Development of an algorithm to estimate reaction forces from bathtub during soaking in the bathwater

Abstract

In order to improve comfort in soaking in the bathwater, the relationship between the biomechanical load and the bathtub shape has to be clarified. For this purpose, many soaking experiments had to be conducted since it was necessary to measure the reaction forces acting on a human from the bathtub experimentally. The objective of this study was to construct an algorithm to estimate reaction forces from a bathtub in order to calculate joint torque as biomechanical loads without experiments for given bathtub shapes and soaking postures. This algorithm calculates distribution of reaction forces which minimizes weighted summation of i) sum of squares of reaction forces, ii) joint torques of hip and knee joints and iii) friction force on the back, considering gravity, buoyancy and passive elastic joint torque. The weighting coefficients were expressed as a function of bathing posture including parameters determined for each person to reflect influence of differences among the individual. In order to examine this algorithm, an experiment to obtain the bathing posture and reaction forces for 10 participants and 24 posture conditions was conducted. It was found that the estimated reaction forces were consistent well with the measured values.