Abstract This paper deals with the development of a mathematical model to evaluate forces in the muscles and ligaments at the hipbone (pelvis) during standing on one leg. This study—a part of the project to establish stresses in the hipbone theoretically, possibly represents the first attempt at such a model. Based on the e.m.g. study of the abdominal muscles and other anatomical observations the model was made statically determinate and presents the forces as a function of the angle θ of the reaction at the sacroiliac joint with the horizontal.