Mossor A M, Austin B L, Avey-Arroyo J A, Butcher M T
Department of Biological Sciences, Youngstown State University, Youngstown, OH USA.
The Sloth Sanctuary of Costa Rica, Limon, Costa Rica.
Integr Org Biol. 2020 Oct 26;2(1):obaa032. doi: 10.1093/iob/obaa032. eCollection 2020.
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 () and three-toed () 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.
肌腱必须能够承受肌肉产生的拉力，以提供支撑，同时避免断裂。因此，哺乳动物肢体中肌腱的特性必须适合适应一系列运动习惯和姿势。肌腱胶原蛋白的组成提供了对负荷的抵抗力，这有助于组织强度，然而，这种强度可以被改变，并非仅仅为了弹性储能/恢复而赋予高强度和刚度。例如，树懒几乎完全是悬垂动物，不能奔跑，并且由于它们低代谢率、体温和消化率的综合影响，它们极度需要保存能量。有可能树懒有一种肌腱“悬垂装置”，其功能类似于直立有蹄类动物的悬垂装置，从而在树枝下方很大程度上被动地支撑它们的体重，同时将肌肉收缩能量消耗降至最低。对二趾树懒和三趾树懒前后肢的指屈肌腱施加拉伸负荷直至断裂，以验证这一假设。总体而言，树懒肌腱的拉伸强度和弹性（杨氏）模量较低，这些材料特性与马的悬垂“韧带”非常相似。这些结果也有助于解释之前在树懒身上的发现，即在姿势悬垂和悬垂行走过程中，指屈肌中的肌肉激活水平相对较低。
