Introduction. Tendons (rotator cuff, Achilles and patellar tendons) are among the most commonly injured soft tissues . Many repairs/reconstructions have been attempted (e.g. sutures, resorbable biomaterials, autografts, and allografts) with varying success. A tissue engineered repair using mesenchymal stem cells (MSCs) is attractive  but tissue stiffness and strength need to be improved . The current study was undertaken to determine how mechanical stimulation in culture of MSC-type I collagen sponge constructs affect the biomechanics and histology of rabbit patellar tendon (PT) defect repairs 12 weeks after surgery. The hypotheses to be tested were that mechanical stimulation would: 1) significantly improve repair biomechanics and histological appearance, 2) result in higher failure forces than measured in vivo forces in normal rabbit PT during an inclined hopping activity , and 3) result in repairs that match the tangent stiffness of normal PT above these peak in vivo forces.