A Wearable Shoulder Skin-Deformation Device Generating Illusory Forces for Posture Correction

The hanger reflex, in which skin shear deformation elicits a directional force sensation, provides a basis for wearable haptic guidance on the shoulders. In this study, we employ a wearable haptic device that uses four pneumatic airbags to apply controlled skin-shear deformation to both shoulders. Previous work on the hanger reflex has demonstrated that shear deformation of the skin can elicit directional force sensations, and such illusory forces are reliably perceived at the shoulder. However, the strength of these perceived forces and their potential influence on upper-body posture have not yet been quantitatively examined. To enable future applications in posture correction during manual or visually demanding tasks, the present study investigates both the perceived force magnitude and the postural effects elicited by shoulder skin-shear deformation. In Experiment 1, we quantified the magnitude of the illusory forces using a psychophysical comparison between forces elicited by skin-shear deformation and real traction forces. The device generated substantially larger perceived forces than typical illusion-based haptic methods, and the relationship between pressure and perceived force was well approximated by a linear model within the tested range. In Experiment 2, we evaluated how these deformation-elicited forces affect upper-body posture. We measured forward-backward trunk inclination under varying initial postures and stimulation directions. Consistent with the perceptual findings, the device produced consistent posture adjustments, particularly in the backward direction, with significant effects of initial posture and stimulus intensity on angular changes. Together, these findings demonstrate that shoulder hanger-reflex device provides quantifiable force sensations while also inducing measurable posture modification. The results offer essential information for individualized calibration and control-parameter design in wearable posture-support and rehabilitation systems, and they provide a foundation for future integration into ergonomic and assistive applications.