One of these, bursitis, can afflict anyone who incurs repetitive stress to a joint, particularly the knee, elbow and shoulder. Basically, bursa sacs allow the skin to slide over the bony prominence. You can think of the bursa as an empty balloon with a drop of oil in it - one side of the balloon is attached to the bony surface and the other to the underside of the surface of the skin and it slides. In athletes, injuries to the bursa sacs are usually caused by repetitive stress to the bursa sac either through overuse or consistent contact with a hard surface. So, in short, bursitis is an inflammation of the bursa sac where it becomes irritated causing pain in the joint.
Secretaries get bursitis in their elbows, plumbers get it in their knees.
The older diagnosis of shoulder bursitis, which I think of now as more of a rotator cuff tendinitis than bursitis in the strict definition, can afflict javelin throwers, pitchers or even electricians doing work over their head. Most tissues of the body will respond to stretching with the symptom of pain. When you have gas in the stomach, it's the stretching of the wall of the stomach that's very painful. In the bursa, it's the stretching of the sac as it fills with fluid that causes the pain in the joint. There is also some streching of the surrounding ligaments.