Plantar Fasciitis

The plantar fascia is a band of connective tissue that runs along the underside of the foot from the heel to the toes. The fascia helps maintain the integrity of the arch, provides shock absorption, and plays an important role in the normal mechanical function of the foot.

Plantar fasciitis is a common cause of heel pain. The common presentation is a sharp, localized pain in the bottom inside part of the heel.

Plantar Fasciitis Is Probably Not Inflammation

Plantar fasciitis was previously believed to be inflammation of the fascia near its insertion on the heel bone. The suffix (-itis) means inflammation. Studies, however, reveal that changes in the tissue associated with the injury are degenerative and not related to inflammation, at least not in the way inflammation is typically viewed 

A sudden onset of heel pain may be related to acute inflammation. For persistent heel pain, the condition more closely resembles long-standing degeneration of the plantar fascia near its attachment than inflammation. This could explain why anti-inflammatory medications and injections have relatively low success rates.

But there is more to heel pain than just the plantar fascia.

Not Just Fascia

The pain associated with plantar fasciitis is typically localized to the underside of the heel towards this inside of the foot. Often overlooked when talking about this injury are the muscles that share an insertion with the plantar fascia. Looking at the muscles in the sole of the foot you can see that the flexor digitorum brevis muscle runs directly above the plantar fascia. On the inside part of the heel the abductor hallucis, an important arch stabilizer muscle, attaches.

It’s not certain that the pain associated with this condition originates in the plantar fascia. Many of the diagnostic features seen with this condition (e.g. bone spurs, thickened fascia) are also found in people without heel pain, and likewise may be absent in people presenting with the injury.

There’s a chance, in my opinion, the pain is the result of a tendon problem of the muscles deep to the fascia, rather than the fascia itself. The role the foot muscles play in this condition will be discussed later in the article.

Overuse Injury?

A common theory on how plantar fasciitis develops is that repeated strain on the fascia causes small tears to develop that eventually lead to pain. Excessive pronation (collapse of the arch) is often cited as the cause of increased mechanical loading of the fascia.

Despite how frequently pronation is blamed, there is not much evidence that arch mechanics play a role in the condition. This may be partly due to the researchers have accurately measuring pronation, especially movement of the arch inside of a shoe. This might also mean though that tensile stress on the fascia from arch movement, at least in itself, is not a cause of plantar fasciitis.

Role of Compression

Animal studies suggest repeated mechanical loading alone may not be enough to cause degeneration of connective tissue. Tissue degeneration is more likely to occur in areas that receive a poor blood supply, or at locations in the connective tissue where the blood supply is cut off.

Another interesting finding is the presence of fibrocartilage near the attachment of the plantar fascia. Fibrocartilage insertions are more common in structures that undergo bending or compressive forces. It may be that bending and compression are a bigger problem than tensile or stretching forces.

Heel spurs were also once thought to be a feature of this condition, with the thinking that they were caused by the fascia pulling away from the bone. New evidence strongly suggests that spurs are unlikely to be a cause of fasciitis, and that they may develop due to compression rather than tensile force on the fascia.

Risk Factors

Currently, no single factor has been reliably identified as contributing to the development of plantar fasciitis.

Two risk factors that have support from current research:

• Decreased ankle dorsiflexion
• Increased Body Mass Index (BMI) in non-athletic populations

When dorsiflexion range of motion (ankle flexibility) is lacking, the body often compensates by increasing movement of the arch. In this way, decreased ankle dorsiflexion influences pronation and places strain on the underside of the foot.

Similarly, having a high BMI causes strain because it places a load on the foot that may be in excess of what the foot can support.

As I mentioned earlier, overpronation is thought to be a contributing factor, but studies on this have so far produced mixed findings.

A lack of ankle flexibility and a high BMI can also both cause increased pressure on the heel in standing. Keeping weight on the heels causes compression under the heel. But it also means the muscles and ligaments in the arch are not being used to balance your body weight.

Looking beyond these potential contributors to heel pain, though, there is another factor that I think is often looked–the way footwear alters the function of the foot.

Can Shoes be a Factor?

There are several features in the design of shoes that can cause abnormal stress on the plantar fascia and potentially affect the muscles in the foot.

Toe Spring

Most shoes curve up in the front in the area of the toes. This upward curve of the sole is called a toe spring. This is engineered into footwear because shoes are crudely made compared to the complexity of the structure and function of the foot.

The curve allows for a rocking motion to occur as a person transfers their weight from their heel to the front of their foot. This rocking motion is a substitute for the bending the foot does when walking barefoot–a motion shoes are unable to replicate.

Having this toe spring means that, for the majority of the time when a person is standing or walking in a shoe, their toes are held elevated off the ground. The curvature of the sole places the toes in an extended position. Because of this, the toes can only make contact with the ground when the heel lifts up.

Keeping the toes splinted in this extended position can hypothetically do several things:

• Holds the plantar fascia and other structures in the sole of the foot in an unnaturally elongated position
• Prevents the toes from gripping the ground
• Prevents the intrinsic foot muscles from contracting to support the arch
• Prevents the muscles and plantar fascia from assisting with shock absorption
• Possibly cuts off circulation on the underside of the foot

When walking barefoot the toes make contact with the ground much sooner in the stance phase of gait than they do inside a shoe. These foot muscles are needed to assist with gripping the ground and stabilizing the arch.

Without support from these intrinsic foot muscles, extra strain may be placed on the plantar fascia. This extra stress can be doubly bad because the toe spring also places the plantar fascia in a stretched position. Toe extension (upward movement) stretches the plantar fascia via the windlass mechanism. With the plantar fascia already stretched by the toe spring at the time the foot contacts the ground, its ability to lengthen farther to absorb forces is limited.

Increased strain on the fascia is not the only problem related to the restricted movement of the toes though. As mentioned earlier, deep to the plantar fascia are those foot intrinsic muscles with tendon attachments to the heel.

Stress Shielding

Stress-shielding is one theory on how connective tissue injuries happen. Stress-shielding suggests that the part of a tendon exposed to a low amount of strain is where problems develop. This contrasts sharply with the conventional theory that excessive strain is the cause of heel pain.

It’s possible that the restricted movement of the toes leads to degeneration of either the plantar fascia or the tendons of the intrinsic muscles because while wearing a shoe those structures are not put under enough tensile stress!

Not only do shoes restrict the ability of the toes to contact the ground, they also limit their ability to fully extend. When going barefoot extension of the big toe is somewhere around 48 degrees. The toe only extends to about 28 degrees when wearing a standard shoe with an orthotic. Because of this, the tissue on the underside of the foot is prevented from moving through a full range of motion. Its reasonable to think that keeping the foot in a fixed position for long periods of time in combination with some of the other factors mentioned could lead to issues.

Built-in Support

The foot arch not only supports the weight of the body, it also provides a space underneath the foot for nerves and blood vessels to pass. A structural arch is supported at its ends, not in the middle. Many shoes have a built-in arch support which takes up space under the arch. This could possible add compressive force near the attachment of the plantar fascia.

Inflexible Sole

Similar to the toe spring, a shoe with a rigid sole also alters the natural movements of the foot. All individuals are built differently. It’s very likely that the few spots where a shoe with a rigid sole bends aren’t matched perfectly with where the foot can bend and twist.

Raised Heel

Most traditional shoes typically are by at least 10mm. This might not directly affect the plantar fascia, but the raised heel does a couple of undesirable things.

• It places the ankle in a plantar flexed (downward pointed) position which can lead to adaptive shortening of the calf muscles. As noted early, calf inflexibility is suspected as major risk factor for developing plantar fasciitis.
• It alters the way a person distributes their weight on the foot.
• Lastly, the extra cushioning under the heel can cause a person to strike the ground harder with the heel! 

My History With Plantar Heel Pain

Around 15 years ago I developed plantar fasciitis in my left foot. A few months earlier I started wearing running shoes and inserts recommended to me by a specialty running store. The “expert” from the store said the shoes and insoles would help with my pronation. I wasn’t running much at the time. My BMI was in the normal range. But I was wearing the shoes for work, standing and walking around in them for eight or nine hours every day.

I tried stretching, ice, taping–all the common recommendations for plantar fasciitis–and none seemed to provide lasting relief. The pain lasted for several months.

What treatment finally worked?

Getting different shoes. A short time after I stopped wearing those particular shoes and insoles the pain went away. Coincidence? Maybe. At the very least that experience made me think that supportive shoes and orthotics are not a great treatment for plantar fasciitis.

Are shoes enough to cause plantar fasciitis by themselves? Maybe.

But I can see how combining a shoe that restricts motion and muscle activity in the foot with increased stress, especially compressive stress, could.

Sources:

1. Wearing SC, Smeathers JE, Urry SR, Hennig EM, Hills AP. The pathomechanics of plantar fasciitis. Sports Med. 2006;36(7):585-611.
2. McPoil TG, Martin RL, Cornwall MW, Wukich DK, Irrgang JJ, Godges JJ. Heel pain–plantar fasciitis: clinical practice guidelines linked to the international classification of function, disability, and health from the orthopaedic section of the American Physical Therapy Association. J Orthop Sports Phys Ther. 2008 Apr;38(4):A1-A18.
3. Orchard J. Plantar fasciitis. BMJ. 2012 Oct 10;345:e6603.