Gravity has a big impact on human life and movement in general. Our bodies are adapted to Earth gravity, which is constant throughout our lives. That however is not true for pilots and astronauts. They must operate in different gravitational environments, which affects their movement and execution of tasks. Researching the changes that arise in altered gravitational environment, especially in weightlessness, is hard to do on Earth. One way of generating weightlessness is with parabolic flights, which generate alternating 20 second phases of weightlessness and enhanced gravity. Our experiment took place on one of such flights and had the goal of researching the changes in kinematics of hand point to point movements and how to minimise those changes. Minimisation was done using a robot that applied force on the hand that during flight compensated the change of gravity force on the arm and during training simulated the weight change, which would happen in the altered gravity.
In this master thesis we analyse the changes between different gravitational environments, without compensation, with compensation, and simulation of altered gravity. The changes in point to point movements were measured with four different parameters: relative time to peak velocity, duration of movement, curvature of trajectory and accuracy of task execution. The task given to the subjects consisted of vertical point to point movements, starting from the middle of a touchscreen, to any of 14 targets that appeared bellow or above the starting position. The perturbation force that compensated or simulated the changes in arm weight was generated by two motors, that pulled on the wrist of the subject via a string while the kinematics of the arm has been measured by an infrared camera system Vicon using passive reflective markers.
Our experiment confirmed a bigger impact of weightlessness on movement kinematics in comparison to enhanced gravity. It is hypothesised that this is due to greater relative change between arm mass and arm weight. The duration of movement turned out to be a good indicator of movements in weightlessness since the movements lasted longer. Likewise the duration of movements showed effective compensation, since compensated movements had a comparable duration with movements in normal gravity. Compensation was also shown to increase the accuracy of task execution. However, in contrast to the literature, the relative time to peak velocity did not carry much information in our experiment. Simulating altered gravity conditions was not as successful as was compensation, since the kinematics of movement did not converge toward kinematics in actual altered environment. We did however notice a peculiar artefact of equalising of kinematic parameters during simulation of altered gravity between upward and downward movements. This observation was not expected and needs further investigation.
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