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Jennifer Lopez was barely recognizable in her grey sweats Saturday as she took daughter Emme to an after-school activity before hitting up her own activity -- sweating it out at the gym with boyfriend Casper Smart!

J Lo even changed into a different sweatsuit half way through the day -- but hey, she's probably got a closet full of them.

The singer didn't look quite as glamorous as she does in her chic American Idol wardrobe ... but hey, she's still Jenny From the Block on the weekends!