Kinematic control of extreme jump angles in the red-legged running frog, Kassina maculata

Abstract

The kinematic flexibility of frog hindlimbs enables multiple locomotor modes within a single species. Prior work has extensively explored maximum performance capacity in frogs; however, the mechanisms by which anurans modulate performance within locomotor modes remain unclear. We explored how Kassina maculata, a species known for both running and jumping abilities, modulates take-off angle from horizontal to nearly vertical. Specifically, how do 3D motions of leg segments coordinate to move the centre of mass (COM) upwards and forwards? How do joint rotations modulate jump angle? High-speed video was used to quantify 3D joint angles and their respective rotation axis vectors. Inverse kinematics was used to determine how hip, knee and ankle rotations contribute to components of COM motion. Independent of take-off angle, leg segment retraction (rearward rotation) was twofold greater than adduction (downward rotation). Additionally, the joint rotation axis vectors reoriented through time, suggesting dynamic shifts in relative roles of joints. We found two hypothetical mechanisms for increasing take-off angle. Firstly, greater knee and ankle excursion increased shank adduction, elevating the COM. Secondly, during the steepest jumps, the body rotated rapidly backwards to redirect the COM velocity. This rotation was not caused by pelvic angle extension, but rather by kinematic transmission from leg segments via reorientation of the joint rotation axes. We propose that K. maculata uses proximal leg retraction as the principal kinematic drive while dynamically tuning jump trajectory by knee and ankle joint modulation.