Data from: Hindlimb muscle function in turtles: is novel skeletal design correlated with novel muscle function?

Variations in musculoskeletal lever systems have formed an important foundation for predictions about the diversity of muscle function and organismal performance. Changes in the structure of lever systems may be coupled with changes in muscle use and give rise to novel muscle functions. The two extant turtle lineages, cryptodires and pleurodires, exhibit differences in hindlimb structure. Cryptodires possess the ancestral musculoskeletal morphology, with most hip muscles originating on the pelvic girdle, which is not fused to the shell. In contrast, pleurodires exhibit a derived morphology, in which fusion of the pelvic girdle to the shell has resulted in shifts in the origin of most hip muscles onto the interior of the shell. To test how variation in muscle arrangement might influence muscle function during different locomotor behaviors, we combined measurements of muscle leverage in five major hindlimb muscles with data on muscle use and hindlimb kinematics during swimming and walking in representative semiaquatic cryptodires and pleurodires. We found substantial differences in muscle leverage between the two species. Additionally, we found that there were extensive differences in muscle use in both species, especially while walking, with some pleurodire muscles exhibiting novel functions associated with their derived musculoskeletal lever system. However, the two species shared similar overall kinematic profiles within each environment. Our results suggest that changes in limb lever systems may relate to changes in limb muscle motor patterns and kinematics, but that other factors must also contribute to differences in muscle activity and limb kinematics between these taxa.骨骼肌肉杠杆系统的变异，是预测肌肉功能多样性与机体运动表现的重要理论基础。杠杆系统结构的改变，往往与肌肉使用模式的变化相关联，并可催生全新的肌肉功能。现存龟类分为两大支系：隐颈龟类（Cryptodires）与侧颈龟类（Pleurodires），二者在后肢结构上存在显著差异。隐颈龟类保留了原始的骨骼肌肉形态：多数髋部肌肉起源于未与龟壳愈合的骨盆带。相较而言，侧颈龟类具有特化的骨骼肌肉形态：其骨盆带与龟壳愈合，导致多数髋部肌肉的起源位点转移至龟壳内部。为探究肌肉排列变异如何影响不同运动行为下的肌肉功能，本研究以代表性的半水生隐颈龟与侧颈龟为研究对象，整合了5块主要后肢肌肉的杠杆力臂测量数据，以及游泳与行走状态下的肌肉使用模式、后肢运动学数据。研究结果显示，两种龟类的肌肉杠杆力臂存在显著差异。此外，两种龟类的肌肉使用模式存在广泛差异，尤以行走状态下最为明显：部分侧颈龟类肌肉展现出与其特化骨骼肌肉杠杆系统相关的全新功能。但在同一运动环境中，两种龟类的整体运动学特征较为相似。本研究结果表明，肢体杠杆系统的改变可能与肢体肌肉运动模式及运动学特征的变化相关，但该类群间肌肉活动与肢体运动学的差异，还需其他因素共同解释。