Achilles Tendon Properties of Minimalist and Traditionally Shod Runners

Context: Tendon adapts to load through alterations in its composition and mechanical properties. Mechanical adaptation to increased load often involves increases in cross-sectional area (CSA), stiffness, and modulus. Runners exhibit these adaptations. Objective: To determine if runners wearing minimalist shoes had larger and stiffer Achilles tendons (AT) than traditionally shod runners. Design: Cross-sectional study of well-trained, traditionally and minimally shod runners. Setting: Laboratory assessment of trained runners. Participants: 23 men (11 traditional, 12 minimalist) and 8 women (6 traditional, 2 minimalist). Runners wearing minimalist shoes had 4.2 +/- 1.6 y of training experience in minimalist shoes. Main Outcome Measures: The authors used diagnostic ultrasound and isokinetic dynamometry to generate a force-elongation curve and its derivatives. Results: Minimalist runners had a greater CSA: mean difference (MD) = 9.2 mm(2), stiffness (MD = 268.1 N/mm), and modulus (MD = 202.9 MPa). ATs of minimalist runners experienced greater stress (MD 8.6 N/mm(2)) during maximal voluntary isometric contraction of the plantar-flexor muscles due to greater force of contraction (MD 798.9 N). Conclusion: The AT in minimalist runners adapts by increasing size, stiffness, and modulus, which is consistent with our understanding of mechanical adaptation of tendon to increased loading. Increased stress to the AT likely requires a slow transition to minimalist running to allow the AT to adapt without evidence of injury.