Intrinsic pedal musculature support of the medial longitudinal arch: an electromyography study. (2003)

Abstract
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.

My comments:

This is a facinating study from 2003, which demonstrated that the smaller muscles inside the foot are very important for supporting the foot arch.  The study demonstrated this by injecting lidocaine into the nerve controlling those muscles, thus temporarily paralyzing them. Doing so they found the the navicular bone drop (used to measure foot arch) increased from ~6mm to 9 mm (a 50% increase).  Thus foot muscle weakness in real life would significantly increase stress on other arch supporting structures including the plantar fascia and posterior tibial tendon,  likely contributing to plantar fasciitis and posterior tibial tendinitis, respectively.

This fits in with other studies I have blogged on that found patients with plantar fasciitis do, in fact, have smaller and weaker foot intrinsic muscles in addition to tighter and weaker calf muscles. It would also explain why current treatment protocols focusing on stretch and orthotics but ignoring strength training are only marginally successful.  So the take home message for those with foot pain, physical therapists, and others treating foot injuries is that foot intrinsic muscle strength needs to be restored. This will lessen stress on the foot arch and help promote optimal recovery from conditions like plantar fasciitis and posterior tibial tendinitis.  This should also go a long way towards preventing more debilitating conditions down the road, such as acquired flat foot deformity.