The Kardashian sisters, who often work out together, both took to Snapchat to document their outdoor gym routine with Rob, 29, as they trained with jump ropes, dumbbells, boxing gloves and battle ropes.
"Today's torture," Kourtney, 37, captioned one post, while Khloé, 32, added, "Back at it."
Clad in a white T-shirt and black sweatpants, the Rob & Chyna star, who is expecting his first child with fiancée Blac Chyna later this month, looked slimmer as he worked up a sweat alongside his svelte siblings.
Kourtney showed off her abs in a black sports bra and matching pants, while Khloé rocked a black sweatshirt and black-and-white printed yoga pants.
At one point, Rob looked fatigued as he took a break from swinging the battle ropes, but he quickly started up again moments later.
Kourtney and Khloé recently shared their workout playlists on their websites and apps, where Kourt wrote, "If there is one thing that gets me through a workout, (besides having Khloé by my side) it's good music! I love to blast '90s pop, rap and Kanye to get me pumped up for a serious exercise session. Of course, Khloé and I have different tastes in music, so we alternate days of who gets to control the music when we workout together."
The former Kocktails With Khloé host's playlist included "Work" by Rihanna and Drake and "No Scrubs" by TLC, while her eldest sister's list featured "Jesus Walks" by brother-in-law Kanye West and "24K Magic" by Bruno Mars.
Rob, who was reclusive for years prior to his relationship with Chyna, took to Instagram last month to tell his 7.4 million followers that he's "snapping back" into fitness as he prepares for the upcoming birth of his daughter. "Baby will be here in 4 weeks and I'm done with carrying this pregnancy weight," he captioned side-by-side throwback photos of himself and his 28-year-old fiancée.
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