Addressing Plantar Pressure Analysis In The Athlete

Plantar pressure analysis can be beneficial in enhancing the diagnosis of common injuries in athletes. Accordingly, this author explores the full potential of pressure mapping systems and how related technologies like video analysis and inertial measurement units can enhance the data of pressure mapping.

The use of a plantar pressure system in evaluating high-level athletes is incredibly beneficial. Unless they are having significant pain, high-level athletes are very gifted at hiding their faults during movement. Video analysis is a valuable tool for the evaluation of both barefoot and shod gait, but being able to “see” within an athlete’s shoe, from the bottom up, is an asset to treatment we should never overlook.

Plantar pressure systems have been available for many years. One of the first and most basic systems that clinicians ever used was a Harris and Beath mat.¹ Use of these mats enabled a practitioner to examine static stance foot pressure data. Some clinicians even had patients actively walk over these ink mats to capture more dynamic gait function. The EDG system was available in the 1980s and eventually evolved into the F-Scan system (Tekscan).² Other available systems are Pedar^® (Novel Electronics), RSScan^® (RSScan International) and now a new fully wireless insole called OpenGo^® (Moticon).

All of these systems offer varying numbers of sensels, individual plantar pressure measuring units, within their foot sensors or pressure mats. Resolutions vary according to the number of sensels available. Resolution can be akin to comparing the number of pixels in a digital camera or comparing analog televisions with the newest 4K HDTVs on the market. The higher the number of sensels, the higher the resolution the data output will tend to have.

The recording speed of these systems, measured in hertz, makes a difference in data collection as well. The higher the hertz of the data capturing system, the more similar the system will be to a high speed video capture in regard to a higher number of frames captured per step. Considering we are talking about working with high-level athletes, higher hertz is more desirable.

A wireless system can also be something to consider when working with athletes. The more opportunity you have to allow athletes to function completely unencumbered in their movement patterns, the more likely you are to be able to acquire a purer stream of actionable data. Several companies offer systems they tout as wireless but they still require the collection of data in a belt-worn unit that has to be wired to cuffs at the athletes’ ankles. OpenGo sensors is the only completely wireless system that I am aware of at this time. This system uses a thin insole that fits into a shoe with no need for any other wires to be worn.

Identifying high pressures with a pressure mapping system is the easy part. Static peak images will combine all the high pressure areas for you and make a very convincing picture for your patients. Watching the full footage of a foot in motion during walking or running gait will not always look the same as a peak pressure image though. You must appreciate that the timing of events has as much, if not often more, to do with the output from pressure mapping than just high pressure areas alone.

For example, the left image is a foot scan of a pro athlete still having knee issues on the right side despite being cleared for play after knee surgery. The early heel off on the right versus the left side is obvious and problematic for the athlete. The difference in timing is an issue as the left foot is functioning normally whereas the right foot has demonstrated an early heel lift and very quick loading of the forefoot and for a longer period of time.

These types of problems prolong injury recovery for athletes and their medical staffs. Pressure mapping can identify these issues very easily. It is then just a matter of figuring out how best to treat the residual problem to return the athlete to play as quickly and safely as possible.

Pertinent Insights On Analyzing Foot And Ankle Segmental Motions

To fully appreciate plantar pressure mapping data, one must first have a very good appreciation for how the foot moves as a unit and at specific individual segmental areas of the foot and the ankle. The best way to appreciate that is by performing a comprehensive segmental foot and ankle exam. This segmental exam is key for fully appreciating why the foot may load in certain ways and at certain speeds or accelerations in comparison to the opposite foot. Since it is unusual for both feet to function exactly alike, one foot’s motions can affect the other during walking and running gait, and even landing after jumps.

The medial and lateral columns are the primary compensation points for forefoot and midtarsal joint motions of the foot. The first and fifth rays can have different stiffness values that we can grade by comparing their dorsiflexion excursion to the next closest segment or ray.^3,4 In podiatry school, we learned to measure the height of the dorsal first metatarsal head to the dorsal aspect of the second metatarsal head to grade hypermobility. Instead, my preference is to compare the plantar aspect of the first metatarsal head to the second metatarsal head. A plantar comparison tends to provide more valuable information, considering that athletes stand, walk and run on the plantar aspect of their feet, not on the dorsal aspect.

You can compare the lateral rays in a similar fashion. The purpose of this is to identify stiffer versus more mobile segments of the forefoot that will usually correspond to higher or lower pressures respectively. For example, a stiff fifth ray will tend to have a higher plantar pressure early after foot contact and later into midstance. This data can often raise a red flag in those athletes who may be at risk of fifth metatarsal fracture or stress reaction. Quantifying prolonged high pressures at the fifth ray is a great way to identify athletes at risk of an injury in this area and test to see if changes to their shoes, orthotics or other treatment plans have had the desired protective effect when one tests them again later.

Measuring the heel position in resting calcaneal stance versus neutral calcaneal stance position can often give some indication of the amount of pronation that an athlete will have during stance. This is affected greatly by other measurable segments as well and heel position is one of the less reliable measures in many different studies.⁵ It is also hard to quantitate this measurement using pressure mapping. It is likely because the degrees of difference for most athletes are very low, regarding the amount of stance phase pronation.

One can detect pronation elements by observing pressures in the medial arch and when viewing force-time curves of the forefoot and heel. Pronation will show a prolonged force versus the time curve in both the heel with delayed offloading and often in the forefoot with early and prolonged loading. Often, though, with the use of a well-cast orthotic, in-shoe pressure will show an increase in medial pressures and improvement in force versus time curves. Any increase in the medial arch area with the use of an orthosis does not mean that the orthotic has necessarily caused the foot to pronate more. Instead, this may mean that the orthosis conforms well to the medial arch when testing with an in-shoe sensor and will therefore have increased pressure in that segment of the foot.

Ranges of motion of joints such as the first metatarsophalangeal joint (MPJ) and the ankle can have great impact on athletes and their plantar pressure measures. Functional range of motion of the first MPJ, also known as functional hallux limitus, will often show up as limited pressures under the first metatarsal head and higher pressures at the hallux. Functional hallux limitus can prolong pronation during midstance in athletes, perpetuating many different types of foot and ankle conditions like peroneal issues, and can also lead to knee and hip issues.

A limited dorsiflexion range of motion at the ankle can lead to Achilles and forefoot pain in athletes as well as an increased risk of knee injury due to instability when patients are up on their toes for a prolonged period. With pressure mapping, limited ankle joint dorsiflexion tends to affect heel contact duration and midstance very greatly. If the ankle cannot follow through with adequate dorsiflexion, then the heel will lift early as indicated in the pressure mapping movies. This will also show up as early loading of the forefoot and lateral column especially.

Issues at the knee and hip can affect foot pressures as well. For example, if an athlete has a fixed flexion of the knee, like Tim Duncan of the San Antonio Spurs, then the heel will lift early and the forefoot will load much earlier and longer in gait.

The right image is also a good visual example of this. As you can see, what happens at the ankle can affect the knee and vice versa. It is best to make sure you evaluate athletes in weightbearing in shorts and when possible, use a brief knee physical exam to identify weaknesses in strength. Watch them walk or use video of them to pick up early knee flexion patterns. Always understand that proximal compensations can affect what is happening at the level of the foot and ankle.

As you can see, it is imperative that you utilize a very comprehensive segmental analysis of the foot and ankle function to fully appreciate what is going on with an athlete prior to doing any pressure mapping. Once you understand these segmental issues, pressure mapping data starts to make much more sense and is easier to understand.

Keys To Understanding Force Versus Time Curves In Pressure Mapping

Appreciating force versus time curves is important in understanding the full meaning of pressure mapping data. With many pressure mapping systems, you can box out certain segments of the foot pressure data. Often blocking out the forefoot, the heel and the entire foot will give you very good information for comparison of right versus left foot data.

Interpreting the force versus time curves has much to do with appreciating that foot segments, like the ankle, subtalar joint and MPJ, can quickly hit functional or structural end range of motion at different stages of gait. The forefoot curves are often affected by structure and function of the stiffness of the lateral and medial columns as well as functional hallux limitus. Blocking the forward motion through the first MPJ can keep the forefoot pressures more lateral and show prolonged duration of forces in the forefoot. Lack of midfoot stability, similar to what would occur with posterior tibial tendon dysfunction or after a midfoot sprain or a Lisfranc injury, can affect both the heel and forefoot curves with delayed forces in both areas.

As I have noted above, if there is limited ankle dorsiflexion range of motion, then the heel will usually lift early and this can be visible in the force versus time curves as a shorter, lower duration curve when the midfoot/forefoot is stable.

In those athletes in whom the midfoot/forefoot is more compliant or unstable, the heel can lift slightly but not fully. This can lead to delayed forward progression at the first MPJ (i.e. functional hallux limitus) because the medial column is unstable.

Understanding force versus time curves in pressure mapping is all about understanding where the primary forces of the foot are acting at a certain time in the gait cycle. Once you appreciate that, it becomes much easier to understand.

What You Should Know About Video Analysis And Inertial Measurement Units

Pressure mapping offers a tremendous amount of data that explains much of what the lower limbs and specific segments of the foot and ankle are doing. This can be difficult to grasp fully without the combined use of video of an athlete’s walking or running gait. When you have both video and in-shoe pressure working together, you can get a better idea of how the function of the foot affects the knees, hips and back, and vice versa. Issues like hip drop, early and prolonged knee flexion, glute and core weaknesses all affect foot function, but are also affected by the function of those foot segments I discussed earlier. This is often the reason why athletes are strong for short periods of time at their core and hips, but over time become weak with prolonged practice and play.

Identifying the foot issues and controlling them as much as possible will usually allow the athletes to stabilize their weaknesses more proximally. This can increase athletes’ performance outcomes and decrease their risk of injury.

Inertial measurement units are becoming much smaller, more useful and affordable. Many inertial measurement units are the size of an automobile key fob that one can attach to different segments of the legs, feet or shoes. These devices provide kinematic or segmental positional data from their gyroscopes and accelerometers that have been tested to within 90 percent of accuracy for video motion capture systems.⁶ With this added data, it is now possible to understand rotational movements at the tibia, femur and pelvis in relation to in-shoe pressure data and video analysis.

Combined, these technologies can give you access to a portable gait analysis laboratory that one can utilize in the athlete’s primary playing environment. Whether athletes are playing football or soccer, baseball, hockey or basketball, all of these units are now mobile and ready for utilization to maximize athlete’s outcomes from treatment.

In Conclusion

Pressure data is a huge assist to those working with or treating athletes on a regular basis. Nothing will give you the high output data on how the foot function affects the gait and movements of high-level athletes. Using in-shoe testing can quickly evaluate taping with accommodation or the use of prescription foot orthotics to see if they are working and assisting athletes as planned. Using a repeatable segmental evaluation system for the foot and ankle is key to understanding exactly what aspects of foot function are contributing to the problems in an athlete’s gait. A quality segmental analysis should provide direct insight into the pressure mapping output. Combining pressure mapping with video analysis and inertial measurement units provides even more actionable data with which to improve an athlete’s injury risk and performance outcomes.

Dr. Williams is the Director of Gait Analysis Studies at the Weil Foot and Ankle Institute and the Weil Foot-Ankle and Orthopedic Institute. He is a Past President and Fellow of the American Academy of Podiatric Sports Medicine. Dr. Williams is a sports medicine professional specializing in the treatment of foot, ankle and movement disorders. He is the Director of Breakthrough Sports Performance, LLC in Chicago.

References

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