate the balance tasks in Lab Activity 11.2, Development of Balance, in the web study guide. Go to www.HumanKinetics.com/LifeSpanMotorDevelopment.
Exercise programs focused on improving strength and balance can reduce the risk of falls in older adults.
Imagine you have a grandparent who comes to live with you. What types of surfaces and conditions around your house could be more likely to lead to falls for him or her than for a young adult? What steps could you take to reduce the chances of a fall?
Balance can be assessed in many ways, both in field settings and in laboratories. Different assessments are used for static balance and dynamic balance. A device used in many laboratory settings is a force plate or force platform. A simple force plate consists of two square plates positioned one over the other with four pressure gauges in between the plates and at the four corners of the plates. The device is placed on the floor, or even set into the flooring to be level with the surface, so that individuals can stand on, walk across, or even jump on or off the plate.
The most basic force plate simply measures vertical force applied in the geometric center of the top plate. More complicated force plates can measure force at, and the location of, a center of pressure. Think about an individual standing on a force plate on one leg. As he or she sways and the body varies from perfect vertical, the center of pressure exerted on the force plate moves. The force plate can detect the location of the center of pressure, how far it moves, and when. The most advanced force plates can break the vector of the force exerted on the force plate into three spatial components. So, force plates can measure changing pressures under the feet as someone stands on or moves across the platform.
When researchers are interested in static balance, they can ask individuals to stand on a force plate and measure both how far the individual sways and the velocity of sway. Individuals can stand on one foot or two feet and in any kind of stance, such as side stride or split stride. Researchers also can ask individuals to lean as far as possible without losing balance to quantify the ability to control the body or the maximum limits of stability.
Computerized dynamic posturography assessments incorporate a force plate. These devices are used to study the reaction of individuals to being slightly thrown off balance by virtue of the force plate tilting. By controlling the surrounding visual field with a three-sided enclosure around the participant, researchers can present conditions with normal vision or no vision and with a surround that is stable or that moves as the individual sways. Researchers can also control whether the force plate moves, and in what direction, by rotating or translating. This allows them to study the visual, vestibular, and somatosensory systems and their interactions in balance, including when balance is perturbed and the person must react to regain or maintain balance. Electromyographs can be used in conjunction with systems such as this to record how the muscles are activated to regain balance.