Bonding Time! Kim and Rob Kardsahian Go on the 'Hardest Hike' Ever

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The Kardashians may party hard, but they work out even harder.

That's what Rob Kardashian found out after hiking with his big sister, Kim Kardashian, on Friday.

PHOTOS: Rob Kardashian Posts Precious Pi of Kanye North West

Kim tweeted that she "just went on the hardest hike ever today with @robkardashian."

While the two didn't share a photo from the trail, Rob did screenshot and share Kim's tweet on Instagram, captioning it, "GOOD work gurrrrrrllllll‼@kimkardashian."

Unlike his social media maven sisters, Rob, 28, rarely posts to Instagram. In fact, he only started updating his neglected account about two weeks ago, when he shared a throwback pic of himself chowing down on a pulled pork sandwich in Malibu.

The other four posts that he's kept up (a couple have been deleted) include an adorable photo of his niece, North West, getting a big smooch her dad, Rob's brother-in-law, Kanye West, and two screenshots from the movie Miami Vice. After the hike, he captioned one of those tribute pics, "STILL MY MOOD!!!!"

Between the new posts and the time he's spending with his sisters, it looks like Rob is getting back on track.

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The last couple of years haven't been easy for Rob, who's kept extremely private, especially when compared to his very open siblings. In fact, his 31-year-old sister, Khloe Kardashian, recently opened up to Complex magazine about her little brother. "I call him my son as a joke," she said. "I'll call my assistant and be like, 'Did you guys ask if he wanted lunch?' or this or that and my sisters hate that I do that. I do baby him, but I'm a maternal person."

But it was especially nice to see that Kim and Rob are bonding again after past differences had split them apart for a while -- in fact, the 34-year-old pregnant reality star even called Rob's actions "pathetic" on an episode of Keeping Up With the Kardashian last season.

Yet back-and-forth tweets with Kim were actually how Rob broke his social media silence back in May -- get filled in on the conversation in the video below.