Locomotion has a clear ecological relevance for many animals. As animals need to move in order to escape from predators, find food, defend territories, etc., it is expected that locomotor performance might affect an individual’s survival and reproduction. Clear links have also been found between an animal’s performance and its morphology. A function analysis can unravel the underlying mechanistic facets of this link. Frogs are interesting model species because they add an extra element to this topic; they have to perform in two entirely different physical environments, particularly semi-aquatic species. However, no trade-off was found between jumping and swimming performance. Morphological correlates were found but, to understand causations, a thorough understanding of the mechanics of locomotion is required. When comparing the mechanics of jumping and swimming, the question arises of how to ensure a sound comparison. Ideally, some measure of ‘effort’ has to be added to the analysis. We suggest the use of the propulsive impulse. Unexpectedly, the impulse of swimming proved to be significantly smaller than the jumping impulses during locomotion in a semi-aquatic frog, Rana esculenta, even when maximal performance had been induced in both media. To verify our impulse calculations, we analysed the external forces during both locomotor modes. The forces during terrestrial locomotion can be measured directly using a force plate; determining the forces in water requires a more complex set-up. For that reason, we performed a DPIV (Digital Particle Image Velocimetry) analysis on the flow generated by swimming frogs. These experiments confirmed the results of our impulse calculations.