What Forces Damage the Plantar Fascia, and Why?

As explained in the previous article, the plantar fascia covers a large area. Originating at the heel and inserting into the base of the toes, it contributes to overall foot stability and function in many ways. In this article, we’ll look closer at the types of forces the plantar fascia is required to withstand, step after step, day after day. And we’ll look at plantar fascia pathomechanics - the faulty function behind the injury.

Is It All About Tension (Tensile Load)?

The first and most obvious of the forces that the plantar fascia withstands is tensile force. Think of pulling on each end of an elastic band, making it longer. You’re applying tensile load (tension) to the elastic band. When the arch flattens, the origin and insertion move further apart, causing the structure that connects them to stretch.

A close look at the cells of the plantar fascia show that it is also subjected to and able to handle:

• Compression

• Bending

• Shear

The Heel Origin of the Plantar Fascia is an Enthesis (& Heel Spurs)

The site where a tendon or ligament inserts into a bone is called an enthesis. The calcaneal entheseal origin of the plantarfascia is fibrocartilagenous in nature, providing a gradual transition from hard bony material to soft tissue (fasia) which serves to dissipate stress evenly within the tissue. Fibrocartilage is able to withstand considerable tensile and shear load.

You may have heard about “heel spurs”. Plantar fasciitis is often referred to as “heel spur syndrome”. Heel spurs are essentially a calcification of an enthesis (more likely of an intrinsic foot muscle rather than the plantar fascia) such that it becomes harder and more like bone. However, not everyone with plantar fasciitis has a heel spur. And not everyone who has a heel spur seen on x-ray has had plantar heel pain.

Heel spurs almost never require treatment in themselves, as they are not the cause of the pain. They are considered to be a secondary finding when there has been long-term excessive tension in the plantarfascia and surrounding intrinsic muscles. Having said that, a calcified enthesis is rendered less able to handle the forces we’ve talked about and so more of that load is transferred to other nearby structures, like the plantar fascia.

Mechanical Factors Related to Forces Applied to the Plantar Fascia

The plantar fascia is designed to withstand the tensile, compression, bending and shear forces exerted on it during gait (running, walking, playing). However, when the plantar fascia becomes painful, we obviously look for ways to decrease these forces, if they are too big. These forces may be higher or lower between individuals, or in the same person at different moments, depending upon many factors:

• Foot alignment - like low arches or high arches. It might be easy to understand how flat arches could increase tension in the plantar fascia. However, surprisingly, research hasn’t been able to confirm this “foot-type” is more prone to developing plantar fasciitis.

• Ground reaction forces - that’s the pressure exerted to the foot when it hits the ground. It might be easy to understand there’s high pressure under the heel when it firsts hits the ground. However, although gait is altered in people with a painful plantar fascia, this may simply be an altered gait due to the pain. In this way, it may simply be a consequence of plantar fasciitis, not the cause of plantar fasciitis.

• Range of motion - like when the calf muscle is tight, or the toes are unable to bend upwards. There is slightly better evidence that a tight calf/Achilles leads to higher tensile loads in the plantarfascia. And there is conflicting evidence for the bend in the big toe.

Cause vs Effect

There are a bunch of other factors that research has found to be more prevalent in some people who are experiencing plantar heel pain. Things like faster pronation velocity, a larger degree of pronation, reduced cushioning ability of the plantar heel fat pad, tight hamstrings, weak toe flexor muscles weak calf muscles and tight calf muscles. But in many cases, preceding or follow-up research failed to find the same link. So the evidence is equivocal for these factors. Not only that, the types of research conducted aren’t able to confirm cause - they can only confirm correlation. In other words, we’re not sure if they are the cause of the heel pain, or simply the result of having heel pain.

It would be great to know that ALL cases of plantar fasciitis are caused by X, Y or Z. It would make our jobs so much easier. The problem is, there is such a large range between people. From foot structure to BMI to age and health issues affecting connective tissue qualities (thresholds for irritation and tolerance to tensile strain loads), to activity levels and injury history, to footwear worn and whether that footwear is a good match given the foot function, occupational activities like standing on hard surfaces or jumping out of trucks or walking on uneven surfaces, and a thousand other variables.

Unfortuantely, we’re not that lucky. But there is a lot of research going on in the field of podiatry - everyday we are learning from this research and allowing it to influence our clinical practice.

So, How Do Podiatrists Fix Plantar Fasciitis If They Don’t Know Exactly What Causes It?

We know that plantar fasciopathy is a mechanical overload of the plantar fascia from either tensile, compressive, bending or shear forces, or more likely combinations of these. And that overload will be the result of a complex interplay of intrinic and extrinsic factors specific to an individual. It’s complicated! And no two situations are the same.

Thankfully, we know we can change the structure of the plantar fascia enthesis, and the whole plantar fascia structure, by applying progressive tensile, compressive, bending and shear loads to it. Not just any amount of load - carefully considered load through specific exercises that gradually increase the capacity of the structure to tolerate the day-to-day loads a person wants it to be able to handle. Again, that will be different for every individual.

Conclusion

Just as it is in other professions, when research does not provide definitive answers, our job is to take our understanding of anatomy, physiology, function and pathology and apply it to the person in front of us, with all their individual traits. We treat the person, not just the pathology.

The art is in applying the science to the individual!