Research and Clinical Applications of Assessing Balance

Returning athletes to competition following injury often creates a dilemma for athletic trainers and team physicians. Most clinicians gather as much data as possible before deciding whether to return an athlete to competition following injury. The status of the postural control system and balance is important for certain pathologies and therefore should be considered in these clinical decisions. As more high-tech balance systems become available, it is important for clinicians to understand not only what is available but what these devices measure. This paper will review the relationship between the postural control system and the kinetic chain, traditional and contemporary techniques for assessing balance, and ways in which clinicians can bridge the gap between balance research and clinical practice. Article: Although a seemingly simple task, maintaining equilibrium or balance while standing upright is an important motor skill. Balance, as described by Nashner (41), is a complex process involving coordination of multiple sensory, motor, and biomechanical components. An individual senses the position of his or her body in relation to gravity and the surroundings by combining visual, vestibular, and somatosensory inputs (41). Balance movements also involve motions of the ankle, knee, and hip joints, which are controlled by coordinated actions along the kinetic chain (Figure 1). These processes are all vital for producing fluid sport-related movements. Postural Control System The postural control system operates as a feedback control circuit between the brain and the musculoskeletal system. The musculature of the legs, feet, and truncus, using this feedback circuit, allows the individual to stand erect against the forces of gravity (22, 31, 38, 55). Postural stability is also greatly influenced by factors such as nervous disorders, dysfunction of optic nerve and vestibular mechanisms, fatigue, and mental status (51). * Kevin M. Guskiewicz is with the Department of Physical Education, Exercise and Sport Science, The University of North Carolina at Chapel Hill, Chapel Hill, NC 275998700. David H. Perrin is with Athletic Training Education and Research, Curry School of Education, University of Virginia, Charlottesville, VA 22903. Feedback obtained from the vestibular, visual, and proprioceptive (somatosensory) sensors relays commands to the muscles of the extremities, which then generate an appropriate contraction to maintain postural stability (26, 31, 38, 50, 51). The vestibular apparatus is the organ responsible for detecting sensations concerned with equilibrium. The apparatus is composed of a system of bony tubes and chambers within the temporal bone called the bony labyrinth. The labyrinth is adjacent to and continuous with the cochlear duct of the inner ear and also consists of three semicircular canals and two large chambers known as the utricle and the saccule (22, 55). Information from the vestibular apparatus can be used in three different ways. First, the information is used to control eye muscles so that when the head changes position, the eyes can stay fixed on one point. Second, vestibular information can be used to maintain upright posture. The vestibular organs are often referred to as the "sense organs of balance," despite research findings to the contrary. That is, very few postural reflexes rely primarily on vestibular input (28, 38, 43, 52). A third use of vestibular information involves conscious awareness of the body's position and acceleration after Information has been relayed to the cortex by the thalamus (55). Investigators (8, 28, 38, 43) agree that the vestibular system is primarily involved in stabilizing slow body sway, which is achieved by a much lower level of leg activation. Much of this research suggests that people rely mostly on visual and somatosensory inputs when maintaining balance under normal conditions. When sudden changes or perturbations are induced causing a person to change his or her direction of movement or head position (i.e., leaning the head sideways, forward, or backward), the automatic control mechanism provided by vestibular input becomes crucial for stabilizing the direction of gaze and ultimately one's equilibrium. Vision is obviously very important for maintaining control of balance, especially under conditions of postural perturbation. Moreover, the eyes would be of little use in detecting an image if they did not remain fixed on an object long enough to gain a clear image. Therefore, when the head is suddenly tilted, signals from the semicircular canals cause the eyes to rotate in an equal and opposite direction to the rotation of the head (22). This is a function of the vestibulo-ocular reflex. Thus, when both the support surface and visual surroundings are tilted, the vestibular input automatically takes precedence (38). In short, the vestibular apparatus mainly contributes to posture by maintaining reflexes associated with keeping the head and neck in the vertical position and allowing the vestibulo-ocular reflex to control eye movement. The proprioceptive system functions via the mechanoreceptive senses of touch, pressure, vibration, and tickle, all of which are commonly referred to as the tactile senses, and the sense of position, which determines the relative positions and rates of movement of parts of the body (22, 55). Muscle spindles and Golgi tendon sensory receptors (proprioceptors) play a vital role in the nervous system's control of posture. They provide the nervous system with continuous feedback about the status of each muscle. Muscle length and changes in length are monitored by stretch receptors embedded within the muscle. These receptors consist of afferent nerve fiber endings that are wrapped around modified muscle fibers, several of which are enclosed in a connectivetissue capsule. The entire structure is called a muscle spindle (22, 55). Muscle spindles send information to the nervous system about either the muscle length or its rate of length. When afferent fibers from the muscle spindle enter the central nervous system, they divide into branches that can take several different paths. One path directly stimulates motor neurons going back to the muscle that was stretched, thereby completing a reflex arc known as the stretch reflex or myotatic reflex. This reflex causes a muscle contraction in response to a muscle being