Effects of Simulated Microgravity and Hypergravity Conditions on Arm Movements in Normogravity

Abstract

The human sensorimotor control has evolved in the Earth's environment where all movement is influenced by the gravitational force. Changes in this environmental force can severely impact the performance of arm movements which can be detrimental in completing certain tasks such as piloting or controlling complex vehicles. For this reason, subjects that are required to perform such tasks undergo extensive training procedures in order to minimize the chances of failure. We investigated whether local gravity simulation of altered gravitational conditions on the arm would lead to changes in kinematic parameters comparable to the full-body experience of microgravity and hypergravity onboard a parabolic flight. To see if this would be a feasible approach for on-ground training of arm reaching movements in altered gravity conditions we developed a robotic device that was able to apply forces at the wrist in order to simulate micro- or hypergravity conditions for the arm while subjects performed pointing movements on a touch screen. We analyzed and compared the results of several kinematic parameters along with muscle activity using this system with data of the same subjects being fully exposed to microgravity and hypergravity conditions on a parabolic flight. Both in our simulation and in-flight, we observed a significant increase in movement durations in microgravity conditions and increased velocities in hypergravity for upward movements. Additionally, we noted a reduced accuracy of pointing both in-flight and in our simulation. These promising results suggest, that locally simulated altered gravity can elicit similar changes in some movement characteristics for arm reaching movements. This could potentially be exploited as a means of developing devices such as exoskeletons to aid in training individuals prior to undertaking tasks in changed gravitational conditions.

Keywords: arm kinematics; exoskeletons; hypergravity; microgravity; parabolic flight; robot assisted training.

Figure 1

(A) Profile view of the experimental setup and (B) view of the screen indicating start (Gray) and end target (Red) locations. The orientation of the coordinate system is marked indicating the positive y coordinate pointing up and the x coordinate pointing out from the screen towards the subject. (C) The measurement window during each parabola consisted of 20 s of normogravity followed by 20 s of hypergravity and finally 22 s of microgravity. (D) The number of parabolas per subject. The parabolas from 5 to 9 during flight were part of another study.

Figure 2

Absolute deviations of the pointing position averaged for all subjects and grouped per condition for upward movements (A) and downward movements (D). Signed deviations for every target in microgravity (B,E) and hypergravity (C,F) for upward and downward movements, respectively. Downward-pointing triangles denote mean values for downward movements, upward-pointing triangles denote mean values for upward movements, the whiskers denote the standard error of the mean. Green represents normogravity, blue represents microgravity and red represents hypergravity conditions, (1g, 0g, 2g) denote in-flight gravitational conditions, (1g S, 0g S, 2g S) denote simulated gravitational conditions. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 3

Movement durations in different gravitational and simulation conditions. Downward-pointing triangles denote mean values for downward movements, upward-pointing triangles denote mean values for upward movements, the whiskers denote the standard error of the mean. Green represents normogravity, blue represents microgravity and red represents hypergravity conditions, (1g, 0g, 2g) denote in-flight gravitational conditions, (1g S, 0g S, 2g S) denote simulated gravitational conditions. *p < 0.05, ***p < 0.001.

Figure 4

Mean velocity profiles for downward and upward movements in all conditions. Solid lines represent the conditions during flight (1g, 0g, 2g) whereas dashed lines represent the simulated conditions (1g S, 0g S, 2g S). Green represents normogravity, blue represents microgravity and red represents hypergravity conditions.

Figure 5

Maximum velocities. Downward-pointing triangles denote mean values for downward movements, upward-pointing triangles denote mean values for upward movements, the whiskers denote the standard error of the mean. Green represents normogravity, blue represents microgravity and red represents hypergravity conditions, (1g, 0g, 2g) denote in-flight gravitational conditions, (1g S, 0g S, 2g S) denote simulated gravitational conditions. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 6

Mean trajectories normalized for target distance for different gravitational and simulation conditions. Left: normogravity and microgravity conditions, Right: normogravity and hypergravity conditions. All trajectories start at the coordinate (0, 0) and end at either (0, 100) for upward movements or (0, −100) for downward movements. Solid lines represent the conditions during flight (1g, 0g, 2g) whereas dashed lines represent the simulated conditions (1g S, 0g S, 2g S). Green represents normogravity, blue represents microgravity and red represents hypergravity conditions.

Figure 7

Maximum displacement in the x direction. Downward-pointing triangles denote mean values for downward movements, upward-pointing triangles denote mean values for upward movements, the whiskers denote the standard error of the mean. Green represents normogravity, blue represents microgravity and red represents hypergravity conditions, (1g, 0g, 2g) denote in-flight gravitational conditions, (1g S, 0g S, 2g S) denote simulated gravitational conditions. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 8

Values of the TPV parameter for different gravitational and simulation conditions. Downward-pointing triangles denote mean values for downward movements, upward-pointing triangles denote mean values for upward movements, the whiskers denote the standard error of the mean. Green represents normogravity, blue represents microgravity and red represents hypergravity conditions, (1g, 0g, 2g) denote in-flight gravitational conditions, (1g S, 0g S, 2g S) denote simulated gravitational conditions. ***p < 0.001.

Figure 9

Normalized integrated EMG for the Trapezius, Pectoralis, Anterior Deltoid, and Posterior Deltoid for all conditions. Downward-pointing triangles denote mean values for downward movements, upward-pointing triangles denote mean values for upward movements, the whiskers denote the standard error of the mean. Green represents normogravity, blue represents microgravity and red represents hypergravity conditions, (1g, 0g, 2g) denote in-flight gravitational conditions, (1g S, 0g S, 2g S) denote simulated gravitational conditions. *p < 0.05, **p < 0.01, ***p < 0.001.