Why can't people jump again in the air?

As we all know, there is triple jump in field events, but we have never heard of double jump or triple jump, because it is impossible. So why can't a man jump in the air a second time?

People stand still on the ground because the gravity they receive is equal to the supporting force of the ground on the human body, and the direction is opposite, so the human body is in a state of balance. However, when the athletes pedal hard and the muscles contract, the generated force increases the pressure on the ground. According to Newton's third law, the ground will give the athletes a supporting reaction force greater than gravity, so that the athletes jump up. This is the superficial truth of the rush high jump. For the human body, the muscle force is the internal force, while the ground support reaction force is the external force. The internal force affects the external force through the hard ground. If the ground is sand, the jumping height of the same athlete is much smaller than that on hard ground, because the retreat of sand reduces the influence of internal force on external force.

When an athlete jumps into the air, his feet are in contact with the air. At this time, it is useless to pedal the air again, because the air resistance is very small, and the muscle force can no longer affect the external force. This is just like suspending a toy car and starting the engine, although the wheels rotate, the car stays in place. We know that after taking off, if there is no influence of other external forces except gravity, the trajectory of the body in the air cannot be changed, and it is futile to pedal several times.

However, it should be noted that although the trajectory of the body center of gravity remains unchanged, the use of different air movements has an impact on the performance, which is the reason why the back style jumps higher than the leap style.