Aperson who sprints for 45 seconds obtains most of his or her energy from

Answer; 
A person who sprints for 45 seconds obtains most of his or her energy from muscle glycogen.

Explanation; 
Muscle glycogen is the glycogen that is stored in the muscles and liver.
Glycogen is the primary source of energy during exercise, and low glycogen levels decreases the ability to gain strength and muscle. An athlete who sprints for 45 seconds obtains most of his or her energy from the muscle glycogen. 

The answer is constant.