Rob Kardashian Baby's on the Way ... I'm Back in the Gym

11/4/2016 8:04 AM PDT

Rob Kardashian's Back in the Gym

Rob Kardashian's breakin' a sweat ... and it's more than just alright. 

Rob joined big sisters Kourtney and Khloe for a CrossFit-style workout -- with some RiRi in the background for inspiration. Rob was working the middle hard ... courtesy of a medicine ball and some TRX bands.

He's lifting himself now, but real soon he'll be lifting 7-10 pounds of baby ... courtesy of Blac Chyna's uterus.