Muscle rigidity refers to an involuntary tensing and stiffness of the muscles. It is caused by neurological conditions, musculoskeletal disorders and reactions to some medications or black widow spider venom, among other things. Doctors usually do many different tests to try to determine the exact cause of muscle rigidity, as there are at least 60 different conditions that could cause it. If a person with involuntary muscle tensing has been bitten by a black widow spider and/or is also experiencing breathing difficulties, intense restlessness or loses consciousness, it's important to get immediate emergency medical help.

A black widow spider bite usually causes stomach pain as well as the abdominal wall and stomach muscles to become rigid. Other causes of involuntary muscle stiffness are anti-psychotic drugs, such as those used to treat psychological disorders such as schizophrenia. Muscle rigidity is often listed in the product packaging of anti-psychotic medications as a possible side effect of taking the drug. Along with muscle stiffness, muscle twitching and sore back muscles may also occur.

Lockjaw, or tetanus, is a nervous system disease caused by a soil bacterium called Clostridium tentani. The main symptom of tetanus is muscle rigidity in the jaw and neck. Lockjaw requires medical attention as it can cause breathing difficulties and heart problems leading to death. After affecting the neck and jaw, the muscle stiffness may affect the stomach and other areas of the body. Other symptoms of lockjaw include fever and sweating.

Parkinson's disease, a degenerative central nervous system disorder, causes a distinctive type of muscle rigidity in facial muscles. The face becomes stiff and mask-like due to a lack of muscle control. Another central nervous system disease, cerebral palsy, causes a loss of muscle control throughout the body. The extreme involuntary muscle tightness in cerebral palsy is called hypertonicity.

Multiple sclerosis is a disease of the brain and spinal cord. Muscle rigidity is a main symptom of multiple sclerosis. Rigid, painful muscles are also caused by the musculoskeletal disorder called fibromyalgia. Many people with fibromyalgia especially experience muscle pain and stiffness upon waking in the morning.

Grand mal seizures associated with the neurological disorder epilepsy can cause extreme muscle rigidity. The body seizes up and the muscles go rigid because muscle control is lost during the seizure. Seizures occur in epilepsy when neurons in the brain are discharging abnormally.