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Pronation is a necessary component of normal running biomechanics, facilitating shock absorption and stabilization. But abnormal levels of pronation, whether restricted or excessive, can alter gait patterns in ways that can potentially increase the risk of running-related injuries.
By Tracy A. Dierks, PhD
Running is one of the most popular forms of physical activity in the U.S. People’s motivation to run can range from the numerous health benefits derived from running for exercise to the desire to compete against others or one’s self when running for sport. Regardless of motivation, people who engage in running are likely to share two similar desires: to get better at running and to avoid sustaining a running injury. Unfortunately, achieving the first goal depends in large part on the second, as a running-related injury can make it difficult to run until the injury subsides.
The incidence of running injuries may be as high as 80%, according to a recent systematic review.1 Consequently, there are numerous options available to runners aimed at decreasing the risk of sustaining an injury while improving running performance. Those options include different athletic shoe designs, which can have a profound influence on running mechanics and injury. Today, more than ever, runners have a myriad of shoe choices, which change each year as new models are released and older models discontinued. Shoe design features range from cushioning to stability to motion control, and, more recently, there is a movement toward barefoot running and minimalist shoes that mimic barefoot running.
This wide spectrum of running shoes is meant to address the various functional demands associated with the differences among runners in running mechanics and foot structure, most notably, pronation and supination and arch type.2-6 Yet the majority of runners do not possess the training or knowledge to adequately determine the most appropriate shoe for their foot and running style.
Experts generally regard a running gait analysis as the most effective method available to identify running mechanics and structure. This analysis can be as basic as viewing a person run on a treadmill, or it can be as sophisticated as a comprehensive analysis performed in a state-of-the-art research or clinical center with 3D motion capture and force platforms mounted directly into treadmills.
Recent advancements in the technology used to perform a more comprehensive gait analysis have made it more accessible to a broader range of people. The equipment continues to become more affordable, smaller, and more portable, and has reached the point where such analyses do not have to be limited to research or clinical settings.7 Practitioners can easily combine equipment options such as pressure sensors, accelerometers, electrogoniometers, and gyroscopes with a high-definition video camera and treadmill to perform a fundamental gait analysis.7
Although runners now have more access to gait assessment than ever before, there is concern that some people performing gait analysis may lack the necessary qualifications. With such a wide range of individuals now able to provide various forms of running gait analyses, practitioners need to consider the training and knowledge base needed to adequately interpret the information from an analysis and make appropriate recommendations.
What is pronation?
Perhaps the most common outcome of any form of running assessment is the classification of a runner as an overpronator. It is also quite common for runners to identify themselves as overpronators. A study by Stefanyshyn et al,8 however, concluded that runners are generally unable to appropriately classify themselves as either over- or normal pronators. Although runners today have a vast amount of resources available to answer questions about pronation, the sheer volume of information can be confusing and overwhelming. Thus, to adequately discuss the role of pronation in running, let us first examine pronation itself.
Pronation is the combination of three movements of the foot, one in each of the three cardinal planes of human movement. During the swing phase, when the foot is in the air and not restricted by ground contact, these motions are eversion (foot rolls inward while lateral side of foot comes up), dorsiflexion (foot/toes move up), and abduction (foot turns out to the side).9
During the stance phase, however, the foot is in contact with the ground and now motion at the foot is largely restricted. To produce pronation now, we must move the segments and joints above the foot. With the foot on the ground, the eversion mechanism is still relatively the same, but the mechanism for dorsiflexion and abduction changes since the foot can’t come off the ground and can’t turn out to the side during most of stance. In this case, the tibia (lower leg) moves toward the toes to produce dorsiflexion and the lower leg rotates inward to produce abduction.
The consequence is that these motions can influence motion throughout the entire leg up to the hips; moving the lower leg can influence knee motion, which in turn can influence hip motion. Since the lower leg is connected to the femur (thigh) at the knee, dorsiflexion of the foot produces knee flexion, while foot abduction ultimately produces knee rotation. What makes pronation unique as a biomechanical phenomenon is that the bones of the foot are oriented in such a way that if a person performs one of these motions, they automatically and simultaneously produce the other two motions to some degree. Thus, pronation during the stance phase of running can influence movement in all three planes within the entire lower extremity.
Figure 1. Rearfoot eversion during the stance phase of running in an uninjured, recreational runner. (A) At heel-strike, the foot is in a neutral or slightly inverted position. (B) At mid-stance, the foot is in a position of maximum eversion.
The obligatory triplanar movements of pronation actually make assessment rather difficult. While each of the three motions can be measured independently with much accuracy, the problem has long been how to represent a triplanar motion with one value. Currently, there are no commonly accepted methods for measuring pronation. Yet, “pronation” is one of the most commonly recognized terms used to characterize foot function, especially in running.
To understand this inconsistency, consider this: when pronation is used in reference to the foot, it is most likely meant to represent eversion at the subtalar joint (the joint just beneath the ankle joint). Since the subtalar joint is located in the rearfoot or hindfoot section of the foot, rearfoot eversion is commonly used to approximate pronation. Thus, when biomechanical studies or clinical assessments provide a numerical value for pronation, the researchers have usually measured rearfoot eversion. To visualize rearfoot eversion, stand behind a runner and watch how both the calcaneus (i.e., the heel) and the medial side of the ankle roll inward relative to the position of the lower leg (Figure 1).
The role of eversion in running
Rearfoot eversion is one of the most investigated and assessed joint motions related to running mechanics. In runners with a heel-strike running pattern, who make up the majority of runners, eversion influences running mechanics the most during the stance phase. Eversion normally begins once the heel of the foot contacts the ground as the stance leg transitions from bearing no weight when it’s in the air to bearing all the runner’s weight once it’s positioned on the ground. A normal pattern of stance phase eversion in uninjured recreational runners can be seen in Figure 2.
Figure 2. Rearfoot eversion curve of an uninjured recreational runner during the stance phase of running. The curve is an ensemble average of 10 trials. A negative eversion value indicates rearfoot inversion.
Typically, runners hit the ground and begin stance with the foot in a neutral position (0° of eversion/inversion) or with some slight inversion (2° or 3°).10-11 The foot then undergoes eversion to a maximum value of between 8° and 12° at the midpoint of stance. In the second half of stance, the foot goes through inversion to return to the neutral position or a slightly inverted position at toe-off at the end of stance. The total excursion or range of motion for eversion during the stance phase is around 10° to 12° when runners wear a typical neutral/cushioning shoe. This entire process is accomplished in a relatively short amount of time, contributing to an average maximum eversion velocity of approximately 115°/sec.11 There is a great deal of variability in eversion from runner to runner, however, with a standard deviation for maximum eversion of about 3° to 5°.10-11 Thus, eversion could be as much as 17° and still be in the normal range.
Eversion also plays a prominent role in the coordinative timing that occurs within the leg. As eversion reaches its maximum at midstance, the maximums for both tibial internal/medial rotation (inward rotation of the lower leg) and knee flexion are reached at the same time (Figure 3).12-15 Once this occurs, the motions in the second half of stance reverse direction such that inversion, tibial external rotation, and knee extension now occur together. The motions of eversion, tibial internal rotation, and knee flexion are quite synchronous and smooth, occurring together in the first half of stance, reaching their maximums almost simultaneously at midstance, and then reversing in the opposite direction for the second half of stance.
The mechanics related to eversion facilitate several functional roles during running. First, eversion acts as one of the primary mechanisms to absorb impact shocks as the foot strikes the ground.16 Second, eversion helps stabilize the ankle joint as the large deltoid ligament on the medial side of the ankle is tensed with eversion, while the wider portion of the talur head locks into the ankle joint mortise with dorsiflexion.9 Stabilization of the foot must occur in order for the stance leg to stabilize as all of the runner’s body weight is transferred onto this foot and leg during stance. The foot must also be flexible during early stance to allow it to conform to the ground, especially when the surface is not entirely flat. As long as rearfoot eversion isn’t extreme, the midfoot and forefoot have some flexibility to either evert or invert, allowing for adaptation to the terrain.9 All in all, rearfoot eversion/pronation is a joint motion that normally occurs during running and allows functional demands to occur. But since eversion has the potential to influence motion throughout the entire leg, it has long been viewed as a potential injury mechanism if abnormalities develop.
Eversion and injury
Figure 3. The synchronous motions between the foot and knee during the stance phase of running. During the first half of stance, rearfoot eversion, tibial internal rotation, and knee flexion occur together, reaching their maximums at about the same time at midstance, then reverse back the other direction for the second half of stance. The dashed box represents a 5% window wherein the maximums occur normally. Each curve is the ensemble average of 10 trials from an uninjured, recreational runner.
If eversion becomes excessive, it can influence mechanics within the entire leg. An overpronator is someone who goes through an excessive amount of eversion. Maximum eversion values in overpronators have been reported to be as high as 22° with a standard deviation of 5°.10 But at what point does eversion become excessive? Past studies have cited either 15° or 18° as the cutoff between normal and excessive eversion.8,10 This means that a runner with a maximum eversion of more than 18° would be identified as having excessive eversion/pronation, i.e., an overpronator. A greater range of eversion would subject the tissues of the foot to more stress and strain. Any increase in force to tissue while the rearfoot is excessively everted might put the tissue over its tolerance threshold.17 In this scenario, overuse injuries involving soft tissues such as plantar fasciitis, Achilles tendinitis, or patellar tendinitis could develop over time.18 Conversely, limited eversion or excessive inversion might load the leg with impact and body weight forces too quickly, subjecting tissue to high forces over a short time, and possibly leading to injuries such as a lateral ankle sprain, stress fracture, or iliotibial band syndrome.18
Due to the link between the foot and lower leg, eversion mechanics can influence mechanics throughout the entire leg. Excessive eversion can result in excessive tibial internal rotation, which can in turn influence knee mechanics and hip mechanics.19 For example, excessive eversion could delay the time point where maximum eversion is reached, pushing it later into stance. This could create a situation of asynchrony in which knee flexion is not timed with eversion and tibial internal rotation. The knee would continue into knee extension as it normally would, but the foot, and subsequently the tibia, would not reverse motion until later in stance. This situation creates a brief window during each gait cycle in which the tibia and femur are out of sync, which could result in the knee experiencing excessive stress and strain forces. Alternatively, this could result in a compensation at the hip and femur that negatively influences mechanics at the patellofemoral joint, potentially increasing the risk of injuries such as patellofemoral pain syndrome.12
While there are certainly several mechanisms in which abnormal eversion could be related to injury, it is important to note that such a cause-effect relationship has not been directly established in the literature to date. Studies have often shown a relationship between abnormal eversion mechanics and injury, but these studies were not designed to determine cause-and-effect outcomes.
In terms of foot structure, Williams et al20 found that runners with very low arches went through a greater range of eversion and more knee flexion compared with runners with very high arches. The runners with this pattern self-reported a higher incidence of previous soft tissue injuries, medial injuries, and knee injuries compared with high-arched runners.18 Conversely, high arch runners self-reported a higher incidence of past ankle injuries, bony injuries, and lateral injuries.18 Interestingly, studies have shown that in people with normal arches, shoes designed to limit eversion do not reduce the risk of injury, 6, 21 but have found that shoe design can alter mechanics.4,5 Although these findings support the idea that abnormal eversion mechanics and foot structure play a role in the different injury patterns runners sustain, exactly how these factors ultimately cause such injuries remains largely unexplained.
Conclusions
The role of pronation in running has long been an area of interest in the running community. Since it is a necessary component of normal running mechanics, it is not surprising that it is a focal point of both running assessments and the design of running shoes. When this motion becomes abnormal, it can alter running mechanics throughout the entire leg and potentially lead to the development of running injuries. Understanding the extent to which abnormal eversion is related to running injuries requires further investigation. Future studies are needed that are designed to investigate the cause-effect relationship between abnormal pronation and the development of running overuse injuries.
Tracy A. Dierks, PhD, is an associate professor in the Department of Physical Therapy at Indiana University in Indianapolis, director of the Motion Analysis Research Laboratory, and director of research at the Riley Hospital for Children’s Robotic Rehabilitation Center at Indiana University Health.
References
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Dr. Dierks correctly states that ” Currently, there are no commonly accepted methods for measuring pronation. Thus when biomechanical studies or clinical assessments provide a numerical value for pronation, the researchers have usually measured rear-foot eversion.” Yet, “pronation” is one of the most commonly recognized terms used to characterize foot function, especially in running.”
I believe that until there is a commonly accepted “scientific” and repeatable method to measure “pronation” clinicians and researchers have absolutely no reference point from which to make their argument about excessive pronation; and for that matter how to clinically manage it.
Nowhere in this article does Dr. Dierks discuss open chain subtalar neutral alignment. Most articles I read that reference “pronation” do not mention the open chain. It seems that everybody focuses only on the closed chain. Additionally he states that “Typically, runners hit the ground and begin stance with the foot in a neutral position (0° of eversion/inversion) or with some slight inversion (2° or 3°)” .
The fallacy that the foot is in a “neutral position” when it is between 0 degrees to 2 degrees of inversion (hind-foot varus?) is another common misconception often used in biomechanical discussions.
I believe clinicians and researchers have to begin their assessment of the foot in the open chain first and then move to the closed chain to fully understand how much “pronation” is taking place in any individual. For example: individual A has an open chain subtalar neutral alignment of 12 degrees hind-foot varus. And if individual A’s heel strike occurs at 0 degrees then A is already pre-positioned at heel strike in an everted position. The pre-positioning of the foot results in an everted movement of 12 degrees from subtalar neutral before the heel even strikes the ground. If at heel strike the heel moves an additional 8 to 12 degrees of eversion then the total motion in runner A from subtalar neutral to full eversion is 20 to 22 degrees. I also believe that many runners are closer to subtalar neutral than to a “neutral” hind-foot position at heel strike. In this paper’s figure 1 picture of the right foot at heel strike I think he looks closer to 4-6 degrees of hind-foot varus than “neutral”.
I have been using and teaching this method for over 20 years. It is reproducible from clinician to clinician. And more importantly I have found (anecdotally!!) that open chain hind-foot varus is a direct proportional indicator of how much an individual pronates. The greater the open chain hind-foot varus value the more the individual pronates in weight bearing: regardless of “arch height.” Arch height is often a misunderstood method for discussing “pronation.” Again I find (anecdotally) that many high arch individuals pronate more than low arch individuals using the method I describe. A quick palpation of the talar head in relaxed stance assists with validation of the open/closed chain assessment of pronation as well as visual assessment of internal tibial rotation and fore-foot abduction in weight bearing.
Finally I find (again anecdotally) that many people never return to a “neutral” (using 0 degree angle definition) calcaneus during mid/late stance phase. This is most likely a result fore-foot position (abducted/dorsiflexed/everted) in late stance.
Assessment of forefoot position in subtalar neutral is equally important as the hind-foot assessment. The synergy between the two is often overlooked in discussion of pronation.
Addition of open chain forefoot assessment/measurements is another “must” when evaluating the whole foot.