The Role of Type I Collagen Molecular Structure in Tendon Elastic Energy Storage

In order to facilitate locomotion and limb movement many animals store energy elastically in their tendons. The formation of crosslinked collagen fibers in tendons results in the conversion of weak, liquid-like embryonic tissues into tough elastic solids that can store energy and perform work. Collagen fibers in the form of fascicles are the major structural units found in tendons. The purpose of this paper is to review the literature on collagen self-assembly and tendon development and to relate this information to the development of elastic energy storage in nonmineralizing and mineralizing tendons. Of particular interest is the mechanism by which energy is stored in tendons during locomotion. In the turkey, much of the force generated by the gastrocnemius muscle is stored as elastic energy during tendon deformation and not within the muscle. As limbs move, the tendons are strained, causing the collagen fibers in the extracellular matrices to stretch. Through the analysis of turkey tendons, collagen fibers, and a molecular model, it is hypothesized that elastic energy is stored in the flexible regions of the collagen molecule. Data from the molecular model, mineralized fibers, and turkey tendons show that the presence of calcium and phosphate ions causes an increase in elastic energy stored per unit strain. Based on the theoretical modeling studies, the increase in stress with strain is a result of the initiation of stretching of the rigid regions of collagen molecules. INTRODUCTION Elastic energy storage is an extremely important mechanical characteristic of collagenous tissues; tendons and ligaments are examples of musculoskeletal tissues that store and transmit energy elastically during mechanical deformation [1-3]. Tendons are necessary for the transference of energy from contracting muscles to bones, leading to the movement of limbs. In the turkey, much of the force generated by the gastrocnemius muscle is stored as elastic energy during tendon deformation and not within the muscle. In animals during normal gait, the body is decelerated as the foot lands on the ground, causing kinetic energy to be stored as strain energy in the muscles and tendons that are stretched by the impact with the ground. Elastic recoil in the tendons converts most of the stored energy into kinetic energy as the foot leaves the ground [1, 2]. In contrast to the energy storage function of tendon, ligaments absorb energy during movement in order to protect joints from damage. The anterior cruciate ligament (ACL) prevents damage to the knee by absorbing energy during movement that could lead to anterior translation of the tibia. Mater. Res. Soc. Symp. Proc. Vol. 874 © 2005 Materials Research Society L1.6.1