The human patellar tendon is mechanically homogenous at its mid-substance

The human patellar tendon contains distinct fascicle bundles across its mediolateral and anteroposterior regions. Studies have suggested region-specific behaviour during in vivo actions, but it is unclear whether such regional differences result from localized variation in composition and mechanical properties within the tendon itself. Furthermore, the viscoelastic properties of any region of the human patellar tendon have not been well described previously. Here, a comprehensive investigation of the composition and material properties of six regions (three mediolateral × two anteroposterior) of the human patellar tendon was performed on tendons harvested from eight cadaver knees. Thorough viscoelastic (stress relaxation and sinusoidal) and elastic (failure) mechanical tests were conducted on dumbbell-shaped samples. Uronic acid (proteoglycan), hydroxyproline (collagen), and water contents were measured from the samples after mechanical testing. No systematic between-region differences were found for any measured biomechanical or biochemical parameter. However, the phase difference between stress and strain decreased as a function of increasing sinusoidal frequency (from 0.1 Hz to 5 Hz), suggesting the human patellar tendon behaves more elastically at higher strain rates. These results indicate the human patellar tendon is a homogenous material at its mid-substance and that other factors, such as geometrical constraints, enthesis properties, and insertion points, may be responsible for any region-specific behaviour in vivo. Additionally, the more elastic behaviour of the human patellar tendon as strain rate increases likely supports improved joint control and enhanced movement economy during fast actions such as sprinting.