The development and evaluation of a three-dimensional musculoskeletal model to investigate jerboa hindlimb dynamics

The lesser Egyptian jerboa, Jaculus jaculus, is a small bipedal rodent with unique morphological features such as disproportionately long hindlimbs and tail, fused metatarsal bones, and the loss of medial and lateral digits. These contribute to an extraordinary repertoire of locomotion including high accelerations and decelerations resulting in unpredictable ricochetal motion. In addition to speed, jerboa are capable of producing immense ground reaction forces, allowing for propulsion of their bodies forward and upward over ten times their hip height. This unmatched performance combined with their unique morphological characteristics separates them from other small mammals and invites great interest in the study of their biomechanics and movement. Investigating muscle interactions and how they ultimately result in whole body movement using in-vivo experimentation alone is not always practical. Therefore, implementing a detailed computational model can provide insights into the musculoskeletal dynamics of the jerboa. This study describes and evaluates the development of the first three-dimensional model of jerboa hindlimb biomechanics based on detailed anatomical measurement collected from micro computed tomography (microCT) scans. Joints are generated for all bones in the hindlimb following International Society of Biomechanics (ISB) standards, with segment mass properties of each geometry measured experimentally and calculated computationally. Tendon insertions and muscle lines of action were validated using microdissections and biomechanics experiments. A sensitivity analysis was conducted to evaluate the model's robustness to changes in input parameters and to identify muscle and joint parameters that are particularly important for locomotor performance. This model combined with measured kinematics, ground reactions forces, and contractile muscle properties can be used to better understand how anatomic and physiological adaptations in the jerboa have evolved to generate the joint moment arms and control mechanisms that give rise to extreme locomotor performance and stability.