Effect of long-duration spaceflight on postural control during self-generated perturbations.

This report is the first systematic evaluation of the effects of prolonged weightlessness on the bipedal postural control processes during self-generated perturbations produced by voluntary upper limb movements. Spaceflight impacts humans in a variety of ways, one of which is compromised postflight postural control. We examined the neuromuscular activation characteristics and center of pressure (COP) motion associated with arm movement of eight subjects who experienced long-duration spaceflight (3--6 mo) aboard the Mir space station. Surface electromyography, arm acceleration, and COP motion were collected while astronauts performed rapid unilateral shoulder flexions before and after spaceflight. Subjects generally displayed compromised postural control after flight, as evidenced by modified COP peak-to-peak anterior-posterior and mediolateral excursion, and pathlength relative to preflight values. These changes were associated with disrupted neuromuscular activation characteristics, particularly after the completion of arm acceleration (i.e., when subjects were attempting to maintain upright posture in response to self-generated perturbations). These findings suggest that, although the subjects were able to assemble coordination modes that enabled them to generate rapid arm movements, the subtle control necessary to maintain bipedal equilibrium evident in their preflight performance is compromised after long-duration spaceflight.

Perceptual-motor exploration as a function of biomechanical and task constraints.

Four experiments are reported that were designed to examine perceptual-motor exploration employed in determining the solution to a dual-axis positioning task under various biomechanical and task constraints. Experiments 1, 2, and 3 used the two elbow joints to examine the impact of varying several geometric features of the relation between visual information and action in this task. Experiment 4 examined the use of within-limb, between-limb, and within-joint axes of motion in a similar task. The exploratory process was analyzed using a symbolic dynamic defined over nominal categories of visual information and actions elicited by the performer. The search strategy used to improve task performance was consistent across all the experimental manipulations imposed. The frequency pattern of nominal action categories demonstrated a preference for single-axis activity except in the within-joint condition which exhibited a preference for dual-axis activity. The pattern of preferred transitions among these action categories was also consistent across conditions, and lag sequential analysis revealed a robust tendency for cyclical activity in that opposite actions were often coupled in sequence. The topologically equivalent (extrinsic geometry) task spaces led to qualitatively similar search strategies when considered at the level of action-information interaction (intrinsic geometry). The physical implementation of this strategy was strongly influenced by the biomechanical constraints of the action system, while the manipulations of the geometric features of the action-information relation served only to influence the quantitative properties of performance outcome.

The effects of practice on limb kinematics in a throwing task.

The effect of practice on limb kinematics in a dart-throwing task was examined to test three current hypotheses regarding limb control: trajectory formation; end-point control; and coordinated joint-space control. Practice was given to both the relatively well-practiced dominant ant the relatively unpracticed nondominant limbs of 5 male subjects to permit analysis of the early phase of coordination acquisition. The nondominant limb demonstrated high absolute joint cross-correlations with high variability throughout practice and consistency in the hand trajectory. The dominant limb exhibit a significant decrease in wrist-elbow and wrist-shoulder cross-correlations over practice while also maintaining a consistent hand trajectory and significantly higher scoring performance. The findings demonstrate that practice effects can be seen in both coordination mode and variability of various parameters of limb motion, but the changing relationship between the variables suggests that control cannot be ascribed to any one of the three hypotheses advanced. It is proposed that the observed invariance or variance in limb trajectories, end-point control, and coordinated joint angles are a reflection of more global parameters emerging from the flow field properties of the organism, environment, and task interaction.

A review of adaptive change in musculoskeletal impedance during space flight and associated implications for postflight head movement control.

We present a review of converging sources of evidence which suggest that the differences between loading histories experienced in 1-g and weightlessness are sufficient to stimulate adaptation in mechanical impedance of the musculoskeletal system. As a consequence of this adaptive change we argue that we should observe changes in the ability to attenuate force transmission through the musculoskeletal system both during and after space flight. By focusing attention on the relation between human sensorimotor activity and support surfaces, the importance of controlling mechanical energy flow through the musculoskeletal system is demonstrated. The implications of such control are discussed in light of visual-vestibular function in the specific context of head and gaze control during postflight locomotion. Evidence from locomotory biomechanics, visual-vestibular function, ergonomic evaluations of human vibration, and specific investigations of locomotion and head and gaze control after space flight, is considered.

Dynamic visual acuity while walking in normals and labyrinthine-deficient patients.

We describe a new, objective, easily administered test of dynamic visual acuity (DVA) while walking. Ten normal subjects and five patients with histories of severe bilateral vestibular dysfunction participated in this study. Subjects viewed a visual display of numerals of different font sizes presented on a laptop computer while they stood still and while they walked on a motorized treadmill. Treadmill speed was adapted for 4 of 5 patients. Subjects were asked to identify the numerals as they appeared on the computer screen. Test results were reasonably repeatable in normals. The percent correct responses at each font size dropped slightly while walking in normals and dropped significantly more in patients. Patients performed significantly worse than normals while standing still and while walking. This task may be useful for evaluating post-flight astronauts and vestibularly impaired patients.

Modeling and simulation for space medicine operations: preliminary requirements considered.

The NASA Space Medicine program is now developing plans for more extensive use of high-fidelity medical simulation systems. The use of simulation is seen as means to more effectively use the limited time available for astronaut medical training. Training systems should be adaptable for use in a variety of training environments, including classrooms or laboratories, space vehicle mockups, analog environments, and in microgravity. Modeling and simulation can also provide the space medicine development program a mechanism for evaluation of other medical technologies under operationally realistic conditions. Systems and procedures need preflight verification with ground-based testing. Traditionally, component testing has been accomplished, but practical means for "human in the loop" verification of patient care systems have been lacking. Medical modeling and simulation technology offer potential means to accomplish such validation work. Initial considerations in the development of functional requirements and design standards for simulation systems for space medicine are discussed.

The use of in-flight foot pressure as a countermeasure to neuromuscular degradation.

The purpose of this study was to determine whether applying foot pressure to unrestrained subjects during space flight could enhance the neuromuscular activation associated with rapid arm movements. Four men performed unilateral arm raises while wearing--or not wearing--specially designed boots during a 81- or 115-day space flight. Arm acceleration and surface EMG were obtained from selected lower limb and trunk muscles. Pearson r coefficients were used to evaluate similarity in phasic patterns between the two in-flight conditions. In-flight data also were magnitude normalized to the mean voltage value of the muscle activation waveforms obtained during the no-foot-pressure condition to facilitate comparison of activation amplitude between the two in-flight conditions. Foot pressure enhanced neuromuscular activation and somewhat modified the phasic features of the neuromuscular activation during the arm raises.

Adaptation of neuromuscular activation patterns during treadmill walking after long-duration space flight.

The precise neuromuscular control needed for optimal locomotion, particularly around heel strike and toe off, is known to he compromised after short duration (8- to 15-day) space flight. We hypothesized here that longer exposure to weightlessness would result in maladaptive neuromuscular activation during postflight treadmill walking. We also hypothesized that space flight would affect the ability of the sensory-motor control system to generate adaptive neuromuscular activation patterns in response to changes in visual target distance during postflight treadmill walking. Seven crewmembers, who completed 3- to 6-month missions, walked on a motorized treadmill while visually fixating on a target placed 30 cm (NEAR) or 2 m (FAR) from the subject's eyes. Electronic foot switch data and surface electromyography were collected from selected muscles of the right lower limb. Results indicate that the phasic features of neuromuscular activation were moderately affected and the relative amplitude of activity in the tibialis anterior and rectus femoris around toe off changed after space flight. Changes also were evident after space flight in how these muscles adapted to the shift in visual target distance.

Body scale and infant grip configurations.

This study examined whether hand/object size ratios define common boundaries to the grip configuration patterns of infants and adults. A group of 5- to 8-month-old infants and a group of adults engaged in a displacement grasping task with inverted cups that varied in size. The findings showed that infant and adult grip configurations varied systematically with object size: More digits were brought into the contact grip configurations with increasing object size. Furthermore, when object size was scaled to hand size, common dimensionless ratios defined the grasping patterns and transitions between grasping patterns in a similar manner for both adults and infants. Consistent with a dynamical view of the development of coordination, the strong role of body scale on the developmental prehensile coordination pattern was observed for a given set of task constraints.

Neuromuscular activation patterns during treadmill walking after space flight.

Astronauts adopt a variety of neuromuscular control strategies during space flight that are appropriate for locomoting in that unique environment, but are less than optimal upon return to Earth. We report here the first systematic investigation of potential adaptations in neuromuscular activity patterns associated with postflight locomotion. Astronaut-subjects were tasked with walking on a treadmill at 6.4 km/h while fixating a visual target 30 cm away from their eyes after space flights of 8-15 days. Surface electromyography was collected from selected lower limb muscles and normalized with regard to mean amplitude and temporal relation to heel strike. In general, high correlations (more than 0.80) were found between preflight and postflight activation waveforms for each muscle and each subject: however relative activation amplitude around heel strike and toe off was changed as a result of flight. The level of muscle cocontraction and activation variability, and the relationship between the phasic characteristics of the ankle musculature in preparation for toe off also were altered by space flight. Subjects also reported oscillopsia during treadmill walking after flight. These findings indicate that, after space flight, the sensory-motor system can generate neuromuscular-activation strategies that permit treadmill walking, but subtle changes in lower-limb neuromuscular activation are present that may contribute to increased lower limb kinematic variability and oscillopsia also present during postflight walking.

Task constraints and infant grip configurations.

The prehensile grip configurations of infants aged 4 through 8 months were examined as they grasped objects that varied in size and shape. The findings revealed that infants as young as 4 months systematically differentiate grip configurations as a function of the object properties in essentially the same way that 8-month-old infants do. However, the younger 4-month-old infants predominantly used the haptic system in addition to the visual system for information pick-up regarding object properties, whereas 8-month-old infants predominantly used information from the visual system alone to differentiate grip configurations according to the object properties. Infants apparently perceive the same action-relevant information through different emphases of the sensory modes to drive the action system with a similar grip configuration for a given object. It is proposed that the traditional description of an orderly sequence to the development of infant prehension (e.g., Halverson, 1931) is too conservative and inflexible to capture the functionally adaptive prehensile behavior of infants to changing task constraints.

Lower limb kinematics during treadmill walking after space flight: implications for gaze stabilization.

We examined the lower limb joint kinematics observed during pre- and postflight treadmill walking performed by seven subjects from three Space Shuttle flights flown between March 1992 and February 1994. Basic temporal characteristics of the gait patterns, such as stride time and duty cycle, showed no significant changes after flight. Evaluation of phaseplane variability across the gait cycle suggests that postflight treadmill walking is more variable than preflight, but the response throughout the course of a cycle is joint dependent and, furthermore, the changes are subject dependent. However, analysis of the phaseplane variability at the specific locomotor events of heel strike and toe off indicated statistically significant postflight increases in knee variability at the moment of heel strike and significantly higher postflight hip joint variability at the moment of toe off. Nevertheless, the observation of component-specific variability was not sufficient to cause a change in the overall lower limb joint system stability, since there was no significant change in an index used to evaluate this at both toe off and heel strike. The implications of the observed lower limb kinematics for head and gaze control during locomotion are discussed in light of a hypothesized change in the energy attenuation capacity of the musculoskeletal system in adapting to weightlessness.