Elderly individuals are likely to develop locomotive disorders such as osteoarthritis or osteoporosis. This increases the risk of falls and makes independent movement difficult. Elderly individuals should better understand walking function to extend their healthy life. We therefore propose a new method for estimating hand and foot reaction forces using only visual markers and a monocular camera. When humans contact the environment with their hands, their hand and feet positions define a convex hull. A proposed “ generalized zero moment point ” is projected on this convex hull, which is approximated as a line or plane, and the distance between this point and each contact point is calculated. Reaction forces are calculated based on the ratios of these distances. Evaluation experiments show high agreement between estimated and measured forces of both hands and feet, confirming the validity of the proposed algorithm.