Indefatigable – Untiring, unyielding to fatigue

(Pronounced in-di-fat-i-guh-buhl)

The walkers indicated that fatigue was not possible, (indefatigable) for they were determined, untiring and wouldn't give up.

Examples of indefatigable in a sentence

The workers were shocked to find out they'd been replaced by indefatigable machines. 

He was indefatigable, even after running a marathon he didn't seem tired at all. 

She was warned not to enter the race as even the most indefatigable athletes struggle to finish.