Kinematic control of extreme jump angles in the red leg 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.

Inverse dynamic modelling of jumping in the red-legged running frogKassina maculata

“…Results for the knee were more complicated: both positive and negative XY and XZ torques significantly increased with jump angle (Table S3). Again, this in line with IK analysis predicting knee extension is important in increasing take-off angles late in the jump (Richards et al, 2017). Increased torque magnitudes were due to higher forces; variability in torque direction was due to the volatile position of the GRF vector relative to the knee.…”

“…Our findings also agree with those of , which suggest horizontal take-off velocity (thrust) is most sensitive to hip extensor torques. In contrast, the ankle contributes equally to thrust and elevation; inverse kinematics (IK) analysis also predicted that ankle extension drives steeper jumps, particularly early in the jump (Richards et al, 2017). Our findings largely support our hypothesis -forward thrust is produced primarily at the hip and ankle whereas elevation is produced primarily at the ankle.…”

“…predicted that increased degrees of freedom at the knee joint allows frogs to bring the foot under the body and doubles the ankle extensor torque producing vertical acceleration of the body. Similarly, IK analysis predicted reorientation of the knee rotation axis is crucial to achieving COM elevation (Richards et al, 2017). Thus, fluctuations in torque direction may reflect the subtle and important role of knee positioning in modulating jump angle by permitting high elevation torques to be produced at the ankle.…”

“…The end of each jump (in which the last toe left ground) was defined as take-off. Jump start was defined as the onset of velocity at the hip marker (closest to the COM; see Richards et al, 2017). Within this interval (i.e.…”

“…These data were used to carry out inverse dynamics analysis and calculate the external moments acting about the hind limb joints during jumping in a walking (as opposed to jumping) frog taxon for the first time. We hypothesize that, based on kinematics analysis (Richards et al, 2017), forward thrust is produced by hip, knee and ankle extension whereas elevation is produced at the ankle and knee; it is at these joints that we expect fine-tuning of jump angle to be achieved. Specifically, steeper jump angles require higher ankle and knee torques to drive downward rotation of the distal limb elements to elevate the body.…”