Anurans are well known for their jumping abilities, making use of their strong hindlimbs. In contrast, the function of the forelimbs during take-off has rarely been studied. We measured the ground reaction forces exerted by forelimbs and hindlimbs during short jumps in the Dybowski's frog Rana dybowskii. Take-off occurred in two phases. Phase one (from the initial time until the forelimbs took off), which lasts a relatively long time (63.2 ± 4.1% of the total take-off phase, N = 20), provides sufficient time for the forelimbs to elevate the body to a suitable posture to deliver the best take-off angle. Phase two (from the forelimbs lift-off until hindlimbs lift-off) was dominated by the hindlimbs which provided a constant and fast elevation. The force angle (angle of the resultant vector from fore-aft and normal force components towards the plane of the substrate) of the hindlimbs and body trajectory was variable before the forelimbs lifted off of the substrate and then primarily followed the direction of the line from the foot-substrate point to the center of mass (COM). The preparation angle adopted when the forelimbs lifted off of the substrate was a good predictor of the take-off angle. The total normal force oscillated around body weight (BW) before the forelimb normal force peaked. The BW shifted from the hindlimbs to the forelimbs during the initial phase of take-off. A simple lever model suggests that the forelimbs are responsible for raising the COM, thus influencing the take-off angle in short jumps.
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