Lewis M. Nashner

The organization of human postural movements: A formal basis and experimental synthesis

A scheme for understanding the organization of human postural movements is developed in the format of a position paper. The structural characteristics of the body and the geometry of muscular actions are incorporated into a three-dimensional graphical representation of human movement mechanics in the sagittal plane. A series of neural organizational hypotheses limit a theoretically infinite number of combinations of muscle contractions and associated movement trajectories for performing postural corrections: (1) Controls are organized to use the minimum number of muscles; (2) frequently performed movements are organized to require a minimum of neural decision-making. These hypotheses lead to the prediction that postural movements are composed of muscle contractile strategies derived from a limited set of distinct contractile patterns. The imposition of two mechanical constraints related to the configuration of support and to requirements for body stability with respect to gravity predict the conditions under which individual movement strategies will be deployed. A complementary organizational scheme for the senses is developed. We show that organization of postural movements into combinations of distinct strategies simplifies the interpretation of sensory inputs. The fine-tuning of movement strategies can be accomplished by breaking down the complex array of feedback information into a series of scalar quantities related to the parameters of the movement strategies. For example, the magnitude, aim, and curvature of the movement trajectory generated by an individual strategy can be adjusted independently. The second half of the report compares theoretical predictions with a series of actual experimental observations on normal subjects and patients with known sensory and motor disorders. Actual postural movements conform to theoretical predictions about the composition of individual movement strategies and the conditions under which each strategy is used. Observations on patients suggest how breakdowns in individual steps within the logical process of organization can lead to specific movement abnormalities. Discussion focuses on the areas needing further experimentation and on the implications of the proposed organizational scheme. We conclude that although our organizational scheme is not new in demonstrating the need for simplifying the neural control of movement, it is perhaps original in imposing discrete logical control upon a continuous mechanical system. The attraction of the scheme is that it provides a framework compatible with both mechanical and physiological information and amenable to experimental testing.

Aging and Posture Control: Changes in Sensory Organization and Muscular Coordination

The following study examined two aspects of balance control in the older adult: 1) the coordination of the timing and the amplitude of muscle responses to postural perturbations, and 2) the ability of the participant to reorganize sensory inputs and subsequently modify postural responses as a consequence of changing environmental conditions. Coordination of muscle activity in postural responses of twelve elderly (sixty-one to seventy-eight years) participants were compared to those of young (nineteen to thirty-eight years) adults using a movable platform and recording the electromyographic activity of muscles of the legs. The following changes were noted in the timing and amplitude of muscle activity within a postural response synergy: 1) increases in the absolute latency of distal muscle responses were observed in all older adults; 2) in five of the twelve older adults temporal reversals of proximal and distal muscle response onset were observed; and 3) there was a breakdown in the correlation of the amplitude of responses within a synergy. The ability of the older adult to balance under conditions of reduced or conflicting sensory information was also impaired. When confronted with functionally inappropriate visual and/or somatosensory inputs, half of the older group lost balance. In most instances, however, the older participants were able to maintain stability during subsequent responses to conflicting stimuli.

Influence of head position and proprioceptive cues on short latency postural reflexes evoked by galvanic stimulation of the human labyrinth.

Abstract This study described the effects of low level (75–250 μA) galvanic stimulation of the labyrinths of the human posture control system. Experiments demonstrated a transient EMG response in gastrocnemius-soleus (GS) muscle with a latency of 100 msec after initiation of galvanic current pulses of 75–250 μA and 50–150 msec duration. These transient EMG responses were modulated by position of the head. With the head turned 90° to one side so that the positive electrode faced posterior resulted in excitation of GS muscles. Turning the head 90° to the other side so that the positive electrode faced anterior resulted in inhibition of GS muscles. The transient EMG response was absent if the head were in its normal forward facing position. A 2 df platform enabled the ankle joints of subjects to be maintained at a fixed angle during sway; thus making vestibular cues critical during control of postural sway. With vestibular cues critical, the transient EMG response caused marked swaying and compensatory responses correlated with galvanic stimulus. Dynamics of these postural responses were tested using both currents of 150 msec duration (to gain an approximate measure of the impulse response of the system) and of 5 sec duration (to measure the step response of the system). A simplified third order model of the results showed that dynamic characteristics of body sway to galvanic stimulation resembled those induced by motion disturbances. The initial postural response to galvanic stimulation was uneffected by the availability of cues from the feet and lower legs, although later components were strongly effected.

Organization of posture controls : an analysis of sensory and mechanical constraints

We analyse two components of posture control in standing human subjects: (1) the mechanical properties which constrain the bodys ability to execute stabilizing postural movements and (2) the mechanical and neural properties which constrain the ability of the vestibular system to sense changes in body orientation. Rules are then proposed to describe the central organization of posture controls within the sensory and mechanical constraints. The organizational rules and knowledge of constraints are combined to predict the effects of selective semicircular canal and utricular otolith lesions on postural stability and the patterns of body and head movements used to maintain balance. Our analysis leads to the prediction that semicircular canal and otolith deficits destabilize patients at different frequencies, and force them to use different patterns of body and head movements. These predictions are compared to posture controls observed in patients with different types of vestibular deficits. The additional steps required to prove or disprove the theory are discussed.

Analysis of Stance Posture in Humans

The generation of a purposeful movement requires that the sensorimotor system regulate and control three kinds of activities: (1) the basic patterns of the movement must be generated in each limb and the patterns of activities among the limbs must be coordinated, (2) the muscular activities generating the basic movement pattern must be adaptively modified to suit the external loading and sensory conditions of the particular task, and (3) the movement patterns must be adjusted to maintain the equilibrium of the body as a whole. The control of upright stance by normal human subjects is one specific example of such a sensorimotor process. Understanding upright posture is made difficult because most of what is known about sensorimotor control has been learned from studies of acute and chronically prepared experimental animals. Apart from the immediate interest in understanding more about ourselves, one might be initially inclined to question how a study in which the experimental paradigms and measurement techniques are severely limited can contribute significantly to the detailed physiological information about the generation and coordination of movements. One aim of this chapter is to show that studies of the performance of awake human subjects are answering important questions about the adaptive and equilibrium controls of the sensorimotor system that have not been addressed in experiments using animal preparations.

Effects of unilateral loss of vestibular function on the vestibulo-ocular reflex and postural control

Long-term recovery from surgically induced unilateral loss of vestibular function was studied in 14 patients. Seven patients underwent surgical extirpation or section of the vestibular nerve, and seven patients underwent labyrinthectomy without vestibular nerve section. The vestibulo-ocular reflex (VOR) and postural control were evaluated preoperatively and monitored for up to 4 years postoperatively with use of pseudorandom rotation (combined sinusoidal frequencies from 0.009 to 1.5 Hz) and moving platform posturography. Immediately following surgery all patients showed minimal reductions in the VOR gain constant, but marked reduction in the time constant, and marked increase in slow eye velocity bias. Bias returned to normal values within about 10 days, but time constants never returned to normal values. Results of standard Romberg tests in these patients were normal throughout the preoperative and postoperative periods. However, all patients showed marked postural control abnormalities in tests of the ability to maintain balance in unusual sensory environments in the immediate postoperative period. Seventy-five percent of the patients eventually recovered normal postural control. Postural control returned to near baseline performance with a time course similar to that of the VOR bias. However, postural control also continued to improve after the recovery of VOR bias was complete.

Abnormal postural control associated with peripheral vestibular disorders

The development of a systematic approach to the diagnosis and management of ataxias of vestibular origin depends critically on the elucidation of the complex sensory and motor interactions involved in human postural control. In this paper, the results of studies of both sensory and motor control of posture in adults and children with peripheral vestibular deficits are summarized and reviewed. In studies of the sensory organization of postural control, normal subjects and patients with peripheral vestibular deficits were exposed to unreliable information from their support surface and/or visual surround during quiet stance. While normal adults and children were able to maintain balance under these conditions, the majority of children and adults with peripheral vestibular deficits showed one or both of the following abnormalities: (1)Vestibular loss patients were unable to maintain equilibrium when forced to rely on vestibular information for postural control. (2)Vestibular distortion patients were unable to select an accurate source of sensory information when exposed to sensory conflicts during quiet stance. Preliminary results of studies of motor coordination in these patients also suggest that vestibular loss patients rely almost exclusively on ankle sway to control posture, even during balance tasks which require hip movements to maintain equilibrium. In contrast, some vestibular distortion patients appear to rely on hip motions, even when not required to do so to maintain balance. The results of these studies are discussed in terms of the implications for both sensory and motor aspects of postural control in patients with ataxias of vestibular origin.

Vestibulo-spinal Control Differs in Patients with Reduced Versus Distorted Vestibular Function

Abnormal vestibular function disrupts a subjects reference to gravity (earth) vertical, and prevents resolution of conflicting or inaccurate visual and somatosensory spatial references. However, errors which patients make when attempting to resolve conflicting visual and somatosensory orientation inputs during upright stance differed markedly in patients with (1) symmetric or asymmetric reduced vestibular function, (2) benign paroxysmal positional nystagmus and vertigo, and (3) a combination of distorted and reduced function. Objective characterization of spatial orientation systems and compensatory strategies under altered sensory conditions is an essential first step toward identifying optimal treatment methods for each of these three types of vestibular deficient patients.

Organization and programming of motor activity during posture control.

Publisher Summary This chapter aims to synthesize a hierarchical model of posture control from experimental observations of stance posture control of normal human subjects. A conceptual synthesis of human experimental observations enhances knowledge about sensorimotor controls derived from the study of animal preparations in several important ways. Studies of chronic and acute cat preparations have revealed much detail about the neural mechanisms within the spinal cord that generate the basic locomotor movement patterns. However, the interruption of many central nervous system functions in these preparations has thus far prevented any study of the adaptive and postural balance controls, which under normal conditions are integrated into the ongoing movement behaviors. The postural adjustments are functionally useful responses elicited by the unexpected movements of a platform upon which a subject stood. Results show that these adjustments are too complexly interwoven into an organizational structure involving many leg muscles to be characterized simply as the response of each individual muscle to its own stretch input. In place of the concept of stretch responses, the hierarchical model suggests that the response activity of each muscle is stereotypically organized into a structure controlled by the pattern of movement inputs from the entire leg.

Head-trunk movement coordination in the standing posture.

The coordination of head and trunk movement during postural sway in the anterior/posterior plane was examined in three normal adults. Postural sway about the ankles or hips was elicited in two ways: (1) In free-fall sway trials, the subject passively fell forward while the feet remained in place on the support surface (ankle sway). (2) In perturbed sway trials, subjects stood on either a flat surface (ankle sway) or a narrow beam (hip sway) which was displaced backwards at the onset of each trial. In all cases, postural responses were initiated before significant horizontal head motion was recorded. For subjects swaying about the ankles, changes in neck angle followed changes in ankle angle as the effect of the postural movement was propagated up the body. Neck muscle activation for ankle sway thus appeared to be elicited by neck stretch resulting from the postural correction. These results suggest that head and body motions may be controlled independently during active postural movements for ankle sway. For subjects swaying about the hip, however, changes in neck and hip angles were coordinated to approximately stabilize the rotational position of the head, and neck and hip muscles were activated simultaneously. These results, in contrast to those for ankle sway, suggest that control of head and body motion is coordinated on a feedforward basis during hip sway.