Reilly Stephen M, Willey Jeffrey S, Biknevicius Audrone R, Blob Richard W
Department of Biological Sciences, Ohio University, Athens, OH 45701, USA.
J Exp Biol. 2005 Mar;208(Pt 6):993-1009. doi: 10.1242/jeb.01473.
Alligator hindlimbs show high torsional loads during terrestrial locomotion, in sharp contrast to the bending or axial compressive loads that predominate in animals that use parasagittal limb movements. The present study integrates new data on hindlimb muscle function with previously obtained data on hindlimb kinematics, motor patterns, ground reaction forces and bone strain in order to (1) assess mechanisms underlying limb bone torsion during non-parasagittal locomotion in alligators and (2) improve understanding of hindlimb dynamics during terrestrial locomotion. Three dynamic stance phase periods were recognized: limb-loading, support-and-propulsion, and limb-unloading phases. Shear stresses due to torsion were maximized during the limb-loading phase, during which the ground reaction force (GRF) and caudofemoralis (CFL) muscles generated opposing moments about the femur. Hindlimb retraction during the subsequent stance-and-propulsion phase involves substantial medial rotation of the femur, powered largely by coordinated action of the GRF and CFL. Several muscles that actively shorten to flex and extend limb joints during stance phase in sprawling and erect quadrupeds act in isometric or even eccentric contraction in alligators, stabilizing the knee and ankle during the support-and-propulsion phase. Motor patterns in alligators reveal the presence of local and temporal segregation of muscle functions during locomotion with muscles that lie side by side dedicated to performing different functions and only one of 16 muscles showing clear bursts of activity during both stance and swing phases. Data from alligators add to other recent discoveries that homologous muscles across quadrupeds often do not move joints the same way as is commonly assumed. Although alligators are commonly considered models for early semi-erect tetrapod locomotion, many aspects of hindlimb kinematics, muscle activity patterns, and femoral loading patterns in alligators appear to be derived in alligators rather than reflecting an ancestral semi-erect condition.
短吻鳄的后肢在陆地运动过程中承受着很高的扭转载荷，这与在进行侧矢状肢体运动的动物中占主导地位的弯曲或轴向压缩载荷形成了鲜明对比。本研究将后肢肌肉功能的新数据与先前获得的后肢运动学、运动模式、地面反作用力和骨应变数据相结合，以便：（1）评估短吻鳄非侧矢状运动过程中肢体骨扭转的潜在机制；（2）增进对陆地运动过程中后肢动力学的理解。识别出了三个动态站立期阶段：肢体加载期、支撑与推进期和肢体卸载期。由于扭转产生的剪应力在肢体加载期达到最大值，在此期间地面反作用力（GRF）和尾股肌（CFL）对股骨产生相反的力矩。在随后的支撑与推进期，后肢回缩涉及股骨的大量内旋，主要由GRF和CFL的协同作用驱动。在 sprawling 和直立四足动物的站立期，一些主动缩短以屈曲和伸展肢体关节的肌肉在短吻鳄中以等长收缩甚至离心收缩的方式起作用，在支撑与推进期稳定膝盖和脚踝。短吻鳄的运动模式揭示了运动过程中肌肉功能存在局部和时间上的分离，并排的肌肉专门执行不同的功能，并且在16块肌肉中只有一块在站立期和摆动期都显示出明显的活动爆发。来自短吻鳄的数据补充了其他近期的发现，即四足动物的同源肌肉通常并不像通常所认为的那样以相同的方式移动关节。尽管短吻鳄通常被视为早期半直立四足动物运动的模型，但短吻鳄后肢运动学、肌肉活动模式和股骨载荷模式的许多方面似乎是短吻鳄特有的，而不是反映祖先的半直立状态。
