Abnormal posturing is an involuntary flexion or extension of the arms and legs, indicating severe brain injury. It occurs when one set of muscles becomes incapacitated while the opposing set is not, and an external stimulus such as pain causes the working set of muscles to contract. The posturing may also occur without a stimulus.^{[failed verification]} Since posturing is an important indicator of the amount of damage that has occurred to the brain, it is used by medical professionals to measure the severity of a coma with the Glasgow Coma Scale (for adults) and the Pediatric Glasgow Coma Scale (for infants).

The presence of abnormal posturing indicates a severe medical emergency requiring immediate medical attention. Decerebrate and decorticate posturing are strongly associated with poor outcome in a variety of conditions. For example, near-drowning patients who display decerebrate or decorticate posturing have worse outcomes than those who do not. Changes in the condition of the patient may cause alternation between different types of posturing.

Three types of abnormal posturing are decorticate posturing, with the arms flexed over the chest; decerebrate posturing, with the arms extended at the sides; and opisthotonus, in which the head and back are arched backward.^{[citation needed]}

Decorticate posturing is also called decorticate response, decorticate rigidity, flexor posturing, or, colloquially, "mummy baby". Patients with decorticate posturing present with the arms flexed, or bent inward on the chest, the hands are clenched into fists, and the legs extended and feet turned inward. A person displaying decorticate posturing in response to pain gets a score of three in the motor section of the Glasgow Coma Scale, caused by the flexion of muscles due to the neuro-muscular response to the trauma.

There are two parts to decorticate posturing.

The effects on these two tracts (corticospinal and rubrospinal) by lesions above the red nucleus is what leads to the characteristic flexion posturing of the upper extremities and extensor posturing of the lower extremities.^{[citation needed]}

Decorticate posturing indicates that there may be damage to areas including the cerebral hemispheres, the internal capsule, and the thalamus. It may also indicate damage to the midbrain. While decorticate posturing is still an ominous sign of severe brain damage, decerebrate posturing is usually indicative of more severe damage at the rubrospinal tract, and hence, the red nucleus is also involved, indicating a lesion lower in the brainstem.^{[citation needed]}

Decerebrate posturing is also called decerebrate response, decerebrate rigidity, or extensor posturing. It describes the involuntary extension of the upper extremities in response to external stimuli. In decerebrate posturing, the head is arched back, the arms are extended by the sides, and the legs are extended. A hallmark of decerebrate posturing is extended elbows. The arms and legs are extended and rotated internally. The patient is rigid, with the teeth clenched. The signs can be present on only one side of the body or on both sides, and they may be present just in the arms, and they may be intermittent.