Final answer:
The lack of joint mobility due to abnormal shortening of muscle is called contracture, which leads to decreased joint flexibility and is different from conditions such as arthritis, sprains, and fractures.
Step-by-step explanation:
Lack of joint mobility caused by abnormal shortening of muscle is known as contracture. This condition occurs when muscles fail to stretch resulting in the stiffening of the muscle without the joint moving, ultimately leading to the reduction in flexibility. It's not arthritis, which involves inflammation and can cause joint pain and stiffness; not a sprain, which involves ligaments; and not a fracture, which is a break in a bone.