Pain under the heel at the first step in the morning is the most characteristic symptom associated with plantar fasciitis. Plantar fasciitis is the tearing, inflammation and degeneration of the plantar fascia, an important connective tissue structure in the bottom of the foot. The plantar fascia is similar in structure to a ligament and is composed of collagen, elastin, glycosaminoglycans (GAGs), fibroblasts and water. Type I collagen adds to the fascia’s considerable strength and the elastin increases elasticity, as the name implies. Glycosaminoglycans are long chains of repeating sugar units which attract water and function to resist pressure.
These structural components allow the fascia to elongate and deform as necessary during standing and walking. The plantar fascia is a fairly stiff structure and designed to resist elongation. Collagen will stretch and deform, but the amount of stretching is dependent on time and load. When a specific load is applied to an object, the object will deform. The amount of deformation is dependent on the object’s structure and composition. Slowly applying a load over a period of time will result in more deformation than quick load application. When the plantar fascia is placed under a significant load (eg: full body weight placed on the foot when stepping down), a large amount of force is placed on the fascia in a very short period of time. The amount of elongation, or deformation, is minimal. When a smaller load is placed on the foot over a longer period of time (eg: during a stretch), the structural components of the fascia have time to respond and the fascia can elongate to a greater degree. The diagram below helps to illustrate the mechanics of collagen fibers elongating when a load is applied.
Ligaments will shorten, contract or tighten when immobilized. This also occurs when ligaments are injured. Connective tissue structures tend to contract with stress deprivation. This means that an injured plantar fascia is prone to shortening at rest. This is commonly seen during the night and even at rest for periods greater than an hour. Upon waking or rising and stepping down, the shortened plantar fascia is subject to a considerable load. The fascia is suddenly stretched and only partially elongates.Due to the quickly applied load, the fascia acts like a cable, tightening under the arch and pulling on the heel bone (calcaneus). This traction force on the calcaneus stimulates a reaction in the outer layer of bone, called the periosteum. The outer layer of bone is innervated with pain fibers which initiate an immediate, sharp pain response. As walking continues, the repeated load applied to the plantar fascia allows it to slowly elongate and the pain eventually dissipates. This creates the common complaint of heel pain at the first step in the morning, or stepping down after long periods of rest, which tends to “work itself out.” The continued pull, over time, from the plantar fascia on the heel bone may cause a heel spur to develop. Although the traction on the heel bone contributes to morning pain, the actual spur is not the cause of pain.
Night splints and stretching exercises are commonly prescribed for treatment of plantar fasciitis. Applying a small load over a long period of time will cause a gradually deformation of the tissue, essentially elongating the plantar fascia. Fascial structures, ligaments and tendons do not undergo as much deformation as muscles. Muscles can be elongated to a greater degree and will remain elongated when a stretch is performed regularly. Night splints also stretch the calf muscles and help improve plantar fasciitis. Tight calf muscles cause the heel to lift early when walking. When the heel lifts early, more stress over a longer period of time is placed on the plantar fascia. Morning calf stretches and use of a night splint will decrease morning heel pain and accelerate the healing of plantar fasciitis.
View our five part series on plantar fasciitis, plantar fasciosis and plantar fascia structure.
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