Take-off and landing forces in jumping frogs

Anurans use a saltatorial (jumping) mode of locomotion. A jumping cycle can be divided into four subphases: propulsion, flight, landing and recovery. We studied the landing phase during locomotion in Rana esculenta by measuring the ground reaction forces during propulsion and landing over a range of distances. Landing performance affects locomotor ability in jumping frogs. Landing and recovery together take up one third of the locomotor cycle. Peak landing forces are on average almost three times larger than propulsive forces. The forelimbs appear to be fully extended when they make contact with the substrate and absorb the first impact peak. The height of this peak varies depending on arm positioning and jumping distance. Since the stiffness of the arms stays constant over the full jumping range, it is possible that this is a limiting factor in the ability of the forelimbs to work as dampers. A spring-dashpot model is used to model the effect of arm angle at touch down. Damping during landing is performed by placing the forelimbs at an optimal angle to cancel frictional forces effectively.