Tendons can function as springs and thereby preserve energy during cyclic loading. They might also have damping properties, which, hypothetically, could reduce risk of microinjuries due to fatigue at sites of local stress concentration within the tendon. At mechanical testing, damping will appear as hysteresis. How is damping influenced by training or disuse? Does training decrease hysteresis, thereby making the tendon a better spring, or increase hysteresis and thus improve damping? 78 female 10 weeks old Sprague-Dawley rats were randomized to three groups. Two groups had botulinum toxin injected into the calf muscles to unload the left Achilles tendon through muscle paralysis. One of these groups was given doxycycline, as a systemic matrix metalloproteinase inhibitor. The third group served as loaded controls. The Achilles tendons were harvested after one or six weeks for biomechanical testing. An increase with time was seen in tendon dry weight, wet weight, water content, transverse area, length, stiffness, force at failure and energy uptake in all three groups (p<0.001 for each parameter). Disuse had no effect on these parameters. Creep was deceased with time in all groups. The only significant effect of disuse was on hysteresis (p=0.004) and creep (p=0.007), which both decreased with disuse compared to control, and on modulus, which was increased (p=0.008). Normalized glucosaminoglycan content was unaffected by time and disuse. No effect of doxycycline was observed. The results suggest that in growing animals, the tendons continue to grow regardless of mechanical loading history, whereas maintenance of damping properties requires mechanical stimulation.