I’m a surgical technologist so I work on my feet most of the time and if it’s busy I can be on my feet for my entire ten hour shift. Google “surgical anti fatigue mats.” It might be exactly what you need. We have a variety at work but they buy the really nice ones specifically for the doctors and because I work night shift and we go down to a skeleton crew everyone on my team uses the nice ones because they are available. Seems like a silly thing, and you’ll feel a little wobbly on them at first but man they make a difference when standing for multiple hours at a time. Price range goes from like $50-$500 depending on what you’re willing to spend. There’s serveral different designs and they come in basically any color you want.
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An alternative and something I see a lot of aging touring bands doing is to bring bring a rug for a similar effect. Lot of the aging national acts that play two hour sets+ sets do that. Picture below is Deftones they are all between 45-48 years old now. Hell they probably have padding under the rugs as well, either way the basic idea is To avoid standing on a hard surface. It’s something I caught onto not only from work but when we’d play gigs and 90 minutes in on a hard stage with a heavier bass I wanted to go lay down but with the same bass at rehearsal on a padded and carpeted floor we’d rehearse for 4-6 hours sometimes.
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My only other suggestion if you still want to stand is to try a lighter bass and see if that helps.
Sitting can absolutely work for some gigs. Acoustic gigs in particular but depending on the style it can be a bit of a turn off to concert goers.