Axial muscle function during lizard locomotion

Area of Science:

• Vertebrate Paleontology
• Comparative Biomechanics
• Evolutionary Biology

Background:

• Epaxial muscles in lizards were previously thought to drive lateral bending during locomotion.
• Recent findings suggested epaxial muscles stabilize the trunk in Varanus salvator, potentially a shared amniote trait.
• This challenged the established understanding of lizard locomotion mechanics.

Purpose of the Study:

• To investigate if epaxial muscle activity in a basal lizard (Iguana iguana) reflects a primitive trunk stabilization role.
• To identify which muscles are responsible for lateral bending if epaxial muscles are not.
• To determine if trunk stabilization by epaxial muscles is a basal lizard or amniote characteristic.

Main Methods:

• Collected synchronous electromyographic (EMG) and kinematic data from Iguana iguana and Varanus salvator during walking and running.
• Recorded EMG activity from epaxial muscles of Iguana iguana.
• Recorded EMG activity from hypaxial muscles of both species.

Main Results:

• Epaxial muscle activity timing in Iguana iguana mirrors that in Varanus salvator, supporting a trunk stabilization role.
• This pattern suggests trunk stabilization by epaxial muscles is a basal feature in lizards and amniotes.
• External oblique and rectus abdominis (hypaxial muscles) showed activity patterns consistent with producing lateral bending.

Conclusions:

• The epaxial muscles of lizards primarily stabilize the trunk during locomotion.
• Trunk stabilization by epaxial muscles is likely a basal trait for lizards and amniotes.
• Hypaxial muscles are the main drivers of lateral bending in lizard locomotion.