This artwork was commissioned by Kassaz, who wrote the following short story about it, part of this anthology:
https://www.fimfiction.net/story/568567/3/sweaty-short-stories-with-pregnant-pony-workouts/bent-out-of-shape

Bent Out of Shape

“Thanks for helping us get our cutie marks as bodybuilders, sis!” Apple Bloom cheerfully trotted down one of the many dirt roads of Ponyville with Scootaloo and Sweetie Belle alongside her. Trailing them by a ways was Applejack—looking nothing like a bodybuilder with her swollen middle—and looking everything like a bodybuilder because of it, as Pinkie Pie had many times told her with a laugh. Applejack was wearing sweat bands around her legs, as were they, and the fancier-looking bands around Sweetie Belle’s reminded her that Rarity could’ve been watching them instead of her. Applejack frowned since nopony would see it.

“No worries, ya’ll just slow down and stay where ah can see ya. I ain’t as fast as I used to be.” Applejack was more than a little annoyed that she had to keep telling them that and, as she felt another blow against her body from the inside, none too pleased about leaving the farm either. According to Granny Smith, an Apple could tell when she was off the farm, even in the womb, and hers didn’t like it at all. It crossed Applejack’s mind that maybe all the walking and talking and everything else she did when leaving the farm caused it instead, but she disregarded the thought and continued to believe in Granny.

She now saw Ponyville’s gym in the distance, and the three fillies dashed over to it and entered, despite her yelling. She sighed and hung her head. The gym was safer than the contraption they’d made on the farm with old wood and wheels and whatever else they could find—Applejack still couldn’t make sense of how they were supposed to use it for exercise—and the realization that the trio was, just for a few minutes, somepony else’s responsibility brought a relieved smile to her face. Somepony in the gym would stop them from maiming themselves, or somepony else, probably. She turned to look at her side before speaking. “Ya better behave better than your aunt when you get outta there, ya hear?” She felt her foal preparing another targeted strike, and sucked in her gut to stop it. Instead of a single hard blow, she felt gentle squirming. “Now what did I just say?” She entered the gym smiling, first walking over to the watering trough and drinking her fill before looking around for the crusaders.

“How much weight should we use?” Sweetie Belle asked Apple Bloom while attaching a harness to Scootaloo.

“Scootaloo could use the exercise, so all of it, I guess.” Apple Bloom slid another piece of metal onto the device. Once Scootaloo nodded, Apple Bloom pushed the rack of metal into place, immediately flinging Scootaloo upwards into the ceiling before she fell back down and was held above the ground; she grit her teeth and began to sweat while her wings buzzed. “C’mon Scootaloo, you can do it!” Applejack decided they would be okay.

Applejack looked around at the other ponies in the gym. There weren’t many. She noticed Twinkle Shine on her way to the watering trough, and chose to start a conversation. “Heya, Twinkle Shine. How’re ya doing?”

Twinkle Shine’s attention was taken from airing out her outfit with her magic, and aimed towards the mare whom she realized to be the sweatiest pony in the entire gym. “I’m fine, Applejack. Are you? Should you be working out, in your condition?”

Applejack wasn’t quite sure what Twinkle meant at first. She looked at her legs, and then her reflection in the water; sweat coated every inch of her fur. Applejack hated to admit it, but she felt a little worn out just from the walk, and could now see that she was still breathing a little more heavily than usual. “Naw, I’m not working out here. I’m just out of shape, I guess.” Applejack was so uncomfortable with her realization that she cracked a joke. “Ya gotta get out of shape when you get a shape in you, you know?” She laughed a little, and Twinkle joined in, whether she found it funny or not. She knew how Applejack could get. It never occurred to Applejack that she had been sweaty from all of her chores outdoors, before she had even noticed the crusaders’ antics for the afternoon.

Applejack walked away from her reflection and absentmindedly leaned against a rack of weights. “I’m just here to watch my sister and her friends while they,” she looked at what they were doing now to see Sweetie Belle red in the face and struggling to lift a single weight with her magic, “try to work out, I guess.” Applejack sighed and relaxed; her belly drooped to rest askew beneath her. She felt another kick reverberate through her to sandwich her body between a little hoof inside and the metal outside. She grunted before standing herself up all the way and walking away from it towards the crusaders.

“Um... Applejack...” Twinkle Shine started.

“Huh?” Applejack stopped next to a hanging sandbag. One of the weights was no longer straight, but bent very conspicuously at the same height as her belly. “Uh,” Applejack had no time to think before another kick struck the sandbag. She waddled mostly out of the way before it swung back and struck her in the rear. It now turned back-and-forth just a little and had a hole in it leaking sand, a hole she realized had been caused by a little hoof. Applejack didn’t even notice her belly swinging underneath her much the same as the sandbag.

“Aw... dangit... that hurt a little bit.” Applejack was sore in more than her body, and didn’t want to think of her coin purse that would be losing some weight soon. She started to suck in her gut again and spoke to the crusaders as she more quickly and awkwardly waddled her way over to them. “Now fillies, I think it’s about time we leave before you break anything, so I don’t have to pay the owner even more.” That last bit was muttered under her breath.

Twinkle Shine nervously looked between the broken weight and leaking sandbag; Twinkle Shine decided she was never going to have an Earth pony’s foals.