By far, the most common cause of heel pain in runners is plantar fasciitis. Until recently, it was assumed that excessive lowering of the medial arch in flat-footed individuals increased tension in the plantar fascia and overloaded the insertion of the plantar fascia on the heel bone.
The plantar fascia functions passively to store and return energy, while the arch muscles play a more dynamic role in “variable load sharing.” Apparently, the intrinsic muscle work with the plantar fascia to prevent lowering of the arch during early stance, while also assisting with arch elevation during propulsion. This explains why the development of plantar fasciitis is not correlated with arch height, and the best predictor of the development of plantar fasciitis is the speed at which the toes move upward during the propulsive period.
In the most thorough study of plantar fasciitis to date, Sullivan et al. took 202 people with heel pain and compared them to 70 asymptomatic (no pain) control participants. They determined that in addition to toe weakness and calf tightness, people with chronic heel pain had significant weakness of the peroneals. This finding was significant, as prior to their research, no one had even considered that the peroneals played even the slightest role in the development of PF.
Whenever possible, runners with PF should avoid making initial ground contact with their midfoot or forefoot, since these strike patterns significantly increase tension in the plantar fascia.