Understanding the wheelbarrow foot
Helping patients understand
Although Root, Orien & Weed's text book "Normal and abnormal biomechanics of the foot", of 1977, gave the podiatry profession a coherent explanation of normal foot function and mechanisms of pathology, it was never easy to relate this to podiatry students let alone patients. Trying to explain that there was a magic position and that if that one joint were operating around that magic position the rest of the foot would be fine, was rather a woolly concept at best. Sadly Root is forgotten for bringing in engineering principles to the foot. Root based his theoretical model on the limited research of the early 20th century, and then attempted to guess the rest. We can hardly blame him or his colleagues for getting things wrong. Nor should we be surprised.
However the upshot of all this is that podiatrists tended to go through the following process with their patient;
"Hello, my knee hurts when I run and I've got these funny shaped toes"
"That's because you are rolling in too much - we call this over pronation. If we stop you over pronating all your problems will go away. So I'll draw some lines on your heels, make some moulds of your feet in a special position and order some expensive insoles for you"
"So that's what is causing my knee pain. Can't wait for my toes to straighten up"!
Sadly this left the patient not really understanding what foot mechanics was all about, and often disappointed them with the results. It might surprise you to realise that where as the average patient has no clue about anatomy, they do have some intrinsic understanding of engineering principles.
This is why Kirby's rotational equilibrium model (Kirby, 2001) is a useful tool to explain why it is difficult to control a dysfunctional tibialis posterior foot. We can explain that the forces from the ground reaction or muscles from the outside of the leg oppose the forces from the muscles on the inside of the leg. We can explain there is a point of rotation in the foot. If the muscles are weak on one side the foot will tend to rotate the other way, rolling outwards or inwards. If the axis moves to the inside of the foot the rotation will be easier inwards. Patients have experiences in life that makes that concept easy to understand. These experiences are unlikely to have anything to do with feet but that doesn't matter. They will easily understand why a medially flared and medially high‐sided shoe will be better than a ballet pump.
This classic picture explains Kirby's model, which although some what questionable in reality due to its overly simplistic reliance on the subtalar joint is quite useful when explaining treatment aims to patients. Taken from Kirby, K.A. (1989) Rotational equilibrium across the subtalar joint axis. Journal of the American Podiatric Medical Association. 79(1): 1‐14.
Rotational equilibrium model is therefore easily understood despite it have little research evidence to back up the idea. The premise of rotational equilibrium is based on the movement around one joint, around its axis, which is not fixed in a joint that is known not to move independently. Take out the reference to the subtalar joint axis, call it a foot axis which is the sum of the soft tissue tensions from one side of the foot opposed by the other and not only will your patients understand this concept more easily and you are probably presenting a model that is far nearer to the truth.
On understanding the rotational axis model of Kirby's this lady was persuaded to change to a pair of antipronating trainers. A shoe type she was initially totally against.
If you want to break down the movements of the foot into its simplest functions then the best approach is to concentrate on its sagittal plane function. Getting a human body from A to B by self‐propulsion is easiest performed in a straight line. It can be no surprise then to find most movement in the foot occurs in the sagittal plane (Hunt & Smith, 2004). It is worth remembering the least occurs in the frontal and the transverse occupies the middle ground (Hunt & Smith, 2004). A model based around this is the "three point rocker", which has certainly allowed practitioners with little previous understanding of gait biomechanics to confidently prescribe foot orthoses. My only complaint with this model is the very little credit being given to the author of this approach to foot function, the gait research legend, Jacquelin Perry.
The threepoint rocker presents a nice easy way to understand how body weight is transferred over our feet. From Perry, J. (1992) Gait Analysis: Normal and Pathological Function. p.33. Thorofare, New Jersey, USA, Slack Incorporated.
The Los Ranchos Amigos approach to gait analysis developed by Perry and colleagues has sadly been neglected in podiatry for far to long. This excellent and simple approach helps practitioners to approach gait in a series of roles that the foot performs over the gait cycle. From this you can understand where pathology might arise. For patients the stance phase of gait can simple be broken down into three roles! These are:
- Weight transmission
Deceleration covers the period from initial contact to loading response. The patient will quickly understand that impacting the ground in the way the foot does bring it to a rapid stop. If we do this through the heel as we should do when we are walking, then we have the plantar fat pad of the heel to cushion the initial heel strike transient (Wearing et al, 2009), and slowed by the action of tibialis anterior firing during loading response (Perry, 1992 p.57). As we load the forefoot we have less fat pad, but an extensive network of ligaments, joints and tendons that allow movement to absorb the impact, which should be slowed by the action of tibialis posterior, a loading response more common in running. All the patient need know is that impact occurs and the breaks need to be put on. During walking at a patient's preferred gait velocity, this is usually a fairly simple process to perform. But if a patient walks further or faster than is usual for the patient their muscle physiology may be unable to cope with the increased velocity of deceleration, which will result in tibialis anterior stains, tendinopathy or more likely in running, anterior fasciitis or compartment syndromes.
Loading response is the last part of the deceleration process, and the stain of this later deceleration is moved from tibialis anterior to tibialis posterior and the proximal plantar ligament. It is loading response when foot medial or lateral instability start to play a part.
Note the body weight vector arrow angle through the heel. As we move into loading response the first part of weight transference begins, Taken from Perry, J. (1992) Gait Analysis: Normal and Pathological Function. P.62. Thorofare, New Jersey, USA, Slack Incorporated.
Once the whole of the foot is loaded the next ask is to transfer body weight from the hindfoot to the forefoot. This is achieved in two section, at the hindfoot via the superficial posterior muscles of the calf, assisted by the deep muscle of the calf, and under the arch by the deep posterior muscles of the calf, assisted by the plantar intrinsics.
The momentum generated by the contraction of the thigh extensors, such as rectus femoris, sartorius and gracilis, in the opposite limb during early swing phase is the initial driver of this movement of body weight force vector. Body weight force vector is drawn forward as the swing limb passes the stance phase limb, pulling the trunk forward as the limb goes into terminal swing. The stance limb needs to provide a stable base without impeding this process. This is achieved by controlling yet allowing ankle dorsiflexion and arch lowering.
Ankle dorsiflexion starts during the early part of single limb support from the plantarflexed position it finds itself in at the end of loading response. AS this dorsiflexion is produced by passive forces in the support limb by the opposite swing limb and trunk movement, the movement needs to be resisted by eccentric muscle action. Essentially the calf muscles pull on the posterior aspect of the tibia, fibula and interrosseous membrane, like a man lowering an object on a rope. This process continues as body weight is shifted anteriorly until the heel needs to leave the ground for further progression.
Note during weight transference the body weight (CoM) force vector gradually moves anteriorly. The foot arch or dome profile lower s to allow easy passage of this force vector. Taken from Perry, J. (1992) Gait Analysis: Normal and Pathological Function. p.63 Thorofare, New Jersey, USA, Slack Incorporated.
The purpose of heel lift is to allow the movement of the body mass that remains over the foot. Normally opposite initial contact is occurring as feel lift begins, so most of the remaining body mass is the weight of the limb itself. This makes sense as who wants to try and move their whole body mass on one foot all the time? A rocking effect therefore occurs as you move one heel off the ground and the opposite one on to the ground. Not only is it a good idea to lift your heel as your body mass lands onto the opposite one, but also to wait until you're opposite foot is accepting body weight before you try to increase your arch height. It would be very hard work in particular for the tibialis posterior to raise the arch before weight was accepted on the opposite side. It is no surprise then to find the arch height at it lowest at heel lift (Hunt& Smith, 2004).
As the remaining mass of the limb moves forward the forefoot experiences increasing load and starts to use the final fulcrum of the foot, the MTP joints! Now everyone knows about the Windlass effect described by Hick (1954), but to only concentrate of the plantarfascia during this critical period of gait is to totally miss the plot. While all the foot proximal to the toes plantarflexed, the toes are forced by ground reaction to dorsiflex. Passive structures like the plantarfascia are merely wound around the circumference of the metatarsal heads, but the active structures are not only passively tightened by this same action but are also trying to contact against the ground reaction forced digital dorsiflexion. This contraction helps tibialis posterior stabilize and raise the longitudinal arc, while flexor hallucis bevis oblique and transversely aligned muscle belly helps stabilize the transverse "arch". Yes this is clever, but it is a risky procedure and getting everything to work right is hard. Acceleration takes a lot of energy, so we should not be surprised that so many patients get problems during this stage of gait, from Achilles tendinopathies, plantarfasciitis, to metatarsalgias, so much pathology can be resolved by stabilizing this important period of gait.
The rest of foot acceleration occurs during early to mid swing phase, and, as the foot is not under strain, it isn't going to experience injury…unless you knock it into something!
During terminal stance the strong gastrocnemius and soleus concentric contraction allows the heel to lift as the vector now lies over the forefoot rocker.Taken from Perry, J. (1992) Gait Analysis: Normal and Pathological Function. p.65. Thorofare, New Jersey, USA, Slack Incorporated.
What to tell your patient
Unless your patient has a lot of anatomy knowledge you are going to start to need a lot of analogies. Fortunately, most people have a crude understanding of mechanical principles. Why? Because they are all around us! We see them working everyday, and often we use then to help us in sport, DIY or gardening. So lets look at the stance phase of gait again but in analogical way.
Deceleration for patients
Your patient is out for a drive, but it's one of those days on the motorway where the traffic keeps speeding up and stopping again. That rush hour heavy traffic time! Your patient has a choice. Either brake over a longer period of time slowly, or keep slamming the brakes on when they get close to the bumper in front. Break slowly and the contents in your car, including yourself still get a smooth rid. Keep slamming the breaks on and we get jerked and bounced around in the car.
Well it isn't so different for our soft tissues when we make ground contact. Our muscles like Tibialis anterior, and to a lesser extent tibialis posterior act as our breaks. As long as the breaks are good, and/or we break slowly, there is no jarring up the body. As research has shown soft tissues vibrate at a frequency of 10‐50 hertz which is nicely in the frequency of vibration that occurs when we impact the ground, especially during running (Wakeling, et al, 2003). So if the muscles don't contract adequately (breaks worn out, sudden impact) or we break too suddenly a shock wave is passed into the soft tissues, like the occupants of that sudden breaking car.
Orthoses and running shoes can reduce the loading rate, thereby breaking over a longer period. This is especially true if we use a high proximal arch on the orthosis or shock attenuating sole. Our loading rate slows. Exercises of the appropriate muscles can essentially improve the breaks!
Failure to put on the breaks strongly can lead to a large impact! Anterior muscle group action needs to be sufficiently strong to prevent this during loading response. Equally putting the breaks on too hard or from too fast a speed will cause tissue damage from soft tissue vibration and torque. A lower loading rate can prevent injury.
Weight Transmission for patients
Weight transmission is a great concept to get the patient to understand why we should have a reducible arch height. Consider a humpbacked bridge. Initially we need to slow down. Hit the bridge to hard and will fly off the top! So having a slope to climb will help us slow down (which helps us during forefoot loading). The car will slow down because it has got to go up a slope, and the car will be at it's slowest at the top of the slope. As you roll down the slope away from the peak of the hump the car will speed up again (which obviously helps the foot speed up weight transmission as we head towards heel lift).
The car represents the body weight force vector (CoM moving anteriorly), and the hump back bridge the human foot dome profile or the arch profile of a foot orthosis. Having a steep hump bridge means that early vector transference is slowed, but later it is sped up down the slope. The human foots ability to lower and raise the height of its hump back bridge means it can control the movement velocity of the vector.
Now wouldn't it be handy if the slope were reduced as we climbed the slope of the hump back bridge? If the hump back bridge could do that then less momentum would be loss as we went up the slope and we wouldn't need to put our foot on the accelerator. Now your patient should be ready to understand why the foot has a hindfoot inclination and a forefoot declination, which is adjustable! Sadly not all patients will have the control over this inclination/declination relationship as well as they should. Take a rigid cavoid foot. In these patients the inclination is too high, and it won't lower enough if at all. Now the car needs more fuel to get up the slope, and the car will be slowed down. In gait this is critical, as if the centre of mass passing over the foot hasn't reached the crest of the slope before the opposite foot goes into late terminal swing increased strain will occur suddenly in the Achilles just prior to heel lift. The faster the loading rate on the Achilles the stiffer it behaves risking injury to the osseous‐tendinous junction. A heel lift effectively reduces the calcaneal inclination, allowing the centre of mass passage smoothly over the foot.
But what if the arch is lowered too far? As we pass through heel lift the arch usually raises up again. Effectively this makes a steeper down slope on our hump back bridge, allowing the car (our centre of mass of the limb) to roll down the slope. But of course this is more part of acceleration!
Acceleration for patients
To help patients you can still use the down slope of the hump back bridge analogy, but there are two even more helpful examples to use. The foot during terminal stance presents the body with it's one and only type two lever system used during gait. Usually we sacrifice mechanical efficiency for increased range of motion and use type three lever systems. That is how we move our knees and hips and even our foot until the terminal stance phase. But during the terminal stance we need to create a powerful acceleration to get the foot off the ground. Now what piece of equipment is the archetype class two lever? The wheelbarrow!
Now the action of the wheelbarrow that typifies the foot in terminal stance is the act of emptying the contents of a wheelbarrow. The wheel of the barrow is the fulcrum. The metatarsal phalangeal joints take this role, and are our fulcrum. The barrow itself becomes the lever, which is the body of the foot. Finally our effort which would usually be achieved by a person lifting the handles is produced by our triceps surae assisted by our peroneals laterally, tibialis posterior medially and our long flexors. The long flexors also help to maintain the integrity of the lever in conjunction with the plantar intrinsics.
Now effectively weight transmission has been about getting the load to the front of the wheelbarrow to make it easier to tip out. So all we need to do is lift the handles (pull up on our calcaneaus through the Achilles) and we trip the load out of the barrow rotating over the wheel, or in the foot rotate through the metatarsal phalangeal joints tipping the centre of mass of the foot and lower limb into swing phase.
Ever wondered why we have a metatarsal declination angle? Well is it easier to empty the contents of a wheelbarrow up a slope, on the level, or down slope? The metatarsal declination gives us the down slope. If you want to trip the contents out of a barrow it is far easier if you trip them out straight ahead.
Terminal stance is about moving the force vector onto the next foot, by tipping in forward. This can be considered like the action of emptying a wheelbarrow. By raising the arch during terminal stance it becomes progressively easier to transfer the vector, like emptying a wheelbarrow down a slope!
Now straight ahead in the foot is through the first and second first MTP joints. This is why the most desirable foot shape has a second metatarsal slightly longer than the first and a small external tibial torsion (seen as malleolar torsion). With this alignment everything is set up through a smooth transfer of centre of mass anteriorly as it rotates over the 1st – 2nd MTP joint fulcrum.
But what if as you go to trip the load out of your barrow you find your wheel is loose? Well then your load will roll out to one side, left or right, medially or laterally in the foot. What stops this situation in the foot is muscle strength of the peroneals and tibialis posterior primarily. If these muscles are weak or traumatized then your wheelbarrow will keep tripping to on side. Now if you want the contents to keep tipping out anteriorly you have two choices. One is to angle the wheelbarrow so that a side ways tip always faces anteriorly. In a medially tipping foot you would abduct the foot. In a laterally tipping you would adduct the foot. The other tactic is to counter balance the twist of the wheel and try and force it the other way. The foot equivalent of this is adducting a medially unstable foot and abducting a laterally unstable. Although the latter example is less common you will find patients who try either of these tactics to keep the centre of mass accelerating anteriorly.
What effect will an unstable mid foot have on all this? Well image someone gave you a joke wheelbarrow where the barrow itself was made of rubber. Every time it's loaded it now sinks in the middle. Every time you lift on the handles it buckles and prevents you tipping the load out of the barrow. It would really hard work using this barrow. Hypermobile pes planus feet behave like this. When a patient damages their plantar ligaments, or has weak plantar intrinsics or long flexors they are developing to one degree or another, a flexible wheelbarrow. If a patient just has a stable anatomical flat foot the situation is different. For them they have just lost the advantage of tipping the wheelbarrow out down a slope. Advise them to wear a slight heel rather than a pair of ballet pumps and they get the same advantage back. They are not in the same situation as the unstable wheel or flexible barrow of the pathological foot problems.
A Final acceleration analogy
One final analogy you may find helpful in foot function is to help explain mechanical metatarsalgias. Because the MTP joints are the fulcrums of acceleration, metatarsalgias is going to be an acceleration problem. The action of the long and short plantarflexors and the plantar intrinsics is quite complex and breaking it all down is going to bamboozle your patient pretty sharply unless they have an anatomy degree. Fortunately we have a great mechanical analogy.
That of a pole vault!
Now our athlete has to use a pole to get himself over a bar, and the fulcrum is the tip of the pole that goes in the hole. So where is the hole when it comes to the foot? Well the contraction of the digital flexors creates a proximally directed force on to the base of the proximal phalanx into the head of the metatarsal, which effectively stabilized the metatarsal head like the hole does to the tip of the pole. Now the metatarsal shaft can start to rotate forward taking the rest of the foot with if like the pole‐vaulter rises on the pole.
Like a polevaulter must get the pole in the hole to get the benefit of a proximal directed force against the forward directed force of the pole, thereby allowing him to lift off the ground, the digital plantar flexors must create a proximal force against the metatarsal head to allow the foot to dorsiflex over the MTP joints.
Now which way does the pole bend when the pole‐vaulter rises? Ever wondered why metatarsals have a concave plantar surface? It is to help resist the anteriorly directed bending moment that is produced during acceleration.
Now imagine your pole‐vaulter missed the hole with the pole. Chances are the pole will shear forward at the tip when it hit the ground. Now you can explain linear plantar callus. Of course it is a little worse for the MTP joint if it is unstable, as the digit will also create a plantar rather than a proximal force into the metatarsal head. But we'll go into that in further detail when we look at matatarsalgia.
Failure to get the pole in the hole causes the pole to shear distally wear it does the ground. The polevaulter is going nowhere! Failure of the plantar flexors to engage in stabilizing the metatarsal head causes distal shear and plantar directed compression. The result is pain and/or callus.
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