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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.

Keywords: ankle, foot, myotonometry, muscle fatigue

The findings of this study provide actionable insights for strength and conditioning specialists, physical therapists, and athletes focusing on foot health and performance. To mitigate the acute increase in PF stiffness following high-intensity calf or posterior tibialis loading, clinicians should prioritize combined (collective) muscle activation rather than isolated fatigue-inducing exercises. Combined exercises appear to promote a “pump effect” that facilitates active recovery and maintains better biomechanical balance. Since isolated fatigue of the EFMs leads to compensatory “stiffening” of the PF, coaches should monitor athletes for signs of localized fatigue during high-volume training. If isolated loading is necessary (e.g., in specific hypertrophy or rehabilitation phases), it should be followed by active recovery bouts or myofascial release techniques (e.g., foam rolling the plantar surface) to restore tissue compliance and reduce the risk of overuse injuries. These recommendations are particularly relevant for athletes involved in high-impact sports (running, jumping), where optimal foot stiffness is crucial for energy return, as well as clinical populations with posterior tibialis tendon dysfunction or pes planus, where managing the load on the PF is vital for long-term therapeutic success.