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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 mm2, stiffness (MD = 268.1 N/mm), and modulus (MD = 202.9 MPa). ATs of minimalist runners experienced greater stress (MD 8.6 N/mm2) 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.