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.
Highlights
Anuran anatomy is unique among vertebrates, owing in part to the elongations and reorientations of pelvic bones during their evolutionary transition from salamander-like tetrapods [1]
Because right kinematics were mirrored from the left, the ‘extra’ medio-lateral force can be interpreted as the additional force required to impose symmetry
In contrast to prior interpretations [5], we propose IS rotation is most important for enhancing jump performance and control, but is not obligatory for torso –ground reaction force (GRF) alignment
Summary
Anuran anatomy is unique among vertebrates, owing in part to the elongations and reorientations of pelvic bones during their evolutionary transition from salamander-like tetrapods [1]. ‘Sagittal-hinge’ jumpers arose multiple times independently [3] hinting that fossil presence of the IS joint is evidence of hopping early in frog evolution [3,5]. To elucidate the role of IS extension, electromyography [2,6], cineradiography [5,7] and inverse dynamics (ID; [7]) have been used to determine that pelvic muscles activate synchronously early in jumps causing IS extensor torque to drive rapid extension of the back. The above studies show neuro-mechanical activity of the IS joint, no analysis has quantified its direct effect on centre of mass (COM) mechanics, joint torques or GRF
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