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.

诸多奔跑型（cursorial）及大型跳跃类物种均为极为高效的运动物种，其演化出的多种形态学适应特征被认为可降低机械负荷、提升运动效率，其中包括延长的肢体远端节段。然而，尽管小型跳跃类群（如沙漠袋鼠鼠（Dipodomys deserti））拥有延长的肢体，其运动效率却低于体型更大的同类，这在一定程度上可归因于它们需要通过爆发式跳跃动作躲避天敌。尽管两种运动模式可能存在相互冲突的机械负荷需求，但袋鼠鼠既是卓越的跳跃者，又能达到较高的跳跃速度，这可能与其特化的后肢肌肉骨骼形态（musculoskeletal morphology）有关。本研究结合实验解剖数据与基于全新开发的肌肉骨骼模型（musculoskeletal model）开展的肌肉力矩生成能力静态分析，对袋鼠鼠后肢的肌肉骨骼结构特征进行了表征，并探究了其形态如何演化以适配跳跃与爆发跳跃的机械负荷需求。该物种的后肢形态似乎偏向于在大范围关节活动范围内维持恒定的力矩臂（moment arms），这可通过减少对特定姿势与运动模式的神经激活需求，平衡相互冲突的运动需求。踝伸肌（ankle extensors）是多数肌肉所呈现的肌肉结构参数（如肌纤维长度（L_fibre））与关节力矩臂之间强正相关关系的一个主要例外。这类肌肉似乎适配跳跃所需的高力矩需求：踝伸肌的双关节（biarticular）特性被用于降低肌腱单元（muscle-tendon unit, MTU）的应变，并形成四连杆机构（four-bar linkage）以促进近端力传递。袋鼠鼠后肢为探究形态如何平衡跳跃与爆发跳跃之间时常存在的相互冲突的需求提供了极具价值的研究案例。