I worked for LA fitness for 3 years and what MuayThai said is correct you do only get paid minimum wage unless you break draw. as far as him saying as a Trainer you can sell training memberships for commission i sold quite a few training memberships and never saw a dime either because the sales people screwed me, or the trainers dont actually get commission because they have two teams the trainers, and the sellers of training.
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if you get a job there you probably wont be breaking draw for a while until the people who sell training fill your schedule up, so what lots of new trainers do is keep a log of all the clients they train, and dont actually charge them for the training session and take the minimum wage then when you have enough clients and start plugging them into your schedule when you have a gap and charge them for the sessions you are owed