Forelimb muscle activation patterns in American alligators: Insights into the evolution of limb posture and powered flight in archosaurs

Iijima, M; Mayerl, CJ; Munteanu, VD; Blob, RW

Abstract

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.e., 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. Electromyographic data from the forelimb muscles of American alligators indicate that alligators modulate the activity patterns of a primary shoulder adductor muscle across a range of limb postures. Among sauropsids, forelimb muscle burst phases from birds and turtles are distinct compared to those of alligators and lizards, suggesting that muscle activation patterns were influenced by major changes in body plan and locomotor modes.image

Citation

Iijima, M., Mayerl, C., Munteanu, V., & Blob, R. (2024). Forelimb muscle activation patterns in American alligators: Insights into the evolution of limb posture and powered flight in archosaurs. Austin Journal of Anatomy, https://doi.org/10.1111/joa.14011

• Journal Article Type: Article
• Acceptance Date: Jan 9, 2024
• Online Publication Date: Jan 19, 2024
• Publication Date: 2024
• Deposit Date: Feb 5, 2024
• Publicly Available Date: Feb 5, 2024
• Publisher: Austin Publishing Group
• Peer Reviewed: Peer Reviewed
• DOI: https://doi.org/10.1111/joa.14011
• Keywords: biomechanics; Crocodylia; electromyography; locomotion; reptile; MOTOR PATTERNS; FUNCTIONAL-ANATOMY; SOFT-TISSUES; TERRESTRIAL LOCOMOTION; HINDLIMB FUNCTION; IGUANA-IGUANA; WALKING; KINEMATICS; MECHANICS; SHOULDER