In order to improve comfort in bathing, the relationship between the biomechanical load and the bathtub shape has to be clarified. For this purpose, many bathing 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 bathtun in order to calculate joint torque as biomechanical loads without bathing experiments. 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 of the back, considering gravity, buoyancy and passive elastic joint torque. Weighting 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, a bathing experiment to obtain the data of bathing posture and reaction forces for 10 subjects and 24 posture conditions was conducted. It was found that the estimated reaction forces were consistent well with the measured values.
