Arnold's teaming up with his physical therapist girlfriend, Heather Milligan, for a fun and informative video that just recently dropped -- and it's all about a topic he's often asked about in his Arnold's Pump Club daily newsletter ... namely, "What do I do about my knees?"

Heather's an expert in the field ... in addition to being Arnold's PT and GF, she's treated a number of pro athletes -- football players, basketball players, UFC fighters, boxers, track & field peeps and all sorts of Average Joes ... and Arnold gives her a ringing endorsement.

Check out the video ... the couple takes folks through a range of exercises and stretches to strengthen knees and joints ... and there's helpful stuff in here for people who hit the gym, slopes or just have bad knees from walking. Seriously ... really good insights here!

Arnold and Heather have been together for over a decade -- they first met in 2012 when she helped him rehab a shoulder injury before filming "Escape Plan" with Sylvester Stallone -- and they come off as a super cute and loving couple in the video.

There's some funny parts too ... like when Arnold hikes up his shorts to show off his leg muscles as he demonstrates a wall sitting exercise designed to help quads, hamstrings, glutes and knees. As Heather says, "Ohhh, showing us your legs ... that's impressive." 😆

Arnold's video advances his upcoming Arnold Classic this weekend in Columbus, Ohio ... where there's guaranteed to be some sore knees at the famous bodybuilding competition. Check out more tips on Arnold's Pump Club.