Force transmission via axial tendons in undulating fish: a dynamic analysis

Area of Science:

• Biomechanics
• Fish Locomotion
• Muscle Physiology

Background:

• Fish swimming exhibits simple beam-like body bending, contrasting with complex internal muscle and tendon structures.
• The precise mechanical role of myoseptal tendons in this phenomenon remains incompletely understood.

Purpose of the Study:

• To investigate the mechanical functions of various myoseptal tendons in fish swimming.
• To understand how tendon structure contributes to the observed whole-body kinematics during locomotion.

Main Methods:

• Developed a computational model of a pumpkinseed sunfish (Lepomis gibbosus) incorporating experimentally derived physical and mechanical properties.
• Simulated swimming with steady muscle activity, accounting for dynamic stiffness, damping, muscle force generation, and tendon geometry.
• Ran multiple simulations varying tendon configurations and stiffness to assess their impact on forces and motion.

Main Results:

• Models with only intrasegmental tendons resulted in unstable forces and motion.
• A simulation including both intra- and intersegmental tendons, enhanced segment stiffness, and a stiffened caudal joint produced stable, large axial and lateral forces.
• Identified four key functions of axial tendons: moment transduction, segment coupling, force entrainment, and flexural stiffening.

Conclusions:

• Axial tendons play essential roles in converting axial muscle forces to moments and transmitting forces between segments.
• Intersegmental axial tendons are critical for distributing forces and stiffening vertebral joints, contributing to stable swimming.
• The integration of these tendon functions explains the seemingly simple, beam-like behavior observed in swimming fish.