Hoffmann's two-toed sloth II: Muscle architectural properties in the thoracic limb of Choloepus (Pilosa: Xenarthra)

Abstract

Two-toed sloths are larger than three-toed forms and potentially have the capacity for greater strength and power associated with their high frequency of suspensory behaviors. However, fiber architecture and whole muscle functional capacities remain completely unknown for either species of Choloepus. Part two of this study provides novel quantifications of muscle architectural properties in the forelimb of Hoffmann's two-toed sloth (C. hoffmanni). A suite of geometric measurements, including muscle mass, belly length, fascicle length, and pennation angle, were used to calculate physiological cross-sectional area (PCSA) and estimate isometric force, joint torque, and instantaneous power. In general, the musculature becomes progressively more pennate from the extrinsic to the distal intrinsic regions of the forelimb, and all flexors are larger than their counterpart extensors. Except for a few pairs of small joint stabilizer muscles, the majority of bellies in each region of the forelimb have greater ability for shortening but limited ability for sized-scaled force production. Nonetheless, several large, strong shoulder (e.g., m. latissimus dorsi) and elbow (e.g., m. brachioradialis) flexors are capable of applying large joint torques over an extended range of contractile excursion by having elongated moment arms. Modification to limb muscle gearing is further exemplified by pairs of synergistic muscles with opposing fast joint rotational velocity versus mechanical advantage arrangements in each functional group. Lastly, the digital flexors have variable architectural properties, but their collectively large PCSA and estimated maximum force production capability (~2.2× bodyweight force) indicates exceptional grip strength. The muscle functional properties determined herein match well with the observed myological traits in Choloepus and provide further evidence of their abilities for frequent and prolonged suspension that are divergent from three-toed sloths.