How to jump without spinning

The remarkable displays of jumping in animals like frogs have inspired many studies on how their muscles generate power. However, in their natural habitats, these animals typically push off against muddy and unpredictably compliant terrains that induce the feet to apply unequal forces on the ground. This may result in substantial angular momentum at take-off, and have dire consequences such as the animal's mouth pointing away from an intended prey, or landing in ways that prevent escaping a predator. We investigated whether morphological features of jumping animals may alleviate this critical problem and help them to jump without spinning. Our analyses and experiments with a brainless passive mechanical jumper focus on the dynamics within the frontal plane as it pushes off unequally using two legs. We find that a flexible pelvis is sufficient to reduce the angular momentum due to unequal leg forces by several orders of magnitude in both our experiments and mathematical analyses. A flexible pelvis acts like a whiffletree mechanism that can balance loads between the legs. Our scaling analyses of these jumpers suggest passive mechanical designs for robotic jumpers and point to the critical role of pelvis morphology in jumping animals for stability.