Although several previous studies investigated bathing comfort from the perspective of thermal effect or physiology, few studies investigated that from the perspective of biomechanics. The objective of this study was to ecaluate bathing postures from the biomechanical viewpoint. Bathing postures and reaction forces from the bathtub to a human was measured for ten healthy male subjects and in two bathtub conditions (new bathtub and conventional one). A three-dimensional motion analysis system and 3D force plates were used to measure the experimental data. A biomechanical model in which a human body is represented as a link of body segments was constructed. The torque due to buoyancy and passive elastic joint torque were considered in the model. The result showed that all the torque components due to buoyancy and gravity, reaction force, and passive elastic joint torque contributed to the joint torque and each torque component changed among bathtub conditions. In addition, joint torques on ankle and knee joints in the new bathtub were smaller than those in the conventional one significantly. These results suggest that the bathing posture in the new bathtub was more comfortable than that in the conventional one.
