Data from: Functional capacity of kangaroo rat hindlimbs: adaptations for locomotor performance

Many cursorial and large hopping species are extremely efficient
locomotors with various morphological adaptations believed to reduce
mechanical demand and improve movement efficiency, including elongated
distal limb segments. However, despite having elongated limbs, small
hoppers such as desert kangaroo rats (Dipodomys deserti) are less
efficient locomotors than their larger counterparts, which may be in part
due to avoiding predators through explosive jumping movements. Despite
potentially conflicting mechanical demands between the two movements,
kangaroo rats are both excellent jumpers and attain high hopping speeds,
likely due to a specialized hindlimb musculoskeletal morphology. This
study combined experimental dissection data with a static analysis of
muscle moment generating capacities using a newly developed
musculoskeletal model to characterize kangaroo rat hindlimb
musculoskeletal architecture and investigate how morphology has evolved to
meet hopping and jumping mechanical demands. Hindlimb morphology appears
biased towards generating constant moment arms over large joint ranges of
motion in this species, which may balance competing requirements by
reducing the need for posture and movement specific excitation patterns.
The ankle extensors are a major exception to the strong positive
relationship exhibited by most muscles between muscle architecture
parameters (e.g. Lfibre) and joint moment arms. These muscles appear
suited to meeting the high moments required for jumping: the biarticular
nature of the ankle extensors is leveraged to reduce MTU strain and create
a four-bar linkage that facilitates proximal force transfer. The kangaroo
rat hindlimb provides an interesting case study for understanding how
morphology balances the sometimes competing demands of hopping and
jumping.