The muscles are often affected in an asymmetrical way with the muscles on one side of the body being weaker than those on the other side. The pattern of weakness in IBM is characteristic. The most frequently affected muscles are the quadriceps (the thigh muscles, which straighten the knee joint) and forearm muscles (that flex the wrists and fingers). Accordingly, people affected by IBM may fall and can have difficulty climbing stairs, getting out of a chair and poor hand-grip. Although many people with IBM do present with this characteristic pattern of weakness, it is not evident in everyone, particularly early in the condition. In most people the quadriceps muscles are affected first but in others the forearm or other muscles are affected first.
Swallowing muscles are affected in some people, but this is a rarely a significant problem early on and most do not encounter severe swallowing problems. The condition does not affect the heart, eyes, gut or bladder. It does not affect the function of the brain or sensation, and speech is rarely affected.
The condition itself does not cause pain. However, weakened muscles can predispose to problems such as falls resulting in injuries affecting bones, joints and soft tissues.