Unloaded rat Achilles tendons continue to grow, but lose viscoelasticity

Abstract

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? Seventy-eight female 10-wk-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 1 or 6 wk 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 decreased 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 with control, and on modulus, which was increased (P = 0.008). Normalized glycosaminoglycan 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.

Mechanical Load and BMP Signaling During Tendon Repair: A Role for Follistatin?

“…This is not as surprising as it seems, because in previous experiments with similar rats, no or very minor effects on intact tendons were seen with unloading by Botox1; a large number of mechanical parameters were unaffected [5]. The tendons even continued to grow and mature normally in spite of dramatic muscle atrophy.…”

Genetic Variation, Protein Composition and Potential Influences on Tendon Properties in Humans

“…Both fibril diameter [29,54,55] and extent of crosslinking [56][57][58][59] are positively correlated with mechanical properties of tendon such as stiffness and Young's Modulus, in animal models. Additionally, this correlation has been reported in vivo in relation to fibril diameter in humans [55].…”

“…It may be that elevated expression levels of the MMP3 gene indicate a degenerative environment, putting the ECM in a state of imbalance with a greater rate of degradation compared to synthesis. The tendon homeostatic abilities would thus be compromised and substrates involved in cross-linking and stabilisation of the collagen fibril (Col V) may be degraded, ultimately weakening the material properties and resulting in a reduction in matrix stiffness [56][57].…”

Stimulation of tendon repair: mechanical loading, GDFs and platelets. A mini-review

“…In the latter, the gastrocnemius and soleus muscle complex had shrunk to a negligible size after 6 weeks. Moreover, the muscle remnants appeared to remain fully pliable, as tested by dorsi-flexion of the foot [9]. In rats with transected Achilles tendons, the botulinum toxin treatment had a drastic negative effect on tendon callus strength: 2 weeks after transection, force at failure was reduced to less than 30% of transected but loaded controls [25].…”