<b>Forelimb muscle activation patterns in American alligators: insights into the evolution of limb posture and powered flight in archosaurs</b>

The evolution of archosaurs provides an important context for understanding the mechanisms behind major functional transformations in vertebrates, such as shifts from sprawling to erect limb posture and the acquisition of powered flight. While comparative anatomy and ichnology of extinct archosaurs have offered insights into musculoskeletal and gait changes associated with locomotor transitions, reconstructing the evolution of motor control requires data from extant species. However, the scarcity of electromyography (EMG) data from the forelimb, especially of crocodylians, has hindered understanding of neuromuscular evolution in archosaurs. Here, we present EMG data for nine forelimb muscles from American alligators during terrestrial locomotion. Our aim was to investigate the modulation of motor control across different limb postures and examine variations in motor control across phylogeny and locomotor modes. Among the nine muscles examined, m. pectoralis, the largest forelimb muscle and primary shoulder adductor, exhibited significantly smaller mean EMG amplitudes for steps in which the shoulder was more adducted (<i>i.e.</i>, upright). This suggests that using a more adducted limb posture helps to reduce forelimb muscle force and work during stance. As larger alligators use a more adducted shoulder and hip posture, the sprawling to erect postural transition that occurred in the Triassic could be either the cause or consequence of the evolution of larger body size in archosaurs. Comparisons of EMG burst phases among tetrapods revealed that a bird and turtle, which have experienced major musculoskeletal transformations, displayed distinctive burst phases in comparison to those from an alligator and lizard. These results support the notion that major shifts in body plan and locomotor modes among sauropsid lineages were associated with significant changes in muscle activation patterns.初龙类（archosaurs）的演化，为理解脊椎动物主要功能转变的内在机制提供了关键背景，例如从匍匐肢体姿势到直立肢体姿势的转变，以及动力飞行的演化获得。尽管针对已灭绝初龙类的比较解剖学与足迹学研究，已为与运动转变相关的肌肉骨骼及步态变化提供了诸多见解，但要重建运动控制的演化历程，仍需现生物种的相关数据。然而，前肢肌电图（electromyography, EMG）数据的匮乏，尤其是鳄类的相关数据，阻碍了学界对初龙类神经肌肉演化的认知。本研究针对美国短吻鳄在陆地运动过程中的9块前肢肌肉采集了EMG数据，旨在探究不同肢体姿势下运动控制的调控模式，并检验运动控制在系统发育与运动模式间的差异。在所检测的9块肌肉中，作为前肢最大肌肉与主要肩关节内收肌的胸肌（m. pectoralis），在肩关节更内收（即直立姿势）的步幅中，其平均EMG振幅显著更低。这表明采用更内收的肢体姿势，有助于减少支撑相期间前肢肌肉的发力与做功。由于体型更大的短吻鳄会采用更内收的肩、髋关节姿势，因此三叠纪时期发生的从匍匐到直立的姿势转变，可能是初龙类体型增大演化的原因，亦或是其结果。对四足动物EMG放电相位的比较分析显示，经历过重大骨骼肌肉改造的鸟类与龟类，其放电相位与短吻鳄和蜥蜴存在显著差异。上述结果支持了这一学术观点：蜥形支系中体型格局与运动模式的重大转变，与肌肉激活模式的显著变化密切相关。