This study suggests a method that, on the basis of concurrent cinematographic and dynamometric measurements, detects the motion of the visceral part of the body, provided that an estimate of its mass has been given in advance and the periodicity of the system dynamics can be assumed. The mathematical model provides equations for both vertical and horizontal displacements of the visceral mass, so that they can be used either separately or in combination to detect its one-, two- and three-dimensional trajectories. To test the model predictions, an external load has been periodically raised and lowered by a subject while standing on a dynamometric platform, and its vertical movement has been estimated accurately. A further experiment on hopping in place (without loads) reveals that the internal visceral mass (assumed equal to about 9 kg) oscillates out-of-phase with respect to the body frame, with a vertical range of about 0.05-0.08 m. A discussion on the possible sources of experimental inaccuracy is included. Also discussed are the applications of the model to work calculations in periodic movements as hopping, walking and running, as well as in investigations about locomotion-respiration coupling.