Comparative mechanical and elastic properties of the dorsal and ventral tendons in the peduncle of harbor porpoise (<i>Phocoena phocoena</i>)

Cetaceans swim via vertical movements of the tail. The tendons located in the caudal peduncle are attached to the caudal vertebrae to generate propulsive oscillations. Arguments have centered on whether the upstrokes and downstrokes of the tail and propulsive flukes are symmetrical or asymmetrical in time. Previous research from kinematics of swimming animals, muscle architecture and histology has supported both conditions. However, the composition and structure of the tendons suggest a potential mechanism to evaluate this disparity. In this study, the tendons of the caudal peduncle of the harbor porpoise (Phocoena phocoena) - specifically, the extensor caudae medialis (ECM) and the extensor caudae lateralis (ECL) from the epaxial muscle, and the medial hypaxialis lumborum (MHL) from the hypaxial muscle - were mechanically tested. Ramp to failure was performed on isolated tendon fascicles. Stress relaxation tests to 3% strain were also performed on fascicles. Polarized light microscopy was used to visualize the fibril crimp as tensile forces were applied to fascicles. Uncrimping of isolated fascicles was visualized at mean strain values between 0.031% and 0.048%. The maximum elastic moduli of fascicles taken to failure were between 1039.5 and 1185.8 MPa. No differences were found in the mechanical performance of the fascicles of the epaxial and hypaxial tendons. The mechanical properties of peduncle fascicles suggest a symmetrical stroke cycle for swimming by the porpoise.