Supination Resistance Test

The determination of the subtalar joint (STJ) axis location in relation to the plantar foot is an important clinical examination technique which allows appreciation of the pronation and supination moments acting across the STJ axis (Kirby, Kevin A.: "Methods for Determination of Positional Variations in the Subtalar Joint Axis", J. American Podiatric Medical Assoc., 77:228-234, May 1987; and, Kirby, Kevin A. Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997, pp. 49-52). Once this examination technique is mastered, it will be noted that patients with medially deviated STJ axes tend to have a maximally pronated STJ in relaxed calcaneal stance position (RCSP) and patients with laterally deviated STJ axes tend to have the STJ neutral or slightly supinated from neutral while standing in RCSP. However, once the foot becomes weightbearing, it is difficult to clinically determin e the spatial location of the STJ axis in relation to the plantar foot. Since the nonweightbearing method for determination of STJ axis location will not work while the patient is weightbearing, I have devised other methods by which to estimate the effects of variations in the location of the STJ axis while weightbearing. One of the methods which I have developed for the purpose of assessing the effects of varying positions of the STJ axis location in relation to the plantar foot is called the supination resistance test. I have been using the supination resistance test for the past twelve years both in my clinical practice and in teaching biomechanics to podiatrists and podiatry students. The test was first detailed in the literature in 1992 (Kirby, Kevin A., and Donald R. Green: "Evaluation and Nonoperative Management of Pes Valgus", pp. 295-327, in DeValentine, S.(ed), Foot and Ankle Disorders in Children. Churchill-Livingstone, New York, 1992, p. 314). In the supination resistance test, the patient is instructed to stand in their angle and base of gait, in RCSP. The patient should also be instructed to relax their feet during the test so that no extrinsic muscular contraction occurs. In other words, the patient should not try to either pronate or supinate their STJ while the test is being performed or the test will give unreliable results. Once the patient is standing in RCSP, the examiner places the tips of two of their fingers directly plantar to the medial aspect of the navicular in the medial longitudinal arch of the foot. (I prefer to use the tips of my index and middle fingers.) Then the examiner should pull directly superiorly on the medial navicular parallel to the long axis of the tibia (Fig. 1). When the examiner starts to pull superiorly on the medial navicular, it should be noted how the patient’s foot responds to this force. In addition, the examiner should note the magnitude of the force which is required to supinate the STJ from its resting position. Care must be taken while performing the test that the patient does not feel as if they are being pushed out of balance since this will make the patient use alternate muscles than what they normally would use while standing in RCSP. In general, if the test is performed as outlined above, the patient will not feel as if they are being pushed out of balance. In a foot which has a normal STJ axis location (i.e. passing posteriorly through the posterior-lateral calcaneus and anteriorly through the first intermetatarsal space area), the examiner will only have to exert a few pounds of digital force to cause STJ supination. As the STJ axis becomes more laterally located, a lesser magnitude of digital force is required to produce STJ supination. As the STJ axis becomes more medially located, a greater magnitude of force is required to produce STJ supination. If the STJ axis is severely medially deviated, so that the STJ axis passes directly over the medial aspect of the navicular, the examiner will not be able to supinate the STJ using the supination resistance test. The biomechanical principles behind the supination resistance test are relatively simple. The more lateral the location of the STJ axis while in RCSP, then the longer is the lever arm (i.e. moment arm) for the examiner to produce supination moment across the STJ axis by pulling superiorly on the medial navicular. The more medial the location of the STJ axis while in RCSP, then the shorter is the moment arm for the examiner to produce supination moment across the STJ axis by pulling superiorly on the navicular. If the STJ axis is so severely medially deviated that the STJ axis passes directly over the medial aspect of the navicular, there will be no moment arm for the examiner to produce supination moment across the STJ axis and the examiner will not be able to supinate the STJ even with a very large amount of digital force on the medial navicular. Like any other clinical test, the supination resistance test must be performed many times on a variety of patients with different foot types before the examiner will feel comfortable with it. However, it becomes obvious after just a few trials of the test, that it is an excellent method of gaining additional insight into the forces acting on the foot in relaxed bipedal stance. This is especially true when one considers that the area on the foot where the examiner pulls, on the medial navicular, is also the point of insertion of the posterior tibial muscle. Of all the muscles of the leg, it is the posterior tibial muscle which has the longest moment arm to produce STJ supination during weightbearing activities. I became impressed early on in performing the test as to how body weight has less effect on the amount of digital force required to supinate the STJ than the position of the STJ axis. Over the past twelve years, I have examined children who weigh 50 pounds with flexible flatfoot deformities which have required much greater magnitudes of digital force to supinate their STJ than 250 pound men with pes cavus deformities. The supination resistance test allows the examiner to "feel" the forces acting on the foot during weightbearing activities which would normally be difficult to measure and appreciate. Kevin A. Kirby, D.P.M. Director of Clinical Biomechanics