Forces in human swimming consist of two components, a drag force and a lift force. The lift force is assumed to be beneficial because of the relative small energy loss to the water. This energy loss can be quantified by determining the propelling efficiency, ep (defined as the ratio of the useful power to the total power output). The first purpose of this study was to investigate whether high values of propelling efficiency can be explained by a relatively high contribution of lift and/or a favorable direction of the generated force in front crawl swimming. Propelling efficiency was estimated using two methods, one based on a physiological approach (epp) by measuring oxygen consumption and one based on a kinematic approach (epk) by calculating forces generated by hands and forearm and power components, from a three-dimensional analysis. The second purpose of this study was to compare epp and epk. The contribution of lift to the total force as well as the direction of the force cannot explain the values of epp. The values of epp and epk did not correlate significantly. In swimming propulsion some processes play a role which cannot be explained at this moment. One of these processes might be the generation of vortices.