Plantar fasciitis is caused by repetitive overloading of the thick band of tissue that runs along the bottom of your foot, connecting the heel bone to the toes. Despite its name (which suggests inflammation), the condition is actually a degenerative breakdown of that tissue. People between 40 and 60 are most commonly affected, and the condition is diagnosed in roughly 0.85% of U.S. adults.
Degeneration, Not Inflammation
The most important thing to understand about plantar fasciitis is that “fasciitis” is a bit of a misnomer. When researchers examine tissue samples from people who’ve had surgery for the condition, they find fragmented, degenerating collagen fibers rather than the inflammatory cells you’d expect from a true “-itis.” The American Academy of Family Physicians now considers “plantar fasciopathy” a more accurate name, reflecting the absence of inflammation. This distinction matters because it shapes how the condition should be treated. Anti-inflammatory strategies may offer temporary pain relief, but they don’t address the underlying tissue breakdown.
What’s actually happening is a failure of repair. Healthy plantar fascia is about 2.2 to 2.5 millimeters thick. When the tissue is repeatedly strained beyond what it can recover from, the collagen fibers start to break down faster than your body can rebuild them. Over time, the fascia thickens and weakens. On ultrasound, a thickness greater than 4 millimeters is considered abnormal, though diagnostic accuracy peaks at a cutoff of about 3.15 millimeters.
Body Weight Is the Strongest Risk Factor
Carrying extra weight places significantly more load on the plantar fascia with every step. Research on risk factors found that people with a BMI over 30 (the clinical threshold for obesity) had 5.6 times the odds of developing plantar fasciitis compared to people with a BMI under 25. That’s one of the largest effect sizes of any risk factor for the condition. Even modest weight gain can tip the balance, because the plantar fascia absorbs forces equal to roughly two to three times your body weight during walking and even more during running.
Time on Your Feet at Work
Occupations that keep you standing or walking on hard surfaces are a well-documented trigger. One study of assembly plant workers found that increased time standing on hard surfaces and increased time walking both raised the risk of developing heel pain. Separate research put a number on it: spending more than 80% of your workday on your feet was associated with 3.6 times higher odds of plantar fasciitis. Factory workers, nurses, teachers, retail employees, and restaurant staff are among those most commonly affected.
The mechanism is straightforward. Prolonged standing compresses the fascia at its attachment point on the heel bone hour after hour, with little opportunity for the tissue to recover. Hard floors like concrete and tile are worse than softer surfaces because they absorb less shock, meaning the plantar fascia has to absorb more.
Foot Mechanics and Arch Shape
How your foot moves during walking and running plays a central role. Runners with plantar fasciitis tend to show greater rearfoot eversion (the heel rolling inward), more toe extension at push-off, and a lower, more flexible arch during movement. A lower arch during running may both trigger and prolong symptoms, because it stretches the fascia further with each stride.
Both flat feet and high arches can contribute, though through different pathways. Flat feet allow the arch to collapse excessively, pulling the fascia taut along its length. High arches create a rigid foot that absorbs shock poorly, concentrating force at the heel. In either case, the plantar fascia ends up handling more strain than it’s designed for.
Limited ankle flexibility also plays a part. When your ankle can’t bend far enough (picture not being able to pull your toes toward your shin), your foot compensates by pronating more, which again increases fascia load. Tight calf muscles are a common contributor to this restricted motion.
Running and Exercise Patterns
Plantar fasciitis is one of the most common running injuries, and it typically stems from doing too much too soon. Sudden increases in mileage, intensity, or hill work can overload the fascia before it has time to adapt. Running on hard surfaces compounds the problem.
Barefoot running and minimalist shoes deserve special mention. Running without shoes encourages a forefoot strike, which increases the load on the calf muscles and the plantar fascia simultaneously. It also increases how far the big toe bends backward at push-off, a motion that directly tensions the fascia through what’s called the windlass mechanism. If you picture your big toe bending upward and pulling the arch tighter like a winch, that’s essentially what’s happening with each step. Shoes with a modest heel-to-toe drop and a firm, less flexible toe section reduce both of these forces.
Age, Sex, and Other Contributing Factors
Plantar fasciitis peaks between ages 40 and 60, with a median age at diagnosis of 56. This timing isn’t coincidental. The fat pad under the heel thins with age, reducing its ability to cushion impact. The fascia itself also becomes less elastic over time, making it more vulnerable to microdamage. Women are affected at higher rates than men, likely due to a combination of footwear differences, hormonal factors that affect connective tissue, and higher rates of certain foot mechanics.
Other factors that increase your risk include spending long hours in unsupportive footwear (worn-out running shoes, flat sandals, or dress shoes with no arch support), having one leg slightly longer than the other, and having tight hamstrings or hip flexors that alter your gait.
Heel Spurs Are a Result, Not a Cause
Many people with plantar fasciitis are told they have a heel spur on X-ray and assume the bony growth is causing their pain. The relationship is actually the reverse. The chronic pulling of the fascia on the heel bone stimulates new bone growth over time, producing the spur. In one study comparing people with plantar fasciitis to age-matched controls, 89% of those with fasciitis had heel spurs, but so did 32% of people with no heel pain at all. The spur is a marker of long-standing fascial stress, not the source of the problem. Removing the spur surgically without addressing the underlying mechanical causes rarely resolves symptoms.