Equinus: A Deformity or a Benefit?

One of the most talked-about but least understood conditions seen in podiatry patients, in my experience, is a restriction of ankle joint dorsiflexion known as “equinus deformity.” Equinus is a Latin term meaning “of or pertaining to horses.”¹ It is likely that the term “equinus” originated from the observation that more severe cases of equinus caused the individual to walk on their toes, simulating the gait of horses.  

In today’s podiatric profession, equinus deformity is defined as less than 10 degrees of ankle joint dorsiflexion with the knee extended and the subtalar joint in neutral position.² This generally accepted value of 10 degrees for “normal” ankle dorsiflexion seems to be based on the observation that the maximum ankle joint dorsiflexion required during walking is 10 degrees, occurring just before the heel-off phase of gait.²

Equinus deformity is generally divided into an osseous type of equinus (“ankle equinus”), and a soft-tissue type (gastrocnemius-soleus equinus). In ankle equinus, there is a bony restriction within the tibiotalar joint that prevents the foot from achieving normal dorsiflexion of the foot relative to the tibia. However, the most common cause of equinus is due to soft tissue structures of the leg, namely the gastrocnemius and soleus muscles and Achilles tendon.

The gastrocnemius and soleus muscles are the largest and strongest muscles of the leg and the Achilles tendon is the largest tendon of the body, together comprising the gastrocnemius-soleus-Achilles tendon (GSAT) complex. The GSAT complex generates the vast majority of the very important ankle joint plantarflexion moments required for normal weight-bearing function. These ankle joint plantarflexion moments will either accelerate ankle joint plantarflexion, decelerate ankle joint dorsiflexion, or stabilize the ankle joint against ankle dorsiflexion moments. As a result, during weight-bearing activities, the GSAT complex is largely responsible for maintaining upright balance during standing, decelerating ankle joint dorsiflexion and accelerating ankle joint plantarflexion during walking, running, jumping, and hopping activities.²

Therefore, restrictions in ankle joint dorsiflexion, or equinus deformity, may have the potential to create abnormalities in foot and lower extremity biomechanical function possibly leading to clinical pathologies. Gait abnormalities such as a “bouncy gait,” rocker-bottom flatfoot deformity, Charcot arthropathy of the midfoot, metatarsalgia, plantar forefoot calluses and ulcerations, genu recurvatum, and low back pain have all been suspected to be associated with equinus deformity.³

Pearls From One Podiatrist’s Experience With Equinus

However, after 37 years of practice as a sports podiatrist, examining athletes from the recreational to professional levels, I have my suspicion that not all “equinus deformities” are such a bad thing. In fact, some of the highest quality running and jumping athletes I have examined over the years have had quite significant equinus deformities, and were perfectly healthy and asymptomatic.  

I suspect that one of the reasons that these athletes were able to run and jump so successfully, and still be free from injury, may have been due to their relative lack of “normal” ankle joint dorsiflexion. My hypothesis is that their “restricted ankle joint dorsiflexion” seems to have been caused by very strong gastrocnemius/soleus muscles which, of course, would not only improve athletic performance but also may cause less than 10 degrees of ankle joint dorsiflexion.

My years of clinical observation leads me to believe that not only do some podiatrists overestimate the pathologies caused by having less than 10 degrees of ankle joint dorsiflexion with the knee extended and the subtalar joint in neutral position, but also that “equinus deformity” may be beneficial in some individuals. This apparent benefit, in my observation and estimation, may be a result of the athletic training and/or due to the genetic structural characteristics of the individual, which may produce improved performance during their running and jumping sports.

Therefore, before we all conclude that equinus is “the root of all foot evils” and start planning elaborate non-surgical and surgical treatments for all individuals with this common clinical finding, it would first be prudent to research why many individuals with equinus deformity seem to not only remain asymptomatic over their lifetimes, but also can perform at a very high level in running and jumping sports for many years without injury. Then, perhaps, we can better understand whether “equinus” is truly a “deformity,” or rather, simply just another structural variation within the lower extremities of the human species that can cause problems in some individuals, but improve athletic performance in other individuals.

Dr. Kirby is an Adjunct Associate Professor within the Department of Applied Biomechanics at the California School of Podiatric Medicine at Samuel Merritt University in Oakland, Calif. He is in private practice in Sacramento, Calif.

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References

1.    Equine. WordSense Online Dictionary. Available at: https://www.wordsense.eu/equine/ . Accessed January 9, 2023.
2.    Root ML, Orien WP, Weed JH. Normal and Abnormal Function of the Foot. Clinical Biomechanics Corp; Los Angeles: 1977.
3.    Clifford C. Understanding the biomechanics of equinus. Podiatry Today. 2014;27(9):38–44. Available at: https://www.hmpgloballearningnetwork.com/site/podiatry/understanding-biomechanics-equinus . Published September 2014. Accessed January 10, 2023.