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

Christopher T. Richards*, Enrico A. Eberhard, Amber J. Collings

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

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Abstract

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.

Original languageEnglish
Article number0367
JournalBiology Letters
Volume14
Issue number9
DOIs
Publication statusPublished - 12 Sep 2018

Keywords

  • frogs
  • inverse dynamics
  • jumping
  • kinematics
  • pelvis

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