What can happen if you move with very high acceleration? You can intuitively consider acceleration as the sudden change in velocity. The greater the change in velocity in a very short time, the greater the acceleration.
A human body is not just a particle but it is a collection of particles and our body can not be considered as a rigid body. Our body can only accept the acceleration up to a certain range. The higher acceleration beyond that range would be fetal and can even be the cause of death.
Let's look a little bit at the Physics behind this. Why the high acceleration is fetal? As already pointed out that our body is a collection of particles and to be honest it is loosely structured. Loosely structured means it is not compact and rigid.
Let's imagine a situation where a soldier jumps from a plane. If the soldier can not open the parachute for some reason, the soldier's velocity continuously increases because of the Earth's acceleration due to gravity. And that final velocity becomes maximum just before reaching the ground.
That huge final velocity suddenly approaches zero when the soldier hits the ground and that causes a large acceleration intolerable to the human body and causes death. That kind of incident has happened in real life situations and it is quiet dangerous.
It's the acceleration that kills not the velocity. You know from Newton's first law that a constant velocity is the equilibrium condition in which the net force is zero. But the acceleration is the state of change in velocity.
The change in velocity is the acceleration and it causes force. The huge negative force caused by the huge negative acceleration potentially rips the body apart both internally and externally when the body strikes the surface from a very high altitude. A great caution must be applied for the soldiers diving with parachutes.
The body does not need to strike a surface to be dangerous. If you move with very high velocity and suddenly stop, that provides a very high acceleration and causes large force.
Where does the inertia play the role here? Inertia is the tendency of a body at rest to stay at rest and the body at motion to stay at motion. So, let's imagine the time just before the body strikes the ground. The body was moving at the final instantaneous velocity, and the body wants to be in motion but the ground does not let it do that and causes a large negative acceleration.
Let's go into the real life situations of high acceleration. There is a term called g-force which is no doubt a misleading term because it is not the force but it is the force per unit mass. The force per unit mass is the acceleration, therefore, g-force is the measure of acceleration.
The most common value of acceleration due to gravity we use is 9.8 m/s2. The g-force is the gravitational acceleration, that is the weight per unit mass whose value is in general 9.8 m/s2. The value of 1g is therefore 9.8 m/s2 where we have used the conventional value of gravitational acceleration despite it is not the same at all places on Earth. We sometimes use 1g equal to 10 m/s2 to roughly estimate the total acceleration.
If the body is not fastened properly, different parts of the body may have the different tendencies of rest and motion. Some part of body may come to rest earlier than other parts and that kind of situation must be overcome to be able to withstand high accelerations.
The g-force (acceleration) a human can withstand depends on the horizontal and vertical acceleration and also on the duration of acceleration. In horizontal acceleration the motion is perpendicular to the spine and in vertical acceleration the acceleration is parallel to the spine. When the acceleration is vertical, the g-force a human can withstand also depends on the upward and downward accelerations.
In normal circumstances, a human can withstand upward vertical acceleration of about 5g for a few seconds. With 5g acceleration your weight is 5 times your weight without acceleration. The upward acceleration forces blood downward towards the feet which normally affects brain and eyes, because the blood is drawn towards the feet away from head. With the increase in acceleration, more complications occur. With appropriate care and g-suits, the pilots can go up to 9g.
The ability to withstand downward acceleration is even lower such as 2g to 3g. This acceleration forces blood towards the head causing excessive blood in the eyes and brain which can cause blood vessel swelling and can even explode. The downward acceleration is more dangerous than the upward acceleration.
The horizontal acceleration is much safer than the vertical acceleration. This acceleration is perpendicular to the spine. The horizontal acceleration can further be divided into forward and backward accelerations. In forward acceleration aka "eyeballs-in acceleration" more acceleration can be tolerated than in the backward acceleration aka "eyeballs-out acceleration" because the blood vessels in the eye are more affected in the backward direction.
In an experiment by US Air Force where a physician named John Paul Stapp went to the speed of as high as 632 mph which is 1,017 km/h and made him the fastest man on Earth. The backward acceleration he went through was 46.2 g with proper harnessing which is the highest known experimentally tested acceleration that a human can withstand.
Other early experiments showed that a human can go through both forward and backward acceleration of 20g for less than 10s, 10g for 1 minute and 6g for 10 minutes without any harm to the body and senses. It was tested not to cause any harm, if the duration of acceleration was longer, it could cause serious harm.