Muscle architectural properties indicate a primary role in support for the pelvic limb of three-toed sloths (Bradypus variegatus).

Tree sloths evolved below-branch locomotion making them one of few mammalian taxa beyond primates for which suspension is nearly obligatory. Suspension requires strong limb flexor muscles that provide both propulsion and braking/support, and available locomotor kinetics data indicate that these roles differ between fore- and hindlimb pairs. Muscle structure in the pelvic limb is hypothesized to be a key anatomical correlate of function in braking/support during suspensory walking and propulsion and/or support during vertical climbing. This expectation was tested by quantifying architecture properties in the hindlimb limb musculature of brown-throated three-toed sloths (Bradypus variegatus: N = 7) to distinguish the roles of the flexor/extensor functional muscle groups at each joint. Measurements of muscle moment arm (rm ), mass, belly length, fascicle length, pennation angle, and physiological cross-sectional area (PCSA) were taken from n = 45 muscles. Overall, most muscles studied show properties for contractile excursion and fast joint rotational velocity. However, the flexor musculature is more massive (p = 0.048) and has larger PCSA (p = 0.003) than the extensors, especially at the knee joint and digits where well-developed and strong flexors are capable of applying large joint torque. Moreover, selected hip flexors/extensors and knee flexors have modified long rm that can amplify applied joint torque in muscles with otherwise long, parallel fascicles, and one muscle (m. iliopsoas) was capable of moderately high power in B. variegatus. The architectural properties observed in the hip flexors and extensors match well with roles in suspensory braking and vertical propulsion, respectively, whereas strong knee flexors and digital flexors appear to be the main muscles providing suspensory support in the pelvic limb. With aid in support by the forelimbs and the use of adaptive slow locomotion and slow muscle fiber recruitment patterns, structure-function in the tensile limb systems of sloths appears to collectively represent an additional mechanism for energy conservation.

A Horse of a Different Color?: Tensile Strength and Elasticity of Sloth Flexor Tendons.

Tendons must be able to withstand the tensile forces generated by muscles to provide support while avoiding failure. The properties of tendons in mammal limbs must therefore be appropriate to accommodate a range of locomotor habits and posture. Tendon collagen composition provides resistance to loading that contributes to tissue strength which could, however, be modified to not exclusively confer large strength and stiffness for elastic energy storage/recovery. For example, sloths are nearly obligate suspenders and cannot run, and due to their combined low metabolic rate, body temperature, and rate of digestion, they have an extreme need to conserve energy. It is possible that sloths have a tendon "suspensory apparatus" functionally analogous to that in upright ungulates, thus allowing for largely passive support of their body weight below-branch, while concurrently minimizing muscle contractile energy expenditure. The digital flexor tendons from the fore- and hindlimbs of two-toed (Choloepus hoffmanni) and three-toed (Bradypus variegatus) sloths were loaded in tension until failure to test this hypothesis. Overall, tensile strength and elastic (Young's) modulus of sloth tendons were low, and these material properties were remarkably similar to those of equine suspensory "ligaments." The results also help explain previous findings in sloths showing relatively low levels of muscle activation in the digital flexors during postural suspension and suspensory walking.

Los tendones deben ser capaces de soportar las fuerzas de tracción generadas por los músculos para proporcionar apoyo evitando el fracaso. Por lo tanto, las propiedades de los tendones en las extremidades de los mamíferos deben ser apropiadas para acomodar una serie de hábitos locomotores y postura. La composición del colágeno de tendón proporciona resistencia a la carga que contribuye a la resistencia del tejido que, sin embargo, podría ser modificada para no conferir exclusivamente gran resistencia y rigidez para el almacenamiento/recuperación de energía elástica. Por ejemplo, los perezosos son tirantes casi obligatorios y no pueden funcionar, y debido a su baja tasa metabólica combinada, temperatura corporal y tasa de digestión, tienen una necesidad extrema de conservar energía. Es posible que los perezosos tengan un tendón «aparato suspensor¼ funcionalmente análogo al de los ungulados verticales, lo que permite un soporte en gran medida pasivo de su peso corporal por debajo de la rama, al tiempo que minimiza el gasto de energía contráctil muscular. Los tendones flexores digitales de las patas delanteras y traseras de los perezosos de dos dedos (Choloepus hoffmanni) y de tres dedos (Bradypus variegatus) fueron cargados en tensión hasta que no se probando esta hipótesis. En general, la resistencia a la tracción y el módulo elástico (de Young) de los tendones perezosos eran bajos, y estas propiedades materiales eran notablemente similares a las de los "ligamentos" suspensivos equinos. Los resultados también ayudan a explicar los hallazgos anteriores en perezosos que muestran niveles relativamente bajos de activación muscular en los flexores digitales durante la suspensión postural y la marcha suspensiva.