What is defined as the immobility of muscles or a joint caused by shortening or wasting of tissue or muscle fibers?

The correct answer is contractures, which refers to the condition characterized by the immobility of muscles or joints resulting from the shortening or wasting of tissue or muscle fibers. Contractures often develop when a muscle or group of muscles is not used for an extended period, leading to tightened tissues around the joints. This can occur due to various reasons, including disuse after injury, neurological conditions, or prolonged immobilization in one position. When a contracture occurs, it limits the range of motion and can cause discomfort or pain, severely affecting a person's mobility and daily activities.

The other terms describe different phenomena related to muscle function or joint mobility. Spasm typically refers to an involuntary contraction of a muscle, which can cause pain but does not inherently result in immobility due to shortening. Rigidity suggests a state of stiffness and resistance to movement, often observed in certain neurological disorders, but it does not specifically imply the shortening of tissue. Stiffness can be a symptom associated with various conditions but does not specifically mean that the tissues have shortened or wasted. Thus, the definition of each other term does not encompass the specific mechanism involved in contractures.