Tendon stiffness

Tendon stiffness is the mechanical property describing how much force a tendon transmits per unit of elongation (ΔForce/ΔLength, typically reported in N/mm); the related material property Young's modulus normalises stiffness for cross-sectional area and resting length. In vivo, stiffness is measured by combining ultrasound imaging of the tendon under isometric muscle contraction with dynamometry. The systematic review and meta-analysis by Bohm, Mersmann and Arampatzis (Sports Medicine - Open, 2015) found that high-magnitude loading (around 80–90 % of maximal voluntary contraction) and longer durations (around 3 seconds per repetition) - as in heavy-slow or isometric protocols - produce the largest gains in stiffness, modulus, and cross-sectional area. Aging, disuse, and tendinopathy reduce stiffness; well-trained tendons store and return elastic energy more efficiently, improving running economy and reducing injury risk.