Your joints are full of connective tissues and lubricating fluids to keep them swinging smoothly. One of the key cushioning agents is the bursae, which are small fluid sacs that absorb shocks and keep bones from rubbing up against each other. Repetitive movements like long-distance running can put too much strain on the bursae. They can become painfully inflamed, a condition known as bursitis. The symptomatic feeling is stiff, achy joints that may be visibly red or swollen.
Runners are most susceptible to bursitis in the hips, knees and heels (baseball pitchers and football quarterbacks tend to get it in their shoulders). In some cases, the bursa sac can be punctured, which can lead to infection and a more serious condition known as septic bursitis.
Treatment is simple: Immobilize and rest the joint, apply ice and take anti-inflammatory medications [source: Mayo Clinic].