Passive Mechanical Properties of Extrinsic Foot Muscles and Achilles Tendon in Adults with and Without Pes Planus

The purpose of the present study was to investigate the changes in stiffness, tone, and elasticity of extrinsic foot muscles and Achilles tendon in adults with pes planus at rest and during standing. The study was conducted with 59 participants, 29 with pes planus and 30 with normal foot posture. The oscillation frequency (indicator of tone), dynamic stiffness (indicator of stiffness), and logarithmic decrement (related to elasticity) of the Achilles tendon, peroneus longus, tibialis anterior, and medial and lateral gastrocnemius muscles were measured with a myotonometer (MyotonPRO, Myoton AS, Estonia). The passive mechanical properties of the selected muscles and tendon were measured at rest and during standing. The oscillation frequency, dynamic stiffness, and logarithmic decrement of the peroneus longus, tibialis anterior, and medial and lateral gastrocnemius muscles were similar in individuals with and without pes planus (p < 0.05). Individuals with pes planus had higher dynamic stiffness of the Achilles tendon at rest (p = 0.042; d = 0.431), whereas they had lower dynamic stiffness of the Achilles tendon with a moderate effect size during standing compared to controls (p = 0.028; d = 0.640). The logarithmic decrement of the Achilles tendon in individuals with pes planus was significantly lower with a large effect size during standing (p = 0.025; d = 0.945). The results obtained suggest that pes planus is not related to the passive mechanical properties of the foot extrinsic muscles. A decrease in stiffness and an increase in elasticity during standing, and an increase in stiffness at rest in the Achilles tendon were found in individuals with pes planus.