The Tesseract

Martha was jogging early in the morning around the MIT campus when Stewart joined him. She was all sweaty and almost out of breath when he tapped her on the shoulder. 

"How many laps have you done so far?" Stewart asked. 

"Ten, how about you?"

"I don't know if I could finish a lap. But I was hoping you can slow down."

"You can always catch up. In the meantime, I am finishing mine." Martha ran faster than her current pace.

"Hey! Wait for me." Stewart grappled with his pace but decided to stop to catch some breath.

Martha was sitting on steps when Stewart arrived after a long walk from the campus oval. Stewart was drenched in sweat as he sat beside Martha. 

"How can you manage to conserve so much energy and expend without losing your breath?"

Martha drank some water from her jug. "Fitness Index dictates all that," Martha responds.