Figure 1. Ankle (L) and hip (R) strategies are used to maintain balance when posture is disturbed by rearward support surface translation.
1996 Project Reports | Home | Contents | Previous |
Christopher F. Runge, MS; Charlotte L. Shupert, PhD; Fay B. Horak, PhD; Felix E. Zajac, PhD
Objective - Although people spend countless, seemingly effortless, hours standing during their lifetime, the task of maintaining balance is actually quite complex. To control balance, the central nervous system (CNS) must assess the state of the body and its environment to judge when changes in movement are necessary to maintain standing. The CNS integrates information from various sensory systems--visual, vestibular (inner ear), and somatosensory--and sends motor commands to muscles to generate force in the proper directions. Individuals with sensory problems experience great difficulty standing under certain conditions and thus are faced with a much greater danger of falling, which can result in serious injury.
Patients with vestibular deficits, which can result from head trauma, viral infections, and certain medical treatments, comprise one group of individuals who have great difficulty maintaining balance (e.g., standing with eyes closed, standing on one foot, or withstanding postural disturbances). The goal of this work is to identify the role the vestibular system plays in maintaining balance, so that rehabilitation for those with vestibular dysfunction can be developed.
Approach - The first step in studying the role of sensory systems in the control of balance is to characterize the responses in subjects with no apparent sensory deficits. Coordination strategies, the building blocks of automatic postural responses, are identified by analyzing their responses to controlled balance perturbations. Various devices (e.g., video cameras, force-sensing platforms, electrodes, and motion sensors) quantify the movement and the muscle activity, helping to characterize these strategies. However, these devices and measurement techniques provide only a subset of the information needed to completely characterize the response. Consequently, some of our efforts are spent designing analytical tools to estimate the unmeasurable quantities, such as joint torques produced by muscles.
Once the coordination strategies are identified, it is necessary to determine what sensory signals trigger the different strategies. Much effort goes into analyzing the early, passive movements of the body, to deduce the responsible sensory signals. If, in a particular controlled situation, the early, passive movements of the body (before the stabilizing response begins) are the same for subjects both with and without vestibular deficits, but aspects of the automatic responses are consistently different, we assert that information from the vestibular system is integral to controlling balance in that situation.
Progress - Subjects with no apparent sensory deficits appear to use two basic coordination strategies to maintain balance when the support surface is perturbed: the ankle strategy and the hip strategy. The ankle strategy controls the body as if it were a single-segment inverted pendulum, and the hip strategy controls the body as if it were a two-segment inverted pendulum (Fig. 1). Vestibular deficit subjects also use the ankle strategy successfully to maintain balance in conditions where it is appropriate, but they appear unable to use the hip strategy successfully. In conditions where subjects with no sensory deficits use the hip strategy, we hypothesize that the CNS uses the vestibular system to somehow control this coordination strategy. Individuals without this sense are thus unable to respond as subjects with this sense, and thus their danger of falling is increased. Our future efforts are directed toward identifying the nature of the balance deficit in situations requiring the hip strategy, and identifying rehabilitation strategies to aid individuals with vestibular deficits.
| Figure 1. Ankle (L) and hip (R) strategies are used to maintain balance when posture is disturbed by rearward support surface translation. |
Republished from the 1996 Rehabilitation R&D Center Progress Report. For current information about this project, contact: Christopher F. Runge.
