Movement Smoothness Metrics in Human-Machine Interaction

The evaluation of motion abilities is crucial to rate human movement control performance in several contexts. In the medical field, for example, smoothness, a feature related to the regularity of movement, is assessed through objective metrics during the execution of free movements in order to support the decision based on clinical scales about the impairment severity. Nevertheless, individuals with and without impairment interact daily with machines to improve their well-being in many contexts: rehabilitation, collaborative robotics, and sport exercises, among others. During these activities, they perform movements in a closed-chain, where inertial or resistance forces introduced by an external tool could affect their motion control. In this study, closed-chain movements performed by three able-bodied and five individuals with coordination impairments were investigated; three different smoothness metrics presented in the literature were applied to analyze the results. The experimental tests consisted in moving a slider mounted on a linear rail with varying velocity and resistance force conditions. Position in the main direction, accelerations and forces in all directions were recorded during the tests. All the metrics detected a smoothness improvement when velocity increased, while only two metrics found an influence of the resistance force on the smoothness.