Effect of cyclic preconditioning on the failure load, strength, and stiffness of multiple rat tendons and ligaments

Purpose: Ligaments and tendons are frequently injured structures in sports activities. It has been well known that stretching before activities largely warms up muscles and prevents athletes from getting injuries. Recent studies have suggested that the tendon might also benefit from such preconditioning stretching; however, it is unclear whether the same benefit exists for all ligament and tendon structures. We hypothesized that the effect of preconditioning on mechanical strength is dependent on the type of ligament or tendon. Methods: Twenty-six anterior cruciate ligaments (ACLs), 22 medial collateral ligaments (MCLs), 20 patellar tendons (PTs), and 18 Achilles tendons (ATs) from euthanized mature Sprague-Dawley rats were used. The specimens in the control group were tested for load to failure without preconditioning. In the preconditioning group, 150 sinusoid cyclic stretchings were performed; the specimens were then consecutively loaded to ultimate failure. Results: Preconditioning significantly increased the mechanical strengths of the MCL, PT, and AT (p = 0.04, p = 0.006, and p = 0.002, respectively); however, it did not affect that of the ACL. There were no significant differences among the mechanical strengths of MCL, PT, and AT in the preconditioning group. Conclusion: Preconditioning significantly enhanced all ligament and tendon structures, except for the ACL. The results suggest that optimal preconditioning exists, but depends on the specific ligament or tendon. Different preconditioning strategies may be needed for different ligament and tendon structures in order to gain maximal protection from injuries.