Imam ibn Qayyim al-Jawziyyah said in his book Inner Dimensions of The Prayer under the subheading Servitude in al-Sujud (prostration) on pg. 50:
The abd (servant) is subsequently prescribed to say "Allah is the Most Great" and then fall down in prostration to the floor to let all his limbs individually demonstrate their servitude to Allah; his forehead placed on the floor before his Lord, his face - the most honorable part of his body - comes together with dust on the floor, and all the while his heart humbles itself to Allah.
The arrangement of this posture is such that his lower body ends up in a higher level than his upper limbs (i.e. the face), emphasizing his submission.
His heart and body are thus rendered humble and submissive before the Greatness and Glory of the Lord.
Moreover, when the state of the heart conforms to the state of the body in this position (i.e. both are humble, submissive and down to earth), it is as if the heart too prostrates to Allah along with the forehead, nose, face, hands, knees and feet.
An abd with such humility and submissiveness shall be among Allah's favoured servants. this is because the closest an abd comes to his Lord is when he is in the state of prostration.
In this posture, it is prescribed for the abd to not have any part of his body depend on any other part for support; that is he should avoid placing weight of his thighs on his legs, maintain space between his belly and his legs, and keep a space between his arms by spreading them to the sides away from his body.
This is so that each part of his physical self can individually demonstrate its slavery to Allah. And it follows from what has been said that the slave (of Allah) should in this posture be at the closest point to his Lord, as the Prophet (pbuh) said: "The closest an abd is to his Lord is when he is prostrating." (Muslim)
The reality of the prostration of the heart is its submission to Allah, and the heart is able to remain in prostration until the Day of Judgment.
One righteous Salaf was asked: "Does the heart prostrate?" He answered: Indeed it does, by Allah it prostrates such that it does not raise its head even once and remains in prostration until it meets Allah, Exalted be He." (Shaykh al-Islam Ibn Taymiyyah attributed this statement to Sahl Ibn Abdullah al-Tustari)