Evaluation of biomechanical loads for three-dimensional soaking postures in bathwater

Abstract

Comfort is required for soaking in bathwater. Although several studies investigated comfort during soaking from a biomechanical viewpoint, these previous studies employed two-dimensional models in the sagittal plane of a bather. The objectives of this study were to take three-dimensional postures of the bather into account and to evaluate the biomechanical loads for three-dimensional soaking postures. A three-dimensional biomechanical model was constructed, and an experiment to measure the soaking postures and reaction forces from the bathtub was conducted for eight participants. In addition, a supplementary experiment to measure the passive elastic joint torques was conducted for the same participants as well. Using the experimental results, the joint torques during soaking were calculated. In addition, the biomechanical load during soaking was defined as the sum of weighted joint torques, and the weighting coefficients were determined so that the tendencies of the biomechanical load were consistent with those of sensory evaluation of comfort. Finally, the biomechanical loads were calculated for various bathtub conditions and evaluated. It was found that the contributions of joint torques at the hip and neck to the comfort were dominant. It was also found that hip joint torques not only in the flexion/extension direction but also in the abduction/adduction direction were affected by the difference in bathtub length. It was suggested that the biomechanical load in the convex foot wall was smaller than those in the flat and concave ones for male bathers.