Intrinsic pedal musculature support of the medial longitudinal arch: an electromyography study.

Much of the work describing support of the medial longitudinal arch has focused on the plantar fascia and the extrinsic muscles. There is little research concerning the function of intrinsic muscles in the maintenance of the medial longitudinal arch. Ten healthy volunteer adults served as subjects for this study, which was approved by the University Investigational Review Board. The height of the navicular tubercle above the floor was measured in both feet while subjects were seated with the foot in a subtalar neutral position and then when standing in a relaxed calcaneal stance. Subtalar neutral was found by palpating for talar congruency. Recordings of muscle activity from the abductor hallucis muscle were performed while the subjects maintained a maximal voluntary contraction in a supine position by plantarflexing their great toes. An injection of lidocaine (1% with epinephrine) was then administered by a Board-certified orthopedic surgeon in the region of the tibial nerve, posterior and inferior to the medial malleolus. Measurements were repeated and compared by using a paired t test. After the nerve block, the muscle activity was 26.8% of the control condition (P =.011). This corresponded with an increase in navicular drop of 3.8 mm. (P =.022). The observation that navicular drop increased when the activity of the intrinsic muscles decreased indicates that the intrinsic pedal muscles play an important role in support of the medial longitudinal arch.