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Variable mechanical effect of anterior tibial muscle contraction

Discussion in 'Biomechanics, Sports and Foot orthoses' started by admin, Feb 25, 2006.

  1. admin

    admin Administrator Staff Member

    Members do not see these Ads. Sign Up.
    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.]
  2. admin

    admin Administrator Staff Member

  3. Anterior tibial muscle and shock absorption

    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.
  4. efuller

    efuller MVP

    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.

  5. davsur08

    davsur08 Active Member

    Does anterior tibial tendon location dictates STJ pronation/supination moments or vice versa?

    Unfortunatelyone 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.

    If Anterior tibial tendon is located lateral to the STJ axis creating a pronatory moment than how would shifting a mediallydeviated STJ to lateral during weightbearing have any impact on non-weightbearing phase? i mean during swing phase the Tib Ant activation would place the foot in a pronated position due to the tendon's lateral location with respect to the STJ axis. I have this doubt because the STJ axis can be located when a dorsally directed force is applied on the plantar foot. and a joint motion in a said plane is dependent on the muslce insertion and spatial location of its tendon (for example a subluxed tendon can alter its primary action at the joint proximal to its insertion). this is my understanding: anterior tibial tendon location is lateral to the talar head creating a pronatory moment during swing, 1) laxity of the extensor retinaculum or 2) bowstringing of the tendon secondary to chronic STJ pronation during weighbearing.
    Dr.Fuller's patient with Post Tib Dysfunction during non-weightbearing did manage to supinate the foot but he couldn't hold it for long before the foot spun in to pronation. In Post Tib Dysfunction the STJ is medially deviated then how did this man invert his foot in non-weightbearing. wouldn't the foot do the same during swing phase before heel strike?

    Iam just not able to grasp the concept. please help:bang:

    Another doubt:

    Because of its location relative to these joint axescontraction of the anterior tibial muscle may causerespectivelyan ankle joint dorsiflexion momenta LMTJ supination moment and a first ray dorsiflexion momentIn additionthe anterior tibial tendon normally passes medial to the subtalar joint (STJaxis which allows it to exert a supination moment across the STJ axis

    The supinatory action of the Tib Ant: i would imagine the first ray would plantarflex with supination then how does first ray dorsiflexion occur? It also decelerates the movement of the leg over the foot by eccentric contraction, i would imagine it would resist first ray dorsiflexion.

    Would really appreciate your help.


  6. davsur08

    davsur08 Active Member

    i think i pressed teh sent button twice and now i cant delete
  7. The location of the anterior tibial (AT) tendon relative to the subtalar joint (STJ) axis will determine whether the AT muscle will produce either a STJ supination moment, a STJ pronation moment or not STJ moment at all. Any rotational motion of the STJ after these moments are created will then change the position of the STJ axis spatial location and the position of the AT tendon relative to the STJ axis.

    Here are some references for you to look at that explain these concepts further.

    Kirby KA: Methods for determination of positional variations in the subtalar joint axis. JAPMA, 77: 228-234, 1987.

    Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989.

    Kirby KA, Green DR: 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.

    Kirby KA: Biomechanics of the normal and abnormal foot. JAPMA, 90:30-34, 2000.

    Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001.
  8. efuller

    efuller MVP

    The location of the tendon relative to the axis dictates the moment from tendon. When the tendon is lateral to the axis it creates a pronation moment and when the tendon is medial to the axis tension in the tendon will create a supination moment.

    Moments (from sources other than the ant. tib) about the STJ axis may cause the STJ move to either a more pronated or a more supinated position and this will effect the moment created by the anterior tibial tendon. Remember there are other sources of moment that may be acting at the same time as the anterior tibial muscle.

    It is quite hard to shift the position of the STJ axis significantly during stance phase. The position of the STJ axis is determined by joint facets on the talus and calcaneus. Then the talus externally rotates, the axis will externally rotate. The talus externally rotates with supination of the STJ. I haven't seen more than a couple of degree change of the STJ with orthoses. That is why I think that is quite hard to shift the location of the STJ axis.

    You only need to know where the tendon is and where the axis when the force is applied to figure out the moment from the tendon.

    The foot has inertia. So if you were to suddenly abduct your leg, the foot would be "left behind" and it would supinate. The intertial force, in this case would be the same as applying an external lateral to medial force on the foot and this would supinate the STJ. That is what happened when I went to test his strength. He shook his leg so that his STJ was in a more supinated position when I went to test his inversion strength. The ant tib muscle was strong, but it had a small lever arm, so it created a small supination moment. That is why it was easy to move the foot into pronation.

    The difficulty of doing this in swing phase of every step is that there is risk of supinating the STJ too much and causing a sprained ankle. It's been a few years since I saw this patient, so I don't remember exactly how he walked.

    Why would the first ray plantar flex with supination. They are two independent joints. The anterior tibial tendon pulls upward on the base of the first met. This upward pull causes a dorsiflexion moment on the first ray. There may be simultaneous moments from some other sorce (e.g. peroneus longus) that could be creating a plantar flexion moment at the same time the ant tib is acting. The muscle that creates the biggest moment will cause the motion seen. The motion seen will depend on the force in the tendon and the lever arm that tendon has at the joint in question.

  9. RobinP

    RobinP Well-Known Member

    I'm going to take this a little off topic from the theoretical to the practical if I may.

    I see quite a lot of race walkers who have a very particular gait. Some use the standard "wiggly hip" race walking technique and others, a "striding out" type of gait. This is usually dependent on the duration of the race. The particular race that I deal with walkers for is 85 miles in 24 hours.

    First of all, I have read the above printed newsletter before without really giving it's practical application much thought. So, my questions are

    1. If STJ axis is medially rotated and the effect of tib ant in terminal stance is negligable or slightly to act as an evertor, how can we influence this in order to improve prepositioning? Taping? But we might then be influencing closed chain moments - is this a problem?

    2. If tib ant is neglibible or working as an evertor, when the heel contacts the ground, pronation should still take place. As the sub talar joint axis will continue to internally rotate, can we treat stance phase problems related to excessive pronation moments by influencing the degree of internal rotation of the STJ axis at initial contact. i.e. taping to assist prepositioning the foot in slight supination and reducing the rate at which the STJ axis internally rotates.(The assumption being that the slower the STJ axis internally rotates, the smaller the likelihood that the residual pronation moments take a tissue outside of it's zone of optimal stress e.g. tib post)

    3. In race walkers, tib ant overusage is a problem, particularly in beginners. Does changing the STJ axis have an effect on the effectiveness off tib ant to a functional degree. Obviously, the greater the lever arm, the more effective the muscle at producing the supination. So can one reduce tib ant overusage in walkers by influencing the STJ axis alignment. I can't think of how the sub talar joint axis is in swing phase and whether the same things apply in open chain as in closed chain.

    I am probably being a little simplistic here but I would appreciate any comment. Also, if anyone sees a lot of race walkers, let me know, I would be interested in your thoughts on common injuries and treatment as it is something I am seeing increasingly

    Many thanks

  10. efuller

    efuller MVP

    If the theory doesn't apply to practice, it's not a very useful theory.

    The patient chooses the position of the STJ in swing and hence at contact. Non weight bearing (swing) just the slightest contraction of the POST tib will supinate the foot to the desired position. The foot is not fighting the ground in swing.

    It is quite difficult to change STJ position in stance more than a couple of degrees. To move the STJ more than a couple of degrees your treatment will often have to move the forces so much that they may become uncomfortable. However, if your treatment actually moves the STJ to a one degree inverted position from where it was in maximal pronation then you will have zero stress in the floor of the sinus tarsi. The great thing about tissue stress is that it explains why the foot feels better even though it did not change position.

    On the other hand if your device shifts too much pressure under the medial arch, this may become uncomfortable and the patient will use their posterior tibial muscle more to increase comfort. This may be ok in sinus tarsi syndrome, but may make things worse in PT dysfunction.

    The ant tib muscle has a much greater effect at the ankle joint, so you should look there when you have pain in the ant tib. The time at which the ant tib is under the most stress is inbetween heel contact and full contact/loading. The center of pressure of ground reaction force is behind the ankle joint axis and causes a plantar flexion moment. The ant tib causes a dorsiflexion moment so that the net plantar flexion moment is smaller and you get a comfortable foot slap.

    A shoe with a posterior flare will increase the plantar flexion moment from the ground. A beginner will have a weaker anterior tib. Those are things that you can address when you see ant tib overuse. Or, as in the case of my patient who was substituting ant tib for post tib because the post tib was damaged, you might see overuse from weakness elsewhere.

    So the importance of the ant tib in moments about the STJ is not that great. There are other muscles that have a much greater effect on the STJ. It is important to understand that. Another, indirect, effect of ant tib is that in stance it shifts the center of posterior posteriorly. This may decrease the pronation moment from the ground. However, to race walk, you need to shift the weight anteriorly to get ankle push off.

  11. I don't think the central nervous system (CNS) would ever recruit the posterior tibial muscle during swing since the posterior tibial is a stance phase muscle and one of its actions is to plantarflex the ankle, which would tend to make the foot plantarflex into the ground during swing, increasing the likelihood of tripping and falling.

    Rather, what I have clinically observed is that when the anterior tibial tendon is on the lateral side of the STJ axis, the CNS will still recruit the anterior tibial muscle, but, instead of the subtalar joint (STJ) supinating in late swing to let heel contact occur in a slightly supinated position, the STJ will stay maximally pronated throughout swing and heel contact will occur with the STJ in the maximally pronated position.
  12. I agree with Eric, if a theory doesn't make things more useful for you in your practice, then it probably isn't a very good theory.

    A well-constructed orthosis that alters the subtalar joint (STJ) axis position from a medially deviated position to a more normal position will allow the anterior tibial (AT) tendon to be medial to the STJ axis at end of stance phase so that it can then be used to allow proper STJ supination motion at the end of swing phase. In other words, by altering stance phase position of the STJ, the swing phase position of the STJ will also likely be altered.

    No, this is wrong. If the STJ is maximally pronated at heel contact, the STJ will not pronate further, since it is maximally pronated.

    As I stated earlier, stance phase forces from an orthosis will alter swing phase positioning of the foot. In addition, there is no reason why strapping therapy can't also be used along with orthosis therapy to influence the moments across the joints of the foot.

    I see a fair number of race walkers here in Sacramento since we have a fairly active race walking club here.

    Race walking causes AT muscle injury since the specific mechanics of race walking is to take longer strides than in normal walking. As a result, at heel contact, the heel will strike the ground more forcefully than in normal walking causing an increase in external ankle joint plantarflexion moment at the instant of heel contact. For beginning race walkers, until the AT muscle has developed adequate strength, then the liklihood of AT muscle/tendon pain and injuries will be increased due to this increased demand on the ankle joint dorsiflexors and the AT muscle, since it is the strongest ankle joint dorsiflexor. I will have the race walkers with AT muscle/tendon pain walk on a softer surface (i.e. treadmill or grass or all-weather track vs concrete and asphalt), or will have them get a shoe with a softer posterior sole or will even grind off a few millimeters of the posterior heel flare of the sole of the race walking shoe to help them with this problem. I typically won't treat these types of AT muscle injuries with orthoses in race walkers unless they have a significantly pronated foot.

    It must be remembered that race walking, even though it is walking (when done properly and not done "illegally" by "lifting" and/or "creeping"), has its own specific set of kinetics and kinematics and, as a result, will produce its own characteristic set of injuries in the race walker.
  13. RobinP

    RobinP Well-Known Member


    Many thanks for your responses.

    I just wrote a lengthy reply to these posts which has just been wiped - I must have been timed out or something.

    It is now 3.12am so I am going to go to bed and will redo the post again tomorrow. Suffice to say that with your detailed answers, i am understanding it better but have a couple of other questions now(always the way!)Goodnight
  14. efuller

    efuller MVP

    I don't think that the muscles are that hard wired that they cannot be used stance and swing. If the PT muscle was stance phase only we couldn't kick a ball as well. The muscles are used to perform the needed task. The force required to supinate the non weight bearing foot are so small it might not make the EMG rise significantlly above baseline levels.

    Also, the PT muscle has essentially 0 lever arm at the ankle. It's line of action is pointing right at the ankle joint axis so its effect on the ankle is negligable. Any plantar flexion effect from the PT would be easily be overcome by the dorsiflexion moment from the AT.

    Last edited: Mar 30, 2011
  15. Phil Wells

    Phil Wells Active Member


    Would it be worth trying to modify the direction and timing of the GRF at initial contact via flares to the footwear and soft heel inserts or SACH's?
    My approach with tib ant problems has always been to try and modify temporal variables - mainly in the sagittal plane - but this approach may also change the duration of moments acting on the stjt.

    I know that duration of load is often the under lying pathological mechanism rather than magnitude of load but have never been 100% how this can be applied when modeling moments of force across an axis e.g. if we accelerate/decelerate the CoP, how does this translate into pathological force?

    Hope this makes sense

  16. efuller

    efuller MVP

    A concept that you may find useful is impulse. Impulse is the magnitude of the force times the time that is applied. You could make a case that some pathology could be related to impulse. So, to reduce impulse you can either reduce time or magnitude of the force. If you had pathology of the anterior tibial muscle, you could decrease the lever arm of ground reaction force at heel contact, to reduce the force needed in the anterior tibial tendon. Yes, a SACH heel will tend to shift the center of pressure forward at heel contact and this will reduce the plantar flexion moment from the ground at the ankle joint at that point in time.

  17. Phil:

    I mentioned this earlier in post #12 of this thread:

    My thought is that by shifting the center of pressure slightly more anteriorly at heel contact, either by grinding off the posterior heel of the shoe or by using a softer heeled shoe or by even walking on a softer surface, then the external ankle plantarflexion moment will be decreased during walking which will lessen the demand on the anterior tibial muscle during early stance phase.
  18. Phil Wells

    Phil Wells Active Member


    Thanks for clarifying impulse in relation to 'real' world mechanics. This approach fits well with Craig's approach to variable density heel raises which I have found effective.

    Would I be correct in assuming that impulse is the underlying mechanism for explaining rocker shoe modifications or are they more about vectors in the sagital plane?


  19. efuller

    efuller MVP

    Phil, Which kind of rocker.

    An anterior rocker that ends just proximal to the metatarsal heads works by making the shoe rotate with the foot as the center of pressure under the foot moves anterior to the rocker. When this happens there is increased duration of force on the heel and decreased magnitude of forces on the forefoot.


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