Fatigue of the plantar intrinsic foot muscles increases navicular drop. (2008)

Abstract
Our purpose was to assess the effect of foot intrinsic muscle fatigue on pronation, as assessed with navicular drop, during static stance. Twenty-one healthy young adults participated. Navicular drop was measured before and after fatiguing exercise of the plantar foot intrinsic muscles. Surface electromyography of the abductor hallucis muscle was recorded during maximum voluntary isometric contractions (MVIC) in order to find the baseline median frequency (MedF). Subjects then performed sets of 75 repetitions of isotonic flexion contractions of the intrinsic foot muscles against a 4.55 kg weight on a custom pulley system. After each set an MVIC was performed to track shifts in MedF. After a MedF shift of at least 10%, navicular drop measurements were repeated. Subjects exhibited 10.0+/-3.8mm of navicular drop at baseline and 11.8+/-3.8mm after fatigue (p<0.0005). The change in navicular drop was significantly correlated with change in MedF (r=.47, p=.03). The intrinsic foot muscles play a role in support of the medial longitudinal arch in static stance. Disrupting the function of these muscles through fatigue resulted in an increase in pronation as assessed by navicular drop.

My comments:

This study goes along with my last blog, but rather than showing paralysis of the foot intrinsic muscles causes a loss of foot arch, these researchers found that merely fatiguing the muscles did the same, though to a lesser degree (~16% vs ~50% in the earlier study). As before, the take home message is that physical therapy programs for conditions such as plantar fasciitis and posterior tibial tendinitis need to address foot intrinsic muscle strength to be fully effective.

The problem is that it is hard to overload the muscles in the feet, especially the foot intrinsic muscles.  This is why I have been using EMS to target and strengthen those muscles. This was a method I first read about in the Charlie Francis training system, where he discussed EMS for the rehabilitation of track and field injuries. He did this specifically by having his athletes do the EMS while they stood on the electrodes (standing on the electrodes keeps your feet from cramping) for 10 minutes for a 10 second on, 50 second off duty cycle.  I tried it myself and have since used it with all my plantar fasciitis patients. Everyone agrees it works the foot intrinsic muscles much harder than can be achieved with active exercise.  The best part is most report they feel better immediately afterwards due to the pain relieving qualities of EMS.