Top Gun: how does the body cope behind the scenes?

Our species is acclimatized to a world placed under the yoke of a gravity constant — in this case, an omnipresent force of acceleration due to the Earth’s attraction (the unit of Earth’s gravity, denoted g, equal to 9.81 m/s²). There are however circumstances where our body is subjected to stronger than the traditional terrestrial gravity… It is there still a matter of acceleration.

In aeronautics or in theautomotivespecialists refer to the G (for Gravitational), or load factor, as a unit of acceleration. And its effects can be dreadful.

As children learning to walk, we very quickly discover that one misstep will eventually result in a painful impact with the ground due, precisely, to gravity. When we get on a plane, this time short of crashing, everything we’ve learned about gravity and what we’re used to suddenly changes. Just watch the latest convolutions Pete “Maverick” Mitchell’s aerials in the latest Top Gun to be convinced.

The flight consists in fact in overcoming gravity in order to rise in the tunesand the speed is essential there. Any aeronautical maneuver can therefore expose our body to significant accelerations, with significant repercussions on the cardiovascular, cerebral and joint levels. Some aircraft are thus capable of reaching 12G, with acceleration climb speeds greater than 15 G/s !

How many Gs do we experience on a daily basis?

Such figures are of course extremes. While remaining motionless on the ground, the acceleration felt is 1G. Everything is fine. At 2G, for example when taking a 60 degree inclined turn, we already have a feeling of moderate compression on our seat, difficulty in moving. A person of 80 kgi.e. a weight of 784.8 N on Earth (considering that it is a situation equal to 1G), will have the sensation of having a weight of 1569.6 N if it undergoes 2G (the Kg being the unit of massthe Newton that of weight). From 8-9G it is impossible to mobilize its limbs, except for the extremities.

In fact, there are three main types of G present in three axes of space. We can experience lateral Gs (Gy) when turning as a result of centrifugal acceleration pushing us outward. For a horizontal acceleration or deceleration, we speak of Gx. Finally, Gz occurs during a descent of the aircraft or following a sudden climb. We are more particularly sensitive to these accelerations undergone in the vertical axis (Gz), that is to say from head to toe, since it is there that we feel the force of the earth’s gravity necessary to maintain its balance.

To further complicate the situation, for the three axes, positive but also negative Gs are possible… Whether in a turn for a car or vertically for an airplane, a resistance opposed to movementthe inertial force, is added to the actual weight due to gravity to give the “apparent” weight of the aircraft in flight. When the apparent weight in motion is greater than the actual weight, the load factor is greater than +1G. On the other hand, if the plane flies on the back for example, the load factor is expressed as negative, -G.

To calculate the G to which they are subjected, airplane pilots, who are particularly exposed, are equipped withaccelerometer three axes: they can thus know in real time what they undergo.

How our bodies normally deal with gravity

The airplane pilot is indeed subjected in flight to a wide variety of physiological effects due to the combinations of acceleration and gravity. They are inherent in the forces ofinertia generated by the accelerations and apply to all the organs of the body, and in particular to thecardiovascular system : the heart (the pump), the vessels (the circuit), the blood (the fluid).

However, blood circulation ensures the transport of oxygen, which is essential for the proper functioning of the organs. The brain is particularly demanding in this area, both in terms of consumption (it is greedy) and the regularity of its supply. He doesn’t like jolts, surpluses or shortages!

On Earth, there is a complex mechanism of control and adaptation of all the machinery that ensures regular and well-oxygenated blood circulation at debit constant to the brain, whether at rest or in full effort: this is cerebral self-regulation. Any variation of the arterial pressure is thus of no consequence. But this beautiful balance has its limits… Acceleration in bends, braking or a fortiori the practice of aerobatics will disturb him greatly.

The ability to maintain a irrigation cerebral blood, resilient to repeated exposures to increased load factors, is therefore a critical issue for pilots exiting the normal conditions Daily.

When our physiological adaptations are no longer enough

The risks were identified, though poorly explained, more than a century ago. In 1918, the first disturbance induced by acceleration was thus felt during the air race of the Schneider Cup where a sharp turn had to be taken. First described as a “malaise in the air”, it is now known as “G-induced loss of consciousness”, or G-LOC and results in confusion and impaired judgment following a temporary abolition of cerebral circulation. A state that occurs from +4.5-6G in the trained pilot.

As the heart is in the thorax, in a vertical position (standing or sitting), the vascularization of the brain, positioned above it, forces the blood flow to fight against its own weight (hydrostatic pressure) to rise from one to the other. In the presence of +Gz, the force of inertia oriented on the head-feet axis will be added to the hydrostatic force and aggravate the situation by opposing the movement of blood from the heart to the head.

Beyond +3Gz maintained for more than ten seconds, our self-regulation mechanisms are overwhelmed with the immediate consequence of a drop in vision and mental performance. This can result in visual disturbances such as the “grey veil” (from 3-4,5Gdue to the decrease in blood circulation in the retina and peripheral vision) and the “black veil” (from 4.5-6G, with cessation of blood flow).

Negative accelerations (-Gz) cause opposite adaptation mechanisms to those of +Gz, accompanied by a more unpleasant feeling and greater perceived fatigue.

But the main problem lies in the rapid succession of -G and +G at high values (effect ” push pull”, or pitch-up), as in aerobatics, which is particularly poorly tolerated. This stems from the disruption of our adaptation mechanisms and our greater sensitivity to the phenomena of veiling and/or loss of consciousness which can occur from +2Gz.

Identify the limits…

If the response of the cardiovascular system does not keep pace with the appearance of the Gs, the pilot’s performance will be degraded to the point of causing loss of consciousness. To avoid this dangerous extremity, studies have helped to better identify the limits of our adaptive capacities and to develop techniques to overcome them.

The establishment of tolerance curves +Gz-time allowed comparison of asymptomatic and symptomatic individuals. The upper limit of these curves, marked by the loss of consciousness (LOC-G), is an essential factor in our physiological response to accelerations.

It appeared that if the increase in acceleration is gradual, the visual symptoms precede the cerebral symptoms. However, for accelerations greater than +7Gz reached quickly, loss of consciousness is not preceded by warning signs. Indeed, if the rate of increase in acceleration is sufficiently low, the cardiovascular reflexes can, at least partially, compensate for the modifications of the circulation. The tolerance threshold is thus increased.

In general, it has also been found that everyone’s sensitivity to these effects is variable and can be modified with practice. Several factors can affect tolerance to accelerations.

If the heat is not too important, a well-rested, hydrated and fit rider physical will be able to tolerate +5Gz. This is explained by the fact that the volume of blood circulating in the body is more important and available: it is then easier for the cardiovascular system to keep the brain perfused with oxygenated blood.

…To overcome them: the training of expert pilots

Expert pilots additionally use musculo-respiratory movements : head tucked into the shoulders, leaning forward, to reduce the height of the hydrostatic column, contraction of the abdominal muscles and lower limbs to slow the flow of blood, intrathoracic overpressure by expelling air or glottis closed with the diaphragm and tight neck muscles.

A regular physical training program including a mix of endurance and strength exercises also increases the pilot’s tolerance to G effects. Important factors to consider are trunk strength and ability aerobic. Any aerobic endurance activity (even apnea or at altitude) is good for the cardiovascular system.

Core-strengthening exercises (cladding, push-ups, pull-ups, sit-ups) and, above all, those that strengthen the neck muscles are a must: high Gs make the head weigh more than normal, and with a helmet, that makes a lot of weight to bear. The pilots of the fastest and most agile aircraft must constantly monitor their outer bearings and modify their leading position during their maneuvers.

Aerobatics is responsible for the onset and/or aggravation of pains spinal. Muscular reinforcement to cope with repeated strong accelerations is essential for these pilots, considered as high-level athletes who, moreover, evolve in extreme environments.

Several tools can also improve individual tolerance to accelerations. Developed very early during the world wars, the anti-g pants, by applying counter-pressure to the lower part of the body in response to accelerations, ensures sufficient venous return. However, these devices only process +Gz and are inappropriate in aerobatic aircraft due to their weight.

Other innovative devices are being developed in research centers and companies in the sector. This is the case with the work carried out by EuroMov Digital-Health in Motion and society Semaxonewho develop algorithms and sensors in order to measure thelive cerebral oxygenation to anticipate the evolution of the pilots’ tolerance to accelerations.