Which term describes the involuntary contraction of muscles, often resulting in stiffness?

The term "rigidity" specifically refers to the involuntary contraction of muscles that leads to stiffness throughout the muscle. This condition is often associated with various neurological disorders, particularly those affecting the brain and spinal cord, such as Parkinson's disease. Rigidity implies a consistent state of muscle tightness, which does not fluctuate and can result in resistance to passive movement.

In contrast, spasticity refers to a condition where certain muscles are continuously contracted, leading to stiffness but with a fluctuating resistance during movement. Weakness signifies a decrease in muscle strength or the ability to generate muscle force, which is different from the involuntary contraction characterized by rigidity. Sarcopenia is related to the loss of muscle mass associated with aging rather than an involuntary muscle contraction.

Therefore, rigidity is the most accurate term that captures the essence of involuntary muscle contraction leading to stiffness.