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I am grateful to Kevin Kirby and Precision Intricast for permission to reproduce this April 2001 Newsletter (you can buy the 2 books of newsletters off Precision Intricast):


The anterior tibial muscle is a very important extrinsic muscle of the foot that has its insertion onto the inferior-medial aspect of the first metatarsal base and the medial aspect of the first cuneiform (Sarrafian, Shahan K.: Anatomy of the Foot and Ankle, J.B. Lippincott Co., Philadelphia, 1983). Its tendon crosses anterior to the ankle joint axis, medial to the longitudinal midtarsal joint (LMTJ) axis and distal to the first ray axis. Because of its location relative to these joint axes, contraction of the anterior tibial muscle may cause, respectively, an ankle joint dorsiflexion moment, a LMTJ supination moment and a first ray dorsiflexion moment. In addition, the anterior tibial tendon normally passes medial to the subtalar joint (STJ) axis which allows it to exert a supination moment across the STJ axis.

Root, Orien and Weed described the anterior tibial muscle as having both accelerating and decelerating functions during walking gait. Accelerating functions of the anterior tibial muscle include 1) assisting ankle joint dorsiflexion at toe off; 2) dorsiflexing the first ray in swing; 3) assisting in toe clearance during midswing; and, 4) supinating the STJ in late swing in preparation for heel strike. Decelerating functions of the anterior tibial muscle include 1) prevention of excessive STJ pronation during swing; 2) supination of the longitudinal midtarsal joint prior to heel strike in preparation for forefoot contact [this action of the anterior tibial muscle actually is an acceleration, not a deceleration function]; 3) deceleration of ankle joint plantarflexion immediately after heel strike, and 4) allowing smooth lateral to medial loading of the forefoot on the ground during the contact phase of gait (Root, M.L., W.P. Orien and J.H. Weed: Normal and Abnormal Function of the Foot. Clinical Biomechanics Corporation, Los Angeles, CA, 1977, pp. 185-189).

Unfortunately, one of the factors that complicate the complete description of anterior tibial muscle function is that its tendon has a variable location relative to the spatial location of the STJ axis. During STJ supination motion, the insertion of the anterior tibial will rotate to a more medial location relative to the STJ axis, greatly increasing the supination moment arm that is available for anterior tibial muscle contraction to cause STJ supination moment. During STJ pronation motion, the insertion of the anterior tibial will rotate to either a less medial or a more lateral location relative to the STJ axis. In effect, lateral rotation of the anterior tibial insertion with STJ pronation either decreases the supination moment arm for the anterior tibial muscle or, if the insertion rotates laterally past the STJ axis, a pronation moment arm for the anterior tibial muscle will result. In addition, a foot with a medially deviated STJ axis may have the STJ axis positioned medial to the anterior tibial insertion that allows the anterior tibial muscle to have a pronation moment arm (Fig. 1). Feet with either a normal STJ axis spatial location or a laterally deviated STJ axis will have their anterior tibial insertion medial to the STJ axis which allows the anterior tibial muscle to have a supination moment arm (Kirby, Kevin A.: "Rotational Equilibrium Across the Subtalar Joint Axis", JAPMA, 79: 1-14, January 1989).



Figure 1. In a foot that has a normal subtalar joint (STJ) axis location (right), the anterior tibial tendon is medial to the STJ axis so that its contraction causes a STJ supination moment. However, in a foot with a medially deviated STJ axis (left), the anterior tibial tendon may be lateral to the STJ axis so that its contraction causes a STJ pronation moment.

As a result of this variable position of the insertion of the anterior tibial muscle relative to the STJ axis, the anterior tibial may function either to cause an increase in supination or pronation moments across the STJ axis. Therefore, the functions of the anterior tibial muscle that have been classically described may not apply to the foot that is maximally pronated at the STJ or that has a STJ axis that is significantly medially deviated. Since feet which are maximally pronated or have significant medial deviation of the STJ axis are quite common and constitute a large proportion of the feet which are treated by podiatrists, then the importance of this variability of anterior tibial muscle function across the STJ axis should not be minimized.

Probably one of the largest effects on walking gait of having the anterior tibial muscle insertion lateral to the STJ axis (e.g. due to a medially deviated STJ axis) is during the swing phase of gait. During swing phase, the anterior tibial muscle is one of the strongest dorsiflexors of the ankle joint, allowing for proper toe clearance. The anterior tibial muscle is also important in producing supination motion of the STJ during the latter half of swing phase in order to prepare the foot for heel contact with the STJ in a slightly supinated position. If the heel does not contact the ground with the STJ slightly supinated (or close to neutral position), then there will be little STJ pronation range of motion available during contact phase to allow the pronation motion which is necessary for normal shock absorption (Root, et al, 1997, pp. 151-153).

It is also important to understand that the anterior tibial muscle is the only one of the ankle joint dorsiflexors that also has a tendon that lies medial to the STJ axis, and therefore, has the potential to cause STJ supination. If the anterior tibial tendon is not medial to the STJ axis when its ankle joint dorsiflexion moment is needed during late swing, then no STJ supination moment can also occur during late swing to supinate the STJ into a slightly supinated position. The likely result is that the foot will stay maximally pronated at the STJ throughout the swing phase and the STJ will also be maximally pronated at heel contact at the beginning of the contact phase of gait.

The resultant lack of available STJ pronation range of motion during contact phase will greatly decrease the vital shock absorbing ability of the foot during the stance phase of walking gait. Increased incidence of shock related symptoms such as degenerative joint disease, muscle spasm and chronic low back pain may result due to the lack of available STJ pronation range of motion during the contact phase of walking gait (Root, et al, 1977, p. 153). Therefore, normal function of the anterior tibial muscle greatly depends on a normal spatial location of the STJ axis since alterations in STJ axis location relative to the anterior tibial muscle insertion can greatly affect the STJ rotational forces that result from its contraction.

[Reprinted with permission from: Kirby KA.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 95-96.]

Kevin's newsletter explains in a great way a previous thread:

Trick question for a podiatry student....

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Even though the anterior tibial muscle rarely becomes a strong pronator of the STJ even with a very medially deviated STJ axis, if the STJ axis only medially deviates enough to cause the STJ axis to pierce through the anterior tibial tendon (losing all STJ supination moment arm), then this will cause significant alteration in gait function, especially during late swing phase.

The anterior tibial muscle is the only ankle joint dorsiflexor that also has a STJ supination moment arm to cause STJ supination during late swing phase at a time when supination should occur to prevent the STJ from being maximally pronated at heel contact. If the STJ supination moment from the anterior tibial is totally lost by significant STJ axis medial devation, then the individual has no ankle joint dorsiflexor muscle that also has the ability to supinate the foot in late swing to prepare the foot to be able to have some pronation range of motion during the contact phase of gait. Without contact phase STJ pronation available, then the body loses one of its most important shock absorbing mechanisms. Without adequate contact phase shock absorption with each walking step, the risk of developing significant pathology within the locomotor apparatus greatly increases over time.

__________________
Sincerely,

Kevin

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Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
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Voice: (916) 925-8111 Fax: (916) 925-8136
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I recall a posterior tibial tendon dysfunction patient who had no contractile ability in the PT tendon clinically. However, when you went to test inversion strength, just before you would grab the foot he would shake his foot so that it inverted. He would contract the anterior tibial tendon and there was some resistance to pronation. It would take little force to pronate the STJ and when it reached a point in the middle of the range of motion he would relax his ant. tib and the STJ would go easily to the end of range of motion in the direction of pronation. This is easily explained by Kevin's paper. The patient "learned" that he could create supination moment with his ant. tib in a more supinated position, when the insertion of the tendon was on the supination side of the axis, and when the joint moved and the insertion moved to the pronation side of the axis he "knew" that he could not generate a supination moment.

Eric