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Upright Posture: how limb biomechanics limits the ability to stand
Arthur Kuo, PhD; Christopher Runge, MS
Objective - The ability to maintain balance while standing upright is compromised in some individuals, such as vestibular-deficient patients (those with inner ear problems), the elderly, and those suffering from nervous system disorders. Among the elderly alone, one-third to one-half of the population aged 65 years and older are at some point harmed by falling. Many of these falls result in serious injury, and for cases in which a hip is broken, the mortality rate is nearly one-half. The goal of this project is to develop diagnostic and rehabilitation tools based on understanding how the central nervous system (CNS) interacts with the musculoskeletal system to control standing.
Approach - We would like to understand the roles played by the CNS and by the biomechanics of the body during posture control. The study is complicated by the fact that the most reliable method for measuring commands sent by the CNS to the muscles is electromyography, which is limited in clinical practice in how reliable it can record from specific muscles. Most motor control studies therefore must rely heavily on external measurements of the actual movements, using cameras, force-sensing platforms, and other sensors. Because these devices provide only a subset of the information needed to completely characterize a movement, some of our efforts have focused on ways to best use this measurement data to represent the behavior. Only with a good representation of the movement can we proceed to study thoroughly how the movement is controlled by the CNS.
Fundamental to the study of balance phenomena is the difficulty in differentiating between events due to the body biomechanics and those due to intentional CNS control. The biomechanics of the body constrain what types of movement can occur. We hypothesize that biomechanical modeling can be used to gain insight into central nervous control since the CNS cannot do more than what the biomechanics will allow.
An engineering model of the human body, including important properties and parameters of the skeleton and muscles, computes inputs for the muscles and simulates the resulting movement (Figure 1). Control systems analysis techniques operate on this model to characterize the constraints on possible motions. These biomechanical constraints reduce the possible ways in which the CNS can maintain balance.
Figure 1. Computer simulations help us understand how the central
nervous system controls posture.
Conclusions - The computer model has shown that control of balance is subject to a large number of biomechanical constraints on movement. The CNS must operate within these constraints to successfully maintain posture. Experiments have shown that changes in CNS control strategies when recovering from a balance disturbance are often due to the effects of these constraints. Understanding how the CNS maintains body balance by controlling the musculoskeletal system, whose properties constrain what type of control will be effective, is critical to the continued development of rehabilitation of persons encountering balance deficits.
Republished from the 1994 Rehabilitation R&D Center Progress Report. For
current information about this project, contact
Christopher Runge.
