The Effect of Acute Body Unloading on Somatosensory Performance, Motor Activation, and Visuomotor Tasks

Abstract

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.

An investigation into the measurement properties of the King-Devick Eye Tracking system

“…The Hecimovich et al 31 study with male youth Australian football participants reported mean saccadic measurements of 151 to 168 and 4 to 7 blinks per test Tejani et al 48 study on baseline eye movements and visual contrast acuity noted an average number of saccades per card (over all three K-DT tests) between 56.9 + 14 and 101 + 41. Marchant et al 47 with their study on somatosensation, muscle activity and visuomotor control and full body loading and unloading reported mean ISI of 257 ms and 287 ms, respectively, between groups. The results from the aforementioned studies are comparable with the current results (Table 1), for example total number of saccades per test ranged from the 150s to 160s, ISIs 230 ms to the 270 ms.…”

“…Recently, research using the K-D ET has been reported. Marchant et al 47 used the K-D ET unit and reported on saccade velocity, and fixation time (ISI) in their study assessing ankle and finger somatosensation and lower limb muscle activity assessing visuomotor control in both conditions. Tejani et al 48 utilized the K-D ET combined unit when assessing baseline differential eye movements and visual contrast acuity in competitive athletes and reported on total number and frequency of saccades and completion time, but not saccade velocities.…”

“…The K-D ET unit provides measurement on duration (time to take to test), number of saccades, average saccade velocity (°/s), peak saccade velocity (°/s), number of fixations, ISI (ms), average fixation polyarea (mm 2 ) and number of blinks. The average saccade velocity and average fixation polyarea are company-based metrics and were reported in the Onge et al 30 and Marchant et al 47 (average saccade velocity only) studies with both using the K-D ET, keeping in mind that the Onge et al 30 study used the older, discontinued, model. Average saccade velocity was mentioned in Tad et al 63 study, however, the usefulness of these two metrics needs to be established in the literature.…”

Evaluation and Utility of the King-Devick With Integrated Eye Tracking as a Diagnostic Tool for Sport-Related Concussion

“…Thus, these 2 metrics cannot be used when assessing eye movements between the K-D ET and other similar research or clinically with other eye-tracking units. Onge et al 26 and Hecimovich et al 18 reported on both of these metrics, and Marchant et al 24 reported on average saccade velocity. All 3 of those studies used the K-D ET unit, and as the metrics are components on the unit, it was determined that the current study report on them for use in future research.…”

Developing Proprioceptive Countermeasures to Mitigate Postural and Locomotor Control Deficits After Long-Duration Spaceflight

“…Body load sensing is important for controlling dynamic balance function because it modulates motor control ( Dietz and Duysens, 2000 ). Even acute unloading appears to decrease ankle proprioception with corresponding decreases in lower leg muscle activation ( Marchant et al, 2020 ). Altering body load perception via water immersion decreases electromyographic activity, maximal voluntary contractions, and spinal reflexes in the antigravity extensors ( Pöyhönen and Avela, 2002 ).…”