Fluidity as a Measure of Movement Quality

In this paper, we present 'fluidity' as a metric for quantifying movement quality, and validate the measurement of fluidity in an unobtrusive, wireless, low-power manner in an operational setting. Full-body movement quality is difficult to unobtrusively quantify due to the high operational complexity and precision often required, but doing so is critical for the effective study of human-system interactions. In the human spaceflight domain, the operational impacts of adaptation to microgravity are not adequately characterized. Spaceflight studies have been focused on single-limb or single-joint kinematics; a gap remains in functional whole-body movement assessment, which is important for risk characterization of spaceflight-degraded physiology, contingency, and planetary exploration scenarios. As a proxy for whole-body movement quality, we propose an adapted definition of biomechanical 'fluidity' from prior literature, improving it to be mathematically meaningful. Coupled with the development of a wearable on-body sensor system, we validated a novel, low-power, wireless kinematic monitoring technique in laboratory and operational (i.e., parabolic flight) environments. Pilot test results demonstrate discriminatory validity of fluidity across gravity levels, as well as between nominal and degraded movements. Beyond spaceflight, assessment of movement quality in an unobtrusive manner can provide real-time, precise knowledge about human performance across applications in exercise physiology, rehabilitation, and human-robotic interaction.