The anti-g straining maneuver, or AGSM, is a physical technique used by fighter pilots, aerobatic pilots, and astronauts to counteract the effects of high acceleration. It combines muscle tensing and a specific breathing pattern to manage blood flow under intense G-forces. This allows individuals to maintain consciousness and tolerate higher levels of acceleration for longer durations.
Physiological Impact of High G-Forces
During maneuvers that produce high positive G-forces, the force of acceleration is directed from head to foot. This causes blood to be pulled away from the head and pool in the lower extremities. The heart struggles to pump blood against this force, leading to a drop in blood pressure in the arteries supplying the brain.
The initial symptoms of oxygen deprivation to the brain are visual. A pilot will first experience a loss of peripheral vision, often described as tunnel vision or a “greyout.” As the G-force increases or is sustained, this can progress to a complete loss of color vision.
If exposure continues without intervention, the result is G-induced Loss of Consciousness (G-LOC). G-LOC occurs when blood flow to the brain ceases, causing a complete loss of function in about four to six seconds. The individual becomes unconscious and can take a minute or more to recover, during which time they are unable to control their aircraft. This loss of control makes a countermeasure necessary.
Performing the Maneuver
The AGSM has two components that work to maintain blood pressure and keep the brain supplied with oxygenated blood. The first is a powerful, continuous isometric contraction where the pilot forcefully tenses the muscles of their legs, buttocks, and abdomen. This muscular clamping action squeezes the blood vessels in these areas. This increases resistance to blood flow into the lower extremities and helps push blood back toward the heart.
This muscular tension is combined with a specialized breathing technique, sometimes referred to as the L-1 maneuver or “Hook Maneuver.” This respiratory component involves a rhythmic cycle that increases pressure inside the chest cavity.
The pilot takes a deep breath, then performs a short, forceful exhalation against a closed or partially closed glottis. This strain is held for approximately three seconds, raising pressure inside the chest and preventing blood from leaving the chest and head. After the hold, the pilot performs a rapid gas exchange—quickly exhaling and inhaling—before repeating the strain. This cycle is repeated for the duration of the high-G turn, creating a pressure lock that keeps blood in the upper body.
Training and Equipment Integration
The AGSM is a standard part of training for pilots of high-performance aircraft and astronauts. This training is conducted in a human centrifuge, a device that simulates the G-forces of flight. The centrifuge allows pilots to practice their AGSM technique in a controlled environment while their physiological responses are monitored.
The maneuver is performed in conjunction with an anti-G suit, a garment with inflatable bladders covering the legs and abdomen. As G-forces increase, the suit automatically inflates, squeezing the lower body. This external pressure provides a base level of G-protection.
While the G-suit is effective, it is not sufficient on its own to prevent G-LOC at very high G-levels. A properly performed AGSM can add approximately 3 Gs of tolerance on top of the protection offered by the suit. The combination allows for sustained operations in environments up to 9 Gs or more.