Ankle and foot biomechanics

Houston Rockets guard James Harden (13) grabs his ankle after taking a shot  Credit: Ken Blaze-USA TODAY Sports.

Ankle function is obvious in any running or jumping-based sport, but if we consider swimming, the ability to push off the starting blocks and propel off the wall can be a major determinant of sporting success. What about a non-weight-bearing sport such as water polo, though? Well, whilst it’s a valid point that not much of the actual match requires force absorption by the foot and ankle (except for holding your opponents at bay), we need to consider all the dry land training that these athletes need to complete, including key lower body strength lifts. So, as we can see, with the exception of stationary target sports such as archery and prone rifle shooting, we need our ankles in good nick.

Fundamental to all injury prevention, rehabilitation, and performance training programs lies a strong underpinning of an understanding of biomechanics, and it is this that I want to flesh out in this article. Rather than a dry approach that looks at force vectors and trigonometry equations, however, what I will aim to articulate here is an applied focus on how we can utilize our knowledge of biomechanics to drive our programming.

It’s complex!

Whilst many people often think of the ankle as existing as simply the hinged talocrural joint, the reality is that this is only part of the equation, which is why we refer to it as an ankle complex. It contains the:

• Talocrual joint – between the talus and both the tibia and fibula;
• Subtalar joint – between the talus and the calcaneus;
• Inferior tibiofibular joint – at the ‘southern’ end of the tibia and the fibula.

All three areas need to work together to allow the foot to lift (dorsiflex), point (plantarflex) and rotate (supinate and pronate).

In a manner similar to the wrist and hand working together to allow the fingers to function well, the various midfoot joints need to work in concert with the various ankle articulations to allow the foot to be positioned well for load absorption and application. It’s complex in both name and nature!

Movement and stability – dual masters

For the ankle to be able to function at its optimal level, it must be both a spring-like lever and a mobile adaptor. Whilst these two aims seem antagonistic, the efficient coordination of these aims is critical to sporting success. 

The body needs a strong and stable base to be able to apply force. In the same way, we cannot fire a cannon out of a canoe; we need to be able to retain stability to execute our movement wishes. The way the foot and ankle complex achieves this is by:

• Keeping structural integrity between the tibia and fibula through a structure known as the syndesmosis;
• Maintaining a “dynamic defense system” orchestrated by the nervous system by applied by the surrounding muscles to ensure that the foot and ankle are not “loose” upon ground impact;
• Maintaining a stiff force bridge known as the plantar fascia allows force to be amplified and applied to the foot in a manner reminiscent of a spring.

At the same time, however, the foot needs to be able to adapt to alterations in the platform upon which it interfaces. If we were too stiff, we would not be able to accommodate these changing demands, and our function would be clearly compromised. Obvious examples of these demands are:

• Changing terrain (such as sand, loose rocks, or even landing from a jump on an opponent’s foot);
• Changing direction when running;
• Manipulating foot position to impart spin on a football when kicking.

As we can see, these demands can be very…well, demanding! This is amplified when the requirement of the sport is to perform the tasks in a split second. Think of a goalkeeper in soccer who, after running off the line to claim a corner, lands on one leg on an opposition player’s foot, then has to change direction out of traffic and kick the ball long to his teammate on the halfway line. This can all take place in the space of a second, and in this time, the foot and ankle have had to execute numerous biomechanical actions, some planned, some unplanned.

The spring-like lever

Now that we appreciate the dual roles of the complex, injuries that affect these functions should make more sense, and we can place our decision-making regarding injury management into context. For example, if we see a syndesmosis diastasis, an injury that affects the structural integrity of the inferior tibiofibular joint, we know that we need to either allow it to heal naturally to ensure that we have appropriate stability or to manage it surgically if conservative treatment is deemed unlikely to achieve this goal.

Similarly, alterations in the dynamic defense system, where the ankle is not appropriately positioned to absorb ground contact, often from poor proprioception, leave us vulnerable to ankle sprains and reduce our ability to transfer force.

Finally, any injury or dysfunction that compromises the “Windlass effect” provided by the integration of the Achilles tendon to the plantar fascia underneath the foot and onto the first metatarsophalangeal (big toe) joint,  will reduce the ability of the spring to leverage us into the propulsive phase of the next step.

Therefore, we can see that we need to assess each of these areas individually to determine where the issue may lie and also understand that frequently there is an overlap, where dysfunctions in one domain can lead to dysfunctions in another.

The windlass effect

This is a mechanical model whereby the plantar fascia, which runs longitudinally from the heel to the toes, supports the medial longitudinal arch of the foot and allows for effective force transition from ground contact (force absorption) to push off (force application).

The mobile adaptor

Many people’s idea of ankle rehabilitation is to get the injured athlete to stand on a wobble board, and that’s it – job done! I know that my own education at university was severely lacking in this sort of area, and so it’s no wonder that this is deemed satisfactory. We know that the wobble board can be an effective tool in improving dynamic balance, but it cannot exist as the sole intervention.

Let’s think about it. Usually, when you are standing on a wobble board, your ankle is in either a dorsiflexed or, at best, neutral position. Now, we know that the majority of ankle sprains occur when the ankle is in its loose-packed position (plantar-flexed). This is because the mechanical restraints of the joints are most open, and the narrow neck of the talus is vulnerable to being forced anteriorly, stretching the lateral ligament complex. With this in mind, surely then it makes sense to ensure that our athlete is both competent and confident in this position before we send them back to battle?

It is with the role of the mobile adaptor in mind that I vary the surface that any retraining takes place on (starting stable and then eventually progressing to, quite literally, shifting sands), but also I vary the position in which the ankle is forced to function in, commencing in a close-packed position (weight-bearing in dorsiflexion) to progressively more vulnerable positions (for example, standing on tip toes).

The role of the calf complex

It seems that everything in this area is called a ‘complex’ and this holds for the calf as well. This is because the calf is comprised of the two heads of the gastrocnemius, the soleus, and, with a minor supporting role, the plantaris. In the anatomy texts, they act to plantarflex the foot, and there is no denying that this is a huge part of their role. In locomotion, they arguably have a greater role; however, one that is not taught well at university and involves the concept of isometry.

We are taught that there are three types of muscle contractions:

1. Concentric (shortening);
2. Eccentric (lengthening);
3. Isometric (holding).

In my mind, the terms eccentric and isometric contraction are oxymorons, and we should instead refer to them as eccentric and isometric muscle actions. Whilst this may seem pedantic and semantic in nature, the reality is that an understanding of what these actions are designed for is fundamental to our program design and delivery. This is especially the case when we consider the role of the calf complex in the foot and ankle.

There have been many studies examining the role that calf strength has in both performance and predisposing us to, or resulting from, an ankle sprain. Many are equivocal, but this is because most do not examine the true isometric nature of calf function.

As we run faster towards a full sprint, we are required to apply more force with even less time in which to apply it because foot contact times are reduced. If, as we were taught, our ankle absorbed the ground contact and applied force by a coupling of eccentric and concentric contractions, it would be impossible for us to move at speed. It would be too slow and also far too metabolically expensive. Take, for example, the ‘burn’ you feel in your calves when performing heel raises over a step. This is an example of an eccentric and concentric muscle action coupling. Now, when was the last time you felt this when running? Never!

The reason for this is that the true role of the calf muscle is to provide a stable base to allow the long Achilles tendon to stretch and recoil. It does this by retaining isometry – acting to hold without lengthening. By doing this, we utilize the tendon to its full extent. The tendon has the ability to recoil significantly faster than muscle can, and because it is non-contractile in nature, it is metabolically efficient.

So, with this in mind, and given that the calf complex attaches to the Achilles and then onto the plantar fascia under the foot, when looking at developing the function of the spring-like lever, we should be doing everything we can to train the isometry of the calf.

How do we do this? There is nothing wrong with doing good old-fashioned heel raises to ensure that the calf has good capacity, but to train function, we need to do plenty of plyometric training. Not only will this train the true nature of calf function in locomotion, but it can be progressed during the rehabilitation period to ensure that landing competencies are enhanced.

It is for this reason that I love skipping as a rehabilitation drill and a core competency that I want from my athletes is to be able to perform 50 ‘double unders’ in 25 seconds. A double under is where the rope travels around twice for every one jump. There is undoubtedly a skill component here and it takes time to develop it, but from an ankle perspective, if you cannot retain calf isometry and the heel lowers down through a slow amortization phase, what ends up happening is the rope hits the shin on the second trip around. I love the rope because there is simply no way to cheat it. It can be a frustrating experience during the learning phase but for the dedicated athlete, the skill acquisition curve is steep.

Think about the fastest animals in the world — cheetahs, horses, Usain Bolt. Do they have big calf muscles or long, rapidly recoiling Achilles’ tendons? Most definitely the latter! Big calves, unless you are a front-row forward in rugby or a bodybuilder, are a waste of mass. We are much better off placing our hypertrophy emphasis upstream at the glutes.

Summary

By having an understanding of the true nature of not just the structure but the function of a joint, we have a greater appreciation of how best to improve performance. Nowhere in the body is this better demonstrated than in the foot and ankle complex.

Through knowledge of both the spring-like lever and mobile adaptor roles of the complex, we can specifically examine each capacity and then be strategic in our interventions. Equally as important are the isometric conditions in which the calf operates most efficiently.

Armed with this information, we can no longer excuse an injury prevention or rehabilitation program that seeks to improve foot and ankle function simply by getting the athlete to stand on one leg, and balance on a wobble board to perform heel raises. We need to examine the positions and conditions in which the complex is expected to operate, and we need to train them specifically here, starting from the most stable when the ankle is least competent (for example, early on in rehabilitation) and then progressing to the most demanding.