What is it?
Plantar fasciitis is a painful condition that affects the bottom of the foot, more precisely in the heel (which is often call heel spur syndrome). In the plantar fasciitis, there is an active inflammation of the origin of the plantar fascia and its structures, which causes pain and tenderness beneath the heel. The plantar fascia is a thick layer of tissue that goes from the heel to the front of the foot and it is this layer of tissue that supports the arch of our foot. In about 30%, both feet are affected, but non-surgical treatment of plantar fasciitis, like physical therapy, is successful in 90% of the cases.  Nevertheless, physical therapy after surgery should be considered to gain mobility of the plantar fascia and reduce the pain or to restore normal sensibility, if it exists.
When the foot is on the ground, it sustains a tremendous amount of force (our body weight), which causes some stress to the plantar fascia where it attaches to the heel. Some small ruptures of the fascia can occur, which are normally repaired by our body. But, in this constant and repetitive process of injury and repair, a bone spur (an outgrowth of the bone) can be formed, in a response of the body to try to firmly attach the fascia to the heel, preventing the total rupture of the fascia. The formation of the bone spur will create an inflammation, which will lead to degeneration of the fat pads.
The primary function of the heel pad is to absorb the shock each time we strike the heel on the floor when we walk or run. As we age, the fat pad becomes thinner and starts to break down, reducing the amount of shock absorption. 
Plantar fasciitis is characterized by a gradual onset of sharp or dull pain deep in the bottom of the heel or arch. Pain is usually more intense with the first few steps in the morning or when trying to walk after extended periods of sitting. Symptoms may decrease after walking for a few minutes. Pain may return of increase towards the end of the day. Positional pain, especially with flexion or extension of the foot or toes. Possible sensations of burning, tingling, or numbness. Interestingly, the spur itself is rarely the cause of pain. 
The symptoms of the plantar fasciitis include pain along the inside edge of the heel near the arch of the foot. The pain is worse when weight is placed on the foot, which is more pronounced in the morning when we first place our foot on the floor. Prolonged standing increases the symptoms, and it is greater at the end of the day. 
The most common cause of plantar fasciitis is related to poor structures of the foot. For example, people who have problems with the foot arches (flat feet or cavus feet), are more prone to develop plantar fasciitis.  In flat feet the plantar fascia may become overloaded during exercise activities, such as running. In cavus feet the plantar fascia is very tight and there may be considerable pressure at the attachment to the heel bone. 
The most frequent cause is an abnormal motion of the foot while walking, called excessive pronation. This will place abnormal stresses on the plantar fascia itself, and its sites or insertion into the bones or feet. Get advice near your physical therapist about postural chances during the gait you should adopt and exercises you can do to correct your posture. Other factors that may contribute to plantar fasciitis and heel spurs include a sudden increase in daily activities, increase in weight, changes in training regimen (intensity and duration), or a change of shoes. 
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 Tennis Injury Free. Heel Pain: Plantar Fasciitis and Heel Spur.
 Physical Therapy At Briarcliff & Jefferson Valley, P.C. Plantar Fascitis and Heel Spurs.