Your experiencing time under tension (TUT) ,rather than a normal short set of shrugs for example. A form of overload for distance (or time in your case). Didn't say how many seconds it took for the 150/250 foot distance, but that is one of the key factors in this type training....TUT.
Even standing in place can produce results, with different lengths of duration. Applied to DL's, squats and mostly any thing else.. Though some men have locked out a heavy bar overhead. Usually taken from a PR or special set up chains set up just a few inches from that lockout position. Heavy bencher's will also do heavy lockouts on occasion.
Don't see to many using the Zercher squat style much any more, but a very all around strength builder. Another good upper back/trap/shoulder girdle builder. Can also affect the lats very well. Will usually try to hit rock bottom every rep, which seems to affect the quads and normal squatting power very well.
Good Luck.