Section: CME Category: Biomechanics

Phasic Activity of the Muscles of the Lower Extremity Biomechanics

Harold Schoenhaus, DPM

Harold Shoenhaus, DPM discusses the effects of extrinsic muscles on the lower leg during gait as well as describing the concept of antagonist and synergistic muscle activity.

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Goals and Objectives
  1. Review the extrinsic muscles of the lower leg
  2. Compare the activity of each muscle that influences foot function during gait
  3. Describe the concept of antagonist and synergistic activity
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    Harold Schoenhaus has nothing to disclose.

  • Lecture Transcript
  • TAPE STARTS – [00:00]


    Male Speaker: Our next speaker comes from Pennsylvania. He's been in practice for 49 years. An old timer who has always had a love for biomechanics and a love for surgery, and I've always felt that in order to be a good surgeon, reconstructive or otherwise, you need to have a thorough foundation and knowledge in biomechanics. And as part of our program for the last few years, we've had talks on biomechanics.

    Sounds somewhat boring at times but extremely important and very influential on some of the surgical procedures we select to do. So what I would thought I would talk about today, the speaker is myself by the way. I won't tell you credentials, it's not necessary but I've been on a platform for a couple of times and I'm going to share with you thoughts on what I refer to as Phasic Activity of Muscle and Lower Extremity.

    And you say, "Well, I think I know what muscles do. We certainly took a course in anatomy and had our days of aggravation and angst about knowing anatomy." And it's almost like you want that behind you except it's the most important course that you all have taken, setting a foundation for everything we do. Of course that changed when you took a course called biomechanics or pathomechanics that I taught for 49 years where I tried to relate to what you've seen or learned in anatomy and I will refer to these types of situations as we go forward. So fasten your seatbelts, we're going to take a quick ride.

    [02:00]

    You may feel like you're in Disney but you're right here in Teaneck. All right, nothing to disclose, learning objectives. We hope you'll know something about muscle function by the time I'm done.

    We start about biomechanics and think about planes of motion and realize that the foot is rather unique because we talk about triplanar motion and the importance of the foot with every step that we take, influenced by intrinsic and extrinsic forces, alteration in terrain, alteration in footgear and obviously structural abnormalities, deformities, some of which we create ourselves. So I'm not going to bother you with all of these, but what is important is the fact the foot has triplanar motion.

    Now, we accept that and say, "Well, what does that mean really?" That means that you either pronate or supinate a foot at the subtalar, midtarsal, even ankle joint in those complexes where the foot either pronates or supinates. You don't have a single plane of motion.

    We think about open chain and closed chain. Closed chain is when weight bearing takes place. From the point that the heel strikes the ground, we have an internal rotary force coming from the leg above which directly has influence on the ankle complex, subtalar, and midtarsal joints. We refer to the subtalar complex as a torque converter because it allows for internal rotation of the leg to continue during the contact phase of gait.

    It's an amazing concept. It's also one of the important factors in why our total ankle joint replacements often fail.

    [04:00]

    There's a lack of understanding of the importance of the subtalar complex and if there's any arthritis in it or the midtarsal joint, the ankle has to take up all of these transverse plane rotation with every step that you take. And I've yet to see a severely arthritic ankle without some influence or effect at the subtalar complex.

    So without the torque converter capability, the rotation has to be taken up in the ankle joint and there is no joint presently in the ankle complex that truly replaces or allows for transverse plane motion to take place to the level that it needs to, even though we talked about mobile bearing joint. So the understanding of biomechanics and function is critical to so many things that we do.

    So if we actually would follow, here's your ankle complex which has that nice shape to it but when the subtalar joint can't move, look what happens to the ankle complex. You're looking at a ball and socket ankle joint. This occurs in patients who have had fusions. This occurs in patients who have had tarsal coalitions early on where he gets functional adaptation of a different joint to accommodate the rotation.

    So I'm not going to bore you. I'm sure you're well aware of the different phases of gait from contact, mid-stance and propulsion, and what the foot is going to do during each one of those phases. Interestingly, influences from the leg are necessary for proper foot function. And what are those influences?

    [06:00]

    Extrinsic muscles of the leg. Every extrinsic muscle of the leg inserts into parts of the foot. And we just have to understand muscle mechanics and muscle function to truly appreciate what these muscles do from the point of strike to the point of toe off.

    We often fail to take into account and consideration muscle function. So we can go through gait cycles and have all these knowledge but we now have to look at the next stage of what we're talking about.

    And let's now concentrate or focus more on antagonistic versus synergistic activity. How do muscles function? There's something called a Blix curve. Muscles function optimally with a length-tension ratio and when you alter that or something alters it, we have faulty muscle function. We affect that every time we try to do a tendon lengthening.

    Think about the fact that you may do an Achilles lengthening or a gastroc lengthening, tibialis posterior lengthening. We take muscle and tendon and transfer it. We do extensor hallucis longus lengthenings, we do Hibbs procedures.

    You think about all of the things that we do to tendon and muscle complexes without truly taking into effect antagonistic or synergistic activity. It happens constantly during the walking and gait cycle.

    We talk about muscle lengthening contractions. We talk about muscle shortening contractions. A muscle lengthening contractions is considered deceleratory.

    [08:02]

    Every step that we take, the forefoot comes down gently to the ground. You don't hear a foot slap. You hear a foot slap when you have paralysis, the anterior crural group of muscles or a neuropathy. Otherwise, it's a smooth transition from the point the heel strike to the forefoot coming down. The anterior crural muscles work in conjunction to decelerate the forefoot from hitting the ground too rapidly. That's a muscle lengthening contraction.

    We have muscle shortening contractions. The same group of muscles, the anterior crural muscles picks the foot up when the heel comes off the ground and we go into toe off, anterior crural activity collectively brings the foot up.

    Those are acceleratory type of activity. So one is lengthening, one is shortening and it's a smooth transition. You watch somebody walk, it's a beautiful phenomena. It's when things become abnormal that you really begin to appreciate how that muscle activity has been affected.

    So there's the Blix curve, that length-tension ratio. I've stated for many years, one of the most difficult things we do is take a tendon and lengthen it. When you're doing a bunion surgery, how many times through the years have you said, "I'm going to lengthen the extensor hallucis longus because I think it's a deforming force holding the hallux over."

    And you take a popsicle stick or whatever method you want to do and do a Z-lengthening of the tendon. When did you stop? How long should you make it? How much tension is necessary to be maintained? Can you determine that on the operating table or is it guesswork? Unfortunately, I think it's guesswork the majority of time.

    [10:03]

    I do a tendoachilles lengthening, when do I stop? We've determined that 10 degrees of dorsiflexion is necessary for normal foot function. So do I lengthen the tendon or a gastroc to the point where I get that 10 degrees of dorsiflexion?

    Well, how do you do that? So you start making cuts and you start pushing on the foot in a dorsiflexion direction. When do you stop? How do you measure it? Intraoperatively, what are you looking at? I think I've gone enough now. What do you mean it's enough? It's improved and that may be enough. Not exact science in any way and we do this constantly.

    We begin to see deleterious effects on tendon function. How many cases of posterior tibial dysfunction have you seen? Peroneal pathology, for whatever reason I've seen a host of peroneal pathology, peroneus brevis tears, peroneus longus, longitudinal tears in the tendon, frank raptures of tendon.

    And what do we do? If you have a rapture of the Achilles tendon that's chronic, we borrow from Peter to pay Paul. Let's take the flexor hallucis longus and insert it into calcaneus and make it a plantar flexor to augment or assist the function of the Achilles tendon or the gastroc soleus complex.

    Well, gee, that sounds good. Technically, it's nice. We've got all the advances to be able to do it, but what was the true function of the flexor hallucis longus? Now that that's gone, are we substituting something else to do what it did or we're just accepting that we need it to perform this function at the ankle complex?

    [12:05]

    But now, are we going to pay the price because of its influence or lack of influence in the foot? If we take a transection through the ankle joint and you take the tendons around the ankle complex and what passes into the foot, you realize everything behind the axis of the ankle, these are all plantar flexors, these are all dorsiflexors.

    We could talk about pronators or supinators because here's the subtalar axis and you could say, "Well, anything on this medial side is going to be a supinator. Anything on the lateral side is going to be a pronator." Well, that's a general concept except we know that the peroneus longus is not a pronator.

    Its influence is going to be on the first ray medially to actually plantarflex it. So it's not a pronator per se, non-weight bearing anatomically. If I take your peroneal tendons and I stimulate them, your foot is going to plantarflex and evert. You say, "Ah, so they're both evertors." So anatomy just gives us one picture of muscle function but doesn't give us what's going on during the gait cycle, which is what we need to know.

    Very interesting, if you look at the tibialis anterior, it actually lies directly on the axis of the subtalar joint. If I ask you anatomically, is the TA supinator or a pronator, what would you say? Probably a supinator, but yet it lies on the axis of the subtalar joint. It's neither a pronator nor supinator, helps to stabilize. Its main influence is on the medial column of the first ray.

    [14:02]

    And we'll talk about that in a moment. Forefoot supinatus, that triplane soft tissue deformity occurs at the longitudinal access in the midtarsal joint. That has impact on the tibialis anterior. That's the tendon that maintains forefoot supinatus, direct influence on the first ray.

    So you can look at every tendon where it passes and it's unbelievably beautiful how tendons pass the ankle complex in certain directions augmented by grooves, held in position by strong ligaments with direct influence and impact with every step that we take.

    Let's just take a quick look at some of these. Peroneus longus brevis, posterior tibial, flexor digitalis longus, flexor hallucis longus, and the triceps all have the same phasic activity. What does that mean? That they all fire at the same time.

    Interesting, you take a look at the peroneus brevis, the strongest pronator of the foot, tibialis posterior, the strongest supinator of the foot are firing at the same time. Why? To stabilize the subtalar and midtarsal complex, so stability and stabilization is critical to be accomplished by tendons which help adjust, and position, and apply forces to neutralize overpowering of the other side. It's interesting. If you look at a Charcot foot, Lisfranc's articulation destruction in the transverse plain, the peroneus brevis is dominant in abducting the forefoot on the rear foot at Lisfranc's joint, because the antagonist, the tibialis posterior is primarily inserting it to the tuberosity navicular.

    [16:14]

    That's influencing the midtarsal joint, not so much able to resist the force at Lisfranc's joint. So, here, we have a concept of antagonism where one will dominate versus another. If you look at tibialis posterior dysfunction, why does that tendon break down over time? That's not a tendon that is easily reconstructed by the way.

    Surgically, I can go in and fix the tibialis posterior but will it function with the same force and strength or is this adult acquired flatfoot, the result of inadequate treatment of flatfoot conditions from the pediatric and adolescent population right into adulthood? Because more times than not, you don't have direct trauma per se. It can happen that somebody tears a posterior tibial tendon or could have a laceration, yeah, something like that, but over time, you watch that tendon break down.

    It's amazing the posterior tibial tendon passes into the foot underneath the spring ligament, dividing and ultimately inserting it to the navicular. So it helps support the spring ligament. What sits on top of the spring ligament? The head of the talus, that ball and socket talar navicular joint.

    If you look on the other side of the foot, its antagonist, the peroneus brevis, deep in the groove behind the fibula passes into the foot, inserts into the base of the fifth metatarsal. Repetitive ankle sprains are one of the causes of disruption and influence on normal function of the peroneal tendons.

    [18:06]

    So patients often complain about continued pain along the peroneal tendons. So what do you do? You get an MRI, identify abnormality of that tendon if it exits. If we look further, the flexor hallucis longus, remember I just told you about, that's the tendon that we like to use to insert into the calcaneus. Where does it pass into the foot? It passes directly under the sustentaculum tali in the foot and then goes forward to the first toe.

    In the propulsive phase of gait when we come up on our toes and up on the hallux, what do you think happens to the distance of origin to insertion of the flexor hallucis? It's shortening because the hallux is dorsiflexing. That helps stabilize the rear foot position which is supination. That's what we need during the propulsive phase, a rigid, supinated foot to propel forward.

    So, here, we're actually finding a tendon that's very important to assist in that and yet what do we do with it? We just destroyed it by inserting into the calcaneus, not the best end result as far as I'm concerned.

    Let's look at anterior muscles of the leg. I love these guys. The anterior crural muscles, acceleratory, deceleratory, they're different than the last muscle group we just looked at because they're considered biphasic. They are swing phase and stance phase muscles. Now, why are these guys so important? We already talked about the tibialis anterior. It's on the axis of the subtalar joint.

    [20:00]

    Its primary function is to help raise the first ray in the propulsive phase. What did we do to enter the propulsive phase, we plantarflex then everted the first ray through activity of the peroneus longus. Peroneus longus had to have a stable calcaneus cuboid articulation to pull down the first ray.

    What happens in a pronating foot? We lose that stabilizing capability. We get dorsiflexion and inversion of the first ray – hypermobility. So, if in fact, we were able to pull down the first ray in normal function, when we start to push forward, something's going to bring that ray back up. That's what the tibialis anterior does. It actually brings up the first ray. That straightens out the hallux and the TA and the extensor hallucis longus, now dorsiflex the foot as we move forward, everything working in sequence and depending upon something else.

    So we have synergistic activity, muscles firing simultaneously to accomplish a task, muscles firing antagonistically to perform a task. We have to take that into account every time we operate on a tendon or transpose it. I love to hear people talk about, "Let's use a certain tendon to repair for an ankle sprain." For years, before the introduction of acellular dermis and other materials, what did we do with chronic ankle instability when the ligaments were torn? We took a tendon. Which one do you use? The peroneus brevis or the peroneus longus?

    [22:01]

    I have always talked about using the longus versus the brevis. The brevis is the strongest pronator, that's going to help resist supinatory motion. Use the longest – the longest is truly a supinator. Plus, it's long enough that I could do whatever I want, split it, maintain function of it, and have it act as an active ligament.

    Now, most people today are not really utilizing either one of those tendons because of the introduction of the orthobiologic type of materials, but those are the thought complexes or thought processes that I go through biomechanically before I elect to change or transfer a tendon.

    How about a drop foot? What would you do for a drop foot? You actually could take the peroneus longus, transfer it to the dorsum of the foot. Early fetal development, the lateral musculature, the peroneus brevis and longus are actually anterior muscles. They're going to fire accordingly. I need that biphasic component.

    If you take a tibialis posterior to treat a drop foot, insert it on to the dorsum of the foot. If you think that is going to dorsiflex the foot when it needs to, it's out of phase. So you'd have to put it under such tension that at least it maintained the foot at a right angle.

    I gave a talk years ago on flat foot. I was in Italy at that time and it was a world congress, and I was talking about hypermobile flat foot, different procedures that you could do. Arthroereisis, osseous procedures, tendoachilles lengthenings, all of these complex things and a gentleman by the name of Valente Valente said to me, "That's all very good doctor but you didn't anything about the tibilais anterior."

    [24:05]

    Now, what procedure used to use a tibialis anterior to try to hold up the arch? Young tenosuspension and the Young tenosuspension is you make a keyhole in the navicular, slide the tension and the tibialis anterior through it, so that it will bring up the height – the arch. And he kept pressing me on this and this gentleman happened to be Valente Valenti, brilliant orthopedist.

    And I said, "Well, I have to be quite frank with you at this point because you keep pressing about this procedure. One, I never use soft tissue to maintain an osseous position. It won't work. You'll destroy the function of the tendon or it'll lengthen over time or shred. And secondly, it's put of phase. It cannot possibly help support the mid-foot. It doesn't fire at that time."

    And the guy looked at me and he said, "You know what, I like you." Now, up to that point, they were shish kebabing me. It was the – European, I guess, orthopedic colleagues like to really dig a speaker. Here, we take it easy on everybody. So, with that, and an explanation of why you shouldn't even consider doing that procedure, it makes sense, biomechanically and function of a tendon.

    So the tibialis anterior works very strongly in conjunction with the extensor hallucis longus to help dorsiflex the foot. But in order for the EHL to be assistive to the TA, the great toe has be straight out. Otherwise, if the first ray was main plantarflexed – or remain plantarflexed, the hallux would come up. It would be a cockup hallux and you would not have the assistive dorsiflexory ability of those two muscles working in conjunction.

    [26:11]

    Now, certainly, the extensor digitorum longus and the peroneus tertius work to assist the TA and EHL to bring the foot up. And you realize they are synergistic and actually antagonistic at the same time, peroneus tertius, extensor digitorum longus, inserting on the lateral side of the forefoot, the other two on the more medial side.

    So they can actually bring the foot up and down, maintaining it in proper position. It's a beautiful thing to watch. So there's a little description of where these tendons pass as they're going through to the great toe and the first ray and the peroneus longus, on the other side, with its influence and recognize that the peroneus longus can only affect the first ray bringing it down if the subtalar complex begins to resupinate.

    So when we look at the triceps, by the way, which I've left out for a few moments, that is a major deforming force or influencing force on the foot. It provides that plantarflexory force through the calcaneus to the calcaneocuboid joint to help supinate the rear foot. It's actually on the supinatory of the joint.

    In flat foot, where the calcaneus can abduct, it becomes more of a pronator. That's why it puts a Giannis procedure – works nicely, get the medial displacement of the calcaneus, get it back over, the TA goes with it, and it's now – it's stabilizing the supinatory direction.

    [28:00]

    The TA – the Achilles tendon is a strong deforming force onto itself. We see it with Charcot. When you see mid-foot breakdown, what's happening to break that mid-foot down with the loss of integrity of the ligamentous structure and osseous component, the Achilles breaks the foot in essence and you now have this convex looking foot which we typically see with the malum perforans ulcer.

    If you ever want that to heal, you have to neutralize or eliminate the Achilles as the primary deforming force in order to recreate or reconstruct the arch and help it stays in that position. We see it as a severe deforming force in flat foot, in pediatric patients. You need 10 degrees of dorsiflexion, why? Because at the end or in mid-stance, when the knee is fully extended, the foot is dorsiflexed 10 degrees to the leg, the thigh 10 degrees extended to the pelvis. That's where 10 degrees comes from.

    It's a major deforming force in uncontrollable hypermobile flatfoot and often needs to be lengthened or a gastrocnemius recession. I just over-cautioned that you do no over lengthen the strongest muscle of the lower extremity in the process.

    Here's a good example showing you the tibialis anterior which is this big guy up on the dorsum of the foot, into the first ray. It actually pulls it up and here's the extensor hallucis longus working in conjunction to help dorsiflex the foot at the ankle. And if the TA does not work, you get a cockup hallux because the big toe will just be pulled up by itself.

    [30:02]

    So the extrinsic muscles are critically important to work as a group. The intrinsic muscles of the foot are a whole different ball game which we're not going to get into, but they all act as stabilizers of the foot in the direction of supination and help assist in the repositioning, reposturing, and stabilizing of the joints, so the extrinsic muscles can perform their function as our body moves forward. The foot is one of the most complex parts of the body to understand and we try to control its function every day. That's yeoman's task. I thank you for your time.

    TAPE ENDS - [30:52]