IMO you shouldnt be standing still lunging but using the whole arena, so you should get exercised

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Everyone I see lunge does so within a small box in the center and pretty much stays put!! How do you lunge - do you just change where you do the circles, or do you get your horse to do some straight lines in between circles etc.?
I still think that long reining at trot is the very best exercise.

While Skeeter's custom saddle was built, Skeeter and I did schooling long rein sessions 4x a week at 30-40 minutes per session for 4 months (!!) - and doing just as much trot work (working trot so quite a bit of forward!) as one would do in a normal schooling ride - and my long reins are not that long so I do more of less move with the horse. Keeping up with a horse in working trot for those lengths of time and for that many sessions a week and for that many months did my aerobic fitness a whole lot of good!!

And as for Skeeter's hind muscle development - wow!! Riding had never really developed his hind (he was an older ex-school horse at the time) - but long reining for 4 months did the trick!!