What Is a “Normal” Amount of Ankle Joint Dorsiflexion?

I read with interest the comments by Kevin Kirby, DPM, in the February issue of Podiatry Today, about some people with so-called “equinus” who can go a lifetime without experiencing pathology. I have to say that I have been fascinated for more than 40 years of practice with trying to diagnose someone as having a pathologically short heel cord.

The theory seems to go back in history to those who tried to come up with a single cause of flat feet. People like George Perkins recognized that there could be more than one reason for flattening of the ach when a person stands.¹ He notes that one reason could be the long-believed cause of a short Achilles tendon and another reason could be a varus condition of the forefoot to the rearfoot. He then proposed a surgical correction of the forefoot varus condition.

It is a lengthy paper that could trace the history of belief in short Achilles tendon causes flattening of the arch.¹ However, my proposal is that we need to better understand the actual mechanics of how tension in the Achilles tendon creates:

1) lifting the heel off the ground and rotation of the foot around the first metatarsophalangeal joint (MTPJ); and
2) pronation torques sufficient to cause abnormal pronation of the subtalar and midtarsal joints.

Unfortunately, in none of the podiatric biomechanics texts do we find such an analysis to a point that would help us better diagnose the person who has a short Achilles tendon that produces pathology. As a result, we have been left with a simple number, 10 degrees, to separate the pathological from the non-pathological and also people trying to either defend or to negate the proposition.

One of the first principles to understand is the collagen tissues of the body do not have the exact mechanical responses to stress between individuals. There are a number of reasons for this, including genetics as well as extrinsic factors of lifestyle and metabolic functions within the body. As a result, the tissues in one person’s body may be more resistant to forces applied than the tissues in another person’s body. Thus, trying to put a single number as differentiating the healthy from the unhealthy is as much of an art as it is science. As a result, we often resort to using an average value to define normality, which many times is correct, but not always.

As I have tried to understand the function of the triceps surae and how it produces heel lift, I realize that the third rocker phase of gait is the rotation of the foot around MTPJs, the first MTPJ being of prime importance. What are the torques that produce first metatarsophalangeal dorsiflexion in closed kinetic chain verses the torques that produce plantarflexion? The first to consider is body weight. The average non-overweight person may weigh from 600–800 Newtons (ie, 60–80 kg). If the center of body mass (CoM) is posterior to the metatarsophalangeal joints then there is a plantarflexion torque on the MTPJs, which keeps the metatarsals from rotating upward around a fixed proximal phalanx that is still on the ground. On the other hand, if the CoM is anterior to the metatarsophalangeal joints, then there is a dorsiflexion torque produced around the MTPJs. So, one of the keys to getting the metatarsophalangeal joints to dorsiflex during gait without pathology behind them is to get the CoM forward of the MTPJs.

My calculations are that given a person of average height and average foot length when leaning forward at the ankle joint, at about 10 degrees of ankle joint dorsiflexion, the CoM moves from being posterior to being anterior to the metatarsophalangeal joints. If, then, a person has a shorter foot length than average for their height, less than 10 degrees of ankle joint dorsiflexion will be needed to move the CoM forward of the MTPJs. If a person likewise has a longer than average foot length for their height, they will need more than 10 degrees of ankle joint dorsiflexion to get their CoM anterior to the MTPJs.

Other factors also play into whether the CoM is posterior or anterior to the MTPJs. Carrying a load in a backpack will move the CoM more posteriorly; therefore, more than 10 degrees of ankle joint dorsiflexion will be needed to get the CoM anterior to the MTPJs. Carrying a load on top of one’s head will move the CoM upward, and as such less than 10 degrees of ankle joint dorsiflexion will be needed. This also applies to basic body morphology as well. People with body proportions that move the CoM upward will need less than 10 degrees of ankle joint dorsiflexion whereas those with body proportions that move the CoM downward will need more than 10 degrees of ankle joint dorsiflexion.

The next consideration is the various joint axes’ locations of the subtalar and midtarsal joint. The Achilles tendon normally inserts slightly medial to the subtalar joint axis, and therefore it produces a direct supination torque around the subtalar joint axis. However, the Achilles also produces a first class lever effect on the forefoot, pushing the metatarsal heads down against the ground, and an upward push of the ground against the forefoot. The proportion of the forefoot in contact with the ground that is medial to the subtalar joint axis versus lateral to the subtalar joint axis is critical in assessing whether there is a net pronation or supination moment around the subtalar joint axis. Thus, a person may easily have 10 degrees of ankle joint dorsiflexion in a non-weight-bearing situation, yet in weight-bearing they may reach a critical point of Achilles tension that will pronate the subtalar joint in gait before they reach 10 degrees of ankle joint dorsiflexion. Likewise, the opposite could be true. Unfortunately, very little work has been done on trying to determine how much force against the metatarsal heads is needed to abnormally pronate the subtalar joint and the midtarsal joints.

The examination itself is problematic and needs to be better addressed. For every person, there is a passive length tension curve of the triceps surae–Achilles tendon complex.

Currently the only way to get any idea of what the passive length–tension curve is for any given person, an isokinetic machine must be utilized, so that one can measure how much torque is being generated around the ankle joint for any given position of the ankle joint. My experience is that most physical therapy departments may have isokinetic equipment that could give this type of data, but they have few who are trained and even fewer who have the interest in obtaining that type of information for the clinician. I am aware that there are people that are working on simple type of measuring instruments to give such information in a clinical situation.

So, if a clinician is measuring the ankle joint dorsiflexion, there is almost no ability of the clinician to determine how much torque they are producing around the ankle joint when taking the measurement. Clinicians with weaker hands may be producing far less torque than clinicians with stronger hands. Asking the patient to assist by contracting the anterior tibial muscle may decrease some of the intertester reliability, but it is still unknown how much dorsiflexion torque is being produced with patient assistance and how the non-weight-bearing passive measurement relates to how much torque is needed before the subtalar and midtarsal joints start abnormally pronating in a closed kinetic chain. Even if the amount of torque is known, one must still consider that total torque is produced by both the active and passive tension generated in the muscle-tendon complex, so that muscle activation is still a variable.

Finally, consider the type of activity that the patient may be involved in. Walking may require 10 degrees of ankle joint dorsiflexion before first MTPJ dorsiflexion starts occurring; however, other activities may require more or less dorsiflexion. Treadmill walking on a level surface shows more ankle joint dorsiflexion occurring. Running or any other activity other than walking may require more or less than 10 degrees of ankle joint dorsiflexing.

In summary, while the 10-degree mark may be considered to be some type of average for normal gait, there is still a tremendous number of variables that enter into the decision of what constitutes a normal versus an abnormal amount of ankle joint available dorsiflexion. Hopefully, this short letter will inspire many of the young practitioners to do a lot more research into this vitally important question of identifying the person who has a pathological versus a normal amount of ankle joint dorsiflexion.

References

1.    Perkins G. Pes planus or instability of the longitudinal arch. Proc R Soc Med. 1948; 41(1):31–40.