Shear wave propagation as a noninvasive metric of loading and microdamage in tendon fascicles.

Tendon damage and injury are often linked with repetitive loading, which can induce microdamage that diminishes the elasticity and strength of the tissue. The current study investigated whether wave propagation metrics could serve as a surrogate measure of microdamage in tendon fascicles. We extracted 18 tail tendon fascicles from tails of 9-month-old F344XBN rats and subjected them to cyclic loading to failure (0-5% peak strain, 0.5 Hz). Transient shear waves were induced via an impulsive tap applied transversely at 50 Hz. Wave propagation along the fascicle was tracked noninvasively using two laser Doppler vibrometers. Second harmonic generation (SHG) imaging confirmed that the fatigue protocol induced microdamage, characterized by kinked fibers, voids, and misalignment in longitudinal images. Wave speeds were well-described by a tensioned beam model, increasing in proportion to the square root of axial stress. Cyclic fatigue loading significantly reduced fascicle elasticity and diminished stress and wave speed at a given stretch. Notably, wave speeds also increased modestly with cyclic loading at a given stress level, suggesting stress may be elevated on intact structures as microdamage accumulates. These findings demonstrate that wave propagation metrics may provide a non-invasive, quantitative approach to tracking stress and fatigue-related changes in tendon material and structural properties.