Objective: Evaluating countermeasures designed to reduce the impact of microgravity exposure on astronaut performance requires the development of effective methods of assessing changes to sensorimotor function in 1g analog systems. In this study, somatosensation at the ankle and fingers, lower leg muscle activity and visuomotor control were assessed using a full body loading and acute unloading model to simulate microgravity. It was hypothesized that the function of the hands and eyes are not constrained to ‘weight bearing’ postures for optimal function and would not differ between the loaded and acute unloaded conditions, whereas lower leg muscle activity and ankle somatosensation would be reduced in the acute unloaded condition. Somatosensation was recorded using the Active Movement Extent Discrimination Apparatus (AMEDA) protocol where participants were required to make an absolute judgment of joint position sense. A score closer to 1.0 demonstrates higher accuracy. Lower leg muscle activity was recorded using electromyography of major lower leg musculature to observe peak muscle activity and duration of contraction. The King Devick infrared eye tracking test was used to asses visuomotor control by monitoring saccade velocity and fixation time. In acute unloading, it was found that ankle somatosensation had decreased accuracy (loaded 0.68, unloaded 0.66, p = 0.045) while finger somatosensation improved (loaded 0.77, unloaded 0.79, p = 0.006). When acutely unloaded, peak lower leg muscle activation reduced ( > 27%) and total contraction time increased (2.02 × longer) compared to loading. Visuomotor assessment results did not vary between the loaded and acute unloaded postures, however the underlying techniques used by the participant to complete the task (saccade velocity and fixations time) did increase in acute unloaded conditions.
Significance: This research provides an insight to how to the human body responds immediately to acute changes of gravitational load direction. It provides insight to the acute affects’ astronauts may encounter when in microgravity.
Methods: The King Devick (KD) infrared eye tracking test was used to assess visual neuromotor control. The KD test is a measure of eye movement control, concentration, and cerebral cortex processing (Davies et al., 2012; Galetta et al., 2016), and has been shown to be sensitive to changes in visual function following concussion and sleep deprivation as well as the effects of hypoxia. The test was conducted via a laptop screen and pupil tracking device, which was positioned approximately 30-40 centimeters in front of the participant for both loaded and acute unloaded conditions.
K-D Discussion: The KD infrared eye tracking test results demonstrated that test completion time is not significantly affected by an acute change in body position. These results compare to those of Dalecki et al. (2013) who demonstrated no change in reaction time or number of errors for a cognitive, visual based task in microgravity. Interestingly, it appears that the approach used by participants to complete the KD infrared eye tracking test of the present study differed significantly between body positions. To scan our surroundings, the visual system uses a combination of both saccadic eye movement and fixation control. In the upright loaded condition, participants demonstrated shorter fixation times with slower saccade angular velocities, whereas the opposite was true in the acute unloaded posture condition. Microgravity environments have been shown to negatively affect visual tracking and gaze hold ability, characterized by slower saccade angular velocity and reduced precision (Kornilova et al., 2012). This increased fixation time may be indicative of the participants’ unfamiliarity with performing screen-based activities in the supine position, and a resultant unanticipated cognitive load.
The current study showed that saccade velocity was slower in the upright loaded position. McColeman and Blair (2014) demonstrated that slowed saccade angular velocity correlated to increased cognitive demand, number of distractions and relevance of the task. This appears to be supported in the present study, as participants may have greater demand on the central motor control systems when in the upright loaded position. McColeman and Blair (2014) also concluded that saccade angular velocity and fixation timing are independent of one another and follow different command systems. If increased cognitive demand slows saccade angular velocity, perhaps upright loaded position could be expected to influence the KD infrared eye tracking test results in the same manner.