Fig. 1.
Importance of hind- and forelimb positioning on minimizing toppling torques at landing. (A) The velocity vector, γ, is the instantaneous tangent to the line describing the position of the snout of the toad throughout the hop. The impact angle, here shown for both a high (γA) and flat (γB) hop, is the instantaneous velocity vector at touchdown. Arm angle, α, was defined as the angle between the plane of the forelimb and the horizontal. Here the arms are shown with an arm angle of αA. (B) Close up view of the toads with points used to digitize kinematics (1-3) and the impact and arm angles for a high hop depicted. (C-E) Simplified model of toad for clarity. (C) Movement of the hind limbs from extended (dashed line) to retracted (solid line) configuration moves the center of mass (COM) anteriorly, reducing torques at impact for an impact angle, γA, and arm angle, αA. (D) For a more acute impact angle, γB, like that depicted in the level hop in A, the same forelimb landing angle of αA will result in net torques around the COM and not allow the toad to stabilize landing. (E) If forelimbs instead were positioned more anteriorly at αB, GRF vector could align more closely with the COM.