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The dynamic role of the ilio-sacral joint in jumping frogs

Richards, C T; Eberhard, E A; Collings, A J


C T Richards

E A Eberhard

A J Collings


A striking feature among jumping frogs is a sharp pelvic bend about the ilio-sacral (IS) joint, unique to anurans. Although this sagittal plane hinge has been interpreted as crucial for the evolution of jumping, its mechanical contribution has not been quantified. Using a model based on Kassina maculata and animated with kinematics from prior experiments, we solved the ground contact dynamics in MuJoCo enabling inverse dynamics without force plate measurements. We altered the magnitude, speed and direction of IS extension (leaving remaining kinematics unaltered) to determine its role in jumping. Ground reaction forces (GRFs) matched recorded data. Prior work postulated that IS rotation facilitates jumping by aligning the torso with the GRF. However, our simulations revealed that static torso orientation has little effect on GRF due to the close proximity of the IS joint with the COM, failing to support the ‘torso alignment’ hypothesis. Rather than a postural role, IS rotation has a dynamic function whereby angular acceleration (i) influences GRF direction to modulate jump direction and (ii) increases joint loading, particularly at the ankle and knee, perhaps increasing tendon elastic energy storage early in jumps. Findings suggest that the pelvic hinge mechanism is not obligatory for jumping, but rather crucial for the fine tuning of jump trajectory, particularly in complex habitats.


Richards, C. T., Eberhard, E. A., & Collings, A. J. (2018). The dynamic role of the ilio-sacral joint in jumping frogs. Biology Letters, 14(9),

Journal Article Type Article
Acceptance Date Aug 15, 2018
Publication Date Sep 12, 2018
Deposit Date Sep 4, 2018
Publicly Available Date Oct 19, 2018
Journal Biology Letters
Print ISSN 1744-9561
Publisher The Royal Society
Peer Reviewed Peer Reviewed
Volume 14
Issue 9
Public URL


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