In a time when science was nothing more than the reasoned conclusions of men who could not yet directly observe what they studied, and imagination filled in the gaps between the observable and its cause.
Chinese philosophers imagined a single energy invigorating all things. Like the Greek philosophers who imagined the four elements composing all matter.
This energy which flowed through and sustained all things was called Chi.
In the Martial arts Chi is used as an abstract construct of the mind. This construct allows us to visualized a preternatural force to reinforce and lend strength to our bodies, creating a mind body connection. Though the Chi may not be real we are still able to produce real, observable, results from its use.
The brain controls all functions of the body, and just as we can consciously take control of our breath, it has been proven we can learn to take control over our other processes as well. In modern science this is called the Placebo effect. Chi is the Martial artists tool to gain that control.
One such effect of the mental movement of Chi is increased circulation to the desired areas, this means that more oxygen is carried to your muscles by the extra blood, which allows your muscles to function at elevated levels. Since Chi is often said to be connected to the breath, this connection to oxygenated blood seems fitting.
"The mind is the commander; the body is the army. Where the mind goes, the Chi flows." - Taoist proverb