Experts believe that when the plantar fascia is put under strain it becomes inflamed or develops tiny tears (micro-tears), or both. This damage is typically associated with one or more of the risk factors described below.
Excessive foot pronation. Normally, when the foot lands on the ground it rolls slightly inward, or pronates. A person who pronates too much, however, puts extra pressure on the inner foot, which can strain the plantar fascia. Excessive foot pronation is common in people who have flat feet (pes planus).
High arches (pes cavus). The plantar fascia helps to maintain the foot’s arch, and maintaining a high arch may put strain on the plantar fascia.
Weak plantar flexor muscles. The muscles that run along the bottom of the foot with plantar fascia are the intrinsic muscles of the feet. If these muscles are weak the plantar fascia may be put under excess strain.
Running. Jogging just a few miles forces the plantar fascia to stretch and contract several thousand times. If the fascia is inflamed or experiencing micro-tears with each step, then running will cause pain.
Prolonged standing or walking. Extended periods of standing or walking can strain the plantar fascia. The tissue needs time to rest and recover.
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Not enough activity. Just as too much activity can cause plantar fasciitis, so can too little. When the foot is at rest the plantar fascia is shortened and it can become less flexible. Regular, intermittent activity can keep the plantar fascia supple and strong.
Tight Achilles tendon. When the Achilles tendon is tight the ankle has a limited range of motion, and it is difficult to fully flex the foot (called dorsiflexion), which may eventually lead to tight plantar fascia. One study estimates that nearly 4 out of 5 people with plantar fasciitis also have a tight Achilles tendon.1
Different leg lengths. When a person has different length legs the plantar fascia and other soft tissue of the foot are stressed asymmetrically and have to work harder to absorb shocks and ground reaction forces.
Obesity. Being overweight or experiencing a sudden weight gain can put extra pressure on the plantar fascia.
Training errors. Over-training or excessive, sudden increases in training (especially increases in running mileage, speed work, or hill running) may strain the plantar fascia.
For many years doctors believed that plantar fasciitis was caused by heel spurs, however, recent research does not support this theory. In fact, experts now believe that overstretching the plantar fasciitis can lead to the development of heel spurs.