Effect of foot orthoses on tibialis posterior activation in persons with pes planus

Purpose: To examine the influence of footwear on tibialis posterior (TP) activation in persons with pes planus. Methods: Six asymptomatic adults with pes planus (arch index of ≤0.16) participated. Subjects performed a resisted foot adduction with plantar flexion exercise (3 sets of 30 repetitions). The exercise was performed barefoot and shod with foot orthoses. The two testing conditions were separated by a week. Magnetic resonance image signal intensity of the tibialis posterior, tibialis anterior, soleus, medial gastrocnemius, and peroneus longus was measured immediately before and after each exercise. Multivariate analyses of variance followed by paired Student's t-test were performed for the signal intensity of each muscle assessed to determine whether TP was selectively activated during the barefoot and shod exercises. Results: When barefoot, five of the six subjects activated other lower-leg muscles in addition to TP. When wearing the foot orthoses and shoes, all five participants activated only TP. Additionally, activation of TP was higher when exercises were performed in shoes with orthoses than when barefoot (P = 0.019). Conclusion: Wearing the foot orthoses and shoes improved selective activation of the TP in persons with flat feet. In cases where selective activation of TP is desirable, such as persons with flat feet or TP tendon dysfunction, use of shoes and an arch supporting foot orthoses may enhance selective activation of the muscle.