Acute Effect of Extrinsic Foot Muscles Fatigue on Plantar Fascia and Achilles Tendon Stiffness: A Case-Crossover Design

ABSTRACT: Dönmez, S and Varol, F. Acute effect of extrinsic foot muscles fatigue on plantar fascia and Achilles tendon stiffness: A case-crossover design. J Strength Cond Res XX(X): 000-000, 2026-The extrinsic foot muscles (EFMs) originate in the leg and insert into the foot, providing dynamic support and movement. Extrinsic foot muscles are anatomically and functionally related to the plantar fascia (PF) and Achilles tendon (AT); therefore, the tissues may exhibit differential mechanical loading characteristics due to increased stresses in the foot. This study aimed to determine the acute effect of fatigue induced in the EFMs on the PF and AT biomechanical properties. Nineteen healthy young subjects were included in this study. The primary outcome measures were stiffness, measured using MyotonPRO at baseline and immediately after the fatigue task. Fatigue was induced by a protocol consisting of unilateral heel raise (UHR), closed chain-resisted foot adduction (CC-RFA), and a combination of these 2 exercises performed on the dominant lower limb in different weeks. Significant increases were found in the stiffness of the PF after fatigue induced by UHR and CC-RFA (p < 0.05). Achilles tendon stiffness did not significantly change after the exercises (p > 0.05). No significant difference was found between exercise types before and after fatigue (p > 0.05). In conclusion, exercise-induced fatigue of the EFMs results in a significant increase in PF stiffness and alters foot biomechanics. Practically, incorporating active recovery or myofascial release techniques following high-intensity EFM loading may help reduce PF tension and the risk of potential overuse injuries. From a preventive perspective, understanding these acute stiffness changes is essential for designing effective conditioning and rehabilitation programs that optimize performance and minimize injury risk.