"C-come on, gotta keep it going!" A rainbow-haired girl panted as she ran in place on her living room treadmill.

Glancing down at the small screen display in front of her, she smirked as she saw the number of miles she'd already run so far today.

"Heh, at this rate, I've got this whole marathon in the bag!" She boasted confidently. "I can see the finish line already!"

Rainbow Dash was in the midst of practicing for the upcoming Canterlot High Marathon, an annual event wherein all the best athletes from every school in the state would come to compete for the title of 'Runner Of The Year'. It was a pretty big competition and one that she had been very eager to participate in ever since learning about it her freshmen year of high school.

Unfortunately, regulations required that only seniors of age 18 or higher were allowed to take part, so in the past, she hadn't been allowed to sign up. But now that she met the requirements, she planned on utterly smoking the competition! Rainbow chuckled to herself at the thought, picking up the pace on her treadmill as she reached up with a small towel to dab some sweat off her forehead.

Of course, with the aid of her magic geode, she'd probably win the whole thing in just a few seconds, which is why Principal Celestia had banned any use of magic whatsoever. Ever since the utter disaster that was the Friendship Games, Principal Celestia and Vice-Principal Luna had been quite anxious about them using magic in big events like that again. As it turned out, giant tentacles and portals to other dimensions popping up everywhere were not quite up to safety standards.

But Rainbow wasn't perturbed. Even before getting her magic geode, she'd always been an avid athlete, so she was sure that she could handle it. Admittedly, she'd gotten a little out of practice since obtaining it, so for the past few months, she'd been tirelessly training. From doing laps around the school grounds before and after her classes to running to and from work each day. Heck, she'd even been trying out some special diets that she'd found on the internet. Though not all of them had been helpful.

*Grroaaaan*

Rainbow gasped as she a recently familiar shifting in her lower gut, very nearly causing her to trip and fall as she was suddenly struck by an intense urge to use the bathroom.

"Ugh! Not again!" She growled, clutching her stomach.

Rainbow quickly turned and lept off of her treadmill, leaving it still running as she bolted for her bathroom. Wrenching the door open, she darted inside. Slapping the toilet seat open, she turned and hastily peeled down her sweat-soaked compression shorts before planting her toned ass onto the cool porcelain.

"Come on, this time for sure!" She grunted, face scrunching up as she started to push.

A few small plops could be heard as tiny chunks of liquidy shit dropped into the bowl almost immediately. An immensely large mass loomed behind them within her bowels, pressing hard against her back door. Rainbow felt her anus start to widen as she kept up the pressure.

"Here it comes!" She gasped, feeling the head of a wide and very dense boulder peek out from her almost painfully wide-spread hole.

But then, just as it seemed like the gargantuan log was going to come out, the pressure it had been putting on her anus suddenly started to weaken at a rapid rate.

"No, no, no! Not again, no come on!" Rainbow begged, still pushing as her body abruptly decided that it didn't feel like giving her any relief just yet.

Though she remained seated, trying her best to coax her bowels into another attempt, eventually Rainbow was forced to admit defeat. The sudden and almost overbearing pressure that had been pressing at her anus mere moments ago had now vanished entirely.

Head in her hands, she growled angrily and kicked the tile floor. Unfortunately, this was nothing new to her.

Since about a week ago, she'd discovered a special diet plan online that had so far ended up being... less than helpful. It promised to cause vast increases in not only her strength and stamina but to her speed as well. Yet, so far, it had only ended up having one very irritating effect on her body.

Severe constipation.

It was a problem had been plaguing her days now, yet no matter what she did, the concrete boulder that had magically formed in her large intestine was refusing to be cleared. She'd tried everything, from laxatives to enemas. Hell, she'd even resorted to using one of her 'toys' to try and make some kind of a dent, but so far nothing had worked.

With a defeated sigh Rainbow glared down into the toilet water circling beneath her. Small chunks of brown mush floated around and around, slowly turning the water a murky brown color. It was about all she ever seemed to be able to produce during these attempts.

Grabbing a few sheets of toilet paper, she begrudgingly wiped away any smudges and flushed the toilet. Pulling her shorts back up, she sighed and walked towards the open door back into her living room. She was hesitant to even bother trying to get back into gear again. No doubt it wouldn't be long before she felt that pressure rear it's ugly head again.

Since it first appeared, her blockage had been nothing but a nuisance. At random times in the day, usually over the span of a few hours, her bowels would start pushing over and over again, making it seem like she was ready to go as it had been earlier. But every trip to a bathroom always ended the same way, with very little to show for it and no real progress being made.

And as much as she wished she could just ignore the sometimes painful urges, it wasn't that simple. Pretty much the only time that she ever felt her bowels start to move on its own was while she was not in a bathroom. It was as if her body knew what she was trying to do wanted to make things more difficult.

Plus, the smaller chunks and spurts of diarrhea that would almost always squeeze out around the blockage were easily capable of getting through her clenched anus and could potentially end up messing up whatever she was wearing at the time. This forced her to make trips to the bathroom every time an urge arose, which in some instances could be only a few minutes apart.

Her boss had already gotten onto her about it once already, though Rainbow had been too embarrassed to admit the reason for her constant trips to the employee bathroom during her shift. Not to mention, the fact that she was late to almost every one of her classes so she could visit a student restroom was becoming a serious issue.

As Rainbow emerged from her bathroom, she glanced over at her still-running treadmill, and with a huff, hopped right back on and continued her workout.

But as the first few minutes went by, all she could think about was the impending urge that was sure raise it's ugly head again any time now. She prayed that her bowels might decide to give her a little break this time around, if only for more than a few minutes. But, as if sensing that she was thinking about it, Rainbow grimaced as she felt the start of a familiar tightness start to grow in her gut.

"Dammit, not again!" She scowled, feeling the heavy sensation in her bowels return in full force, just as like it had before. With a frustrated huff, Rainbow reached up to stop her treadmill, only to pause as her finger hovered an inch away from the button.

She knew all too well what would happen should she stop and sit her butt right back down on that accursed toilet seat. She'd get so close, just like always, right on the rim of sweet, sweet, relief, only to be denied yet again. Then she would get cleaned up, flush whatever tiny progress she hadn't really made, and then wait for the urge to strike a third time...

"Screw it." She eventually sighed, lowering her arms and continuing to run, even as she felt the pressure mount against her.

She knew that ignoring the urge like this would more than likely result in a few marks being left in her shorts from whatever managed to leak out of her, but at this point, she was too frustrated to care. There were more clothes in the dryer anyway and had nowhere to be today. Rainbow decided to focus on her running, tuning out everything else that got in the way of her training.

So, mostly out of spite, she kept running, not even bothering to put any effort into holding anything back.

As expected, it wasn't long before she felt a bit of liquid mush start to leak out her as the massive log she'd been struggling with for the past week began to peek out. The filth spread and smeared between her ass cheeks as they rubbed together. Rainbow found herself blushing at the sticky sensation that resulted. Despite no one being around, the fact that she was knowingly letting her shorts get messy felt awkward and a little embarrassing.

But then, just as her asshole stretched to about the same amount as it had when she was on the toilet, Rainbow gasped as something new suddenly started to happen. It kept coming out. Before Rainbow could really even register what was happening, she felt the immense log start to slowly emerge all on its own!

'This is my chance!' She thought.

With optimistic haste, Rainbow lept off her treadmill and darted back into her bathroom. Peeling down her shorts as fast as she could, she felt them peel away from her crack and, as expected, a brown skid mark ran along the inside. But that didn't matter right now, she was inches away from victory!

Hovering her ass over the toilet and reaching back to spread her stained cheeks apart, she pushed with everything she had. In response, she felt the immense log slide out another inch, but then, the worst started to happen. Though it had extended further than she'd ever gotten it to before this point, Rainbow gasped in horror as she felt the boulder start to recede back inside her once again.

"No!" She shouted, still pushing with all her strength. But in the end, she was left back at square one. "No! I was so close!"

She was left powerless as her bowels pulled the blockage back in, the pressure that had allowed her to make so much progress a moment ago weakening just as it always seemed to do. And now she had nothing to show for her efforts but a smelly pair of shorts and a messy crack.

"Gahh!" She growled loudly, slapping the wall in frustration as she let herself fall down onto the toilet seat.

She didn't know what to do at this point. She certainly wouldn't be able to run in the marathon like this, and with how much this was already impacting her training, she wouldn't be ready in time for it anyway. But as Rainbow grumbled in despair, staring down at the slightly soiled shorts around her ankles, she suddenly had an idea.

For whatever reason, she seemed to be making the most progress while she was running on her treadmill. The urges only actually seemed to stop after she reached the bathroom. So... what if she just didn't stop running? If her bowels were refusing to loosen unless she was being active, why not give in to its demands? At this point, she was desperate enough to try just about anything if it meant relief.

A sense of optimism rose over her as she considered the new idea, like a light at the end of a hopelessly dark tunnel. The possibility of finally getting the relief she so desired sounded nothing short of fantastic. Though, at the same time, she didn't exactly feel like shitting all over her treadmill or something. She would have to think of some way to avoid making a big mess like that.

She briefly considered placing a bucket underneath or behind but decided that it would be too risky. There was a high chance she'd miss and end up getting shit all over the carpet. But as she stared downwards in thought, she realized that the answer lay around her ankles.

"I could just shit in my shorts..." She mumbled, quirking an eyebrow.

Just saying it aloud felt strange. Granted, it wasn't too far off from what she'd already done, but this would be on a much larger scale. She hadn't pooped her pants since she was very little, and the idea of doing it purpose just felt so very wrong.

But, the more she thought about the potential benefits, the more won over to the idea she was getting. It wasn't like anyone was ever going to know she did it anyway. Heck, she wouldn't even have to stop exercising to do it. Staring down at the long line of brown mush that already stained the inside of her tight compression shorts, she sighed and shrugged.

'Might as well just use these again...'

Standing back up, she felt an odd shiver run up her spine as the remaining filth between her ass cheeks met. There would be no point in wiping with what she was planning to do...

Bending down, she hesitantly pulled her shorts back up. The tight fabric clung to her body, digging in deep between her cheeks as it was inevitably smeared even more. Looking over into her bathroom mirror, she could see the faintest trace of a brown line following down her ass from the outside. The sight made her blush.

Despite how gross this was, she was surprised that it didn't gross her out more. In fact, the warm mushiness felt kinda... good, in a weird way...

Shaking herself out of that odd train of thought, she left the bathroom.

"Well, here goes nothing," Rainbow said apprehensively, hesitating before hopping back up onto her still-running treadmill.

Rainbow found herself waiting anxiously for the urge to return yet again, the idea that in just a few minutes she'd purposefully be trying to shit herself never ceasing to weird her out. Part of her still hoped the urges would just go away so she could just focus on training.

But like clockwork, it only took a few minutes of constant running before the tightness in her bowels showed itself once again.

'Here we go...' She gulped.

As before, she felt the weight of the massive boulder make contact with the inside of her anus and start to slowly spread it apart. All the while, she made no effort to resist its movements, instead focusing on running above all else. Her breath hitched as the log reached the same point of as before, and she found herself fighting the urge to turn and spring to the bathroom. Instead, she forced herself to keep running, betraying years of training herself not to do what she was now actively trying to encourage.

Her breathing began to get shaky as she felt her anus stretch to its limit, the log inside her slowly starting to slither out unimpeded. Her gait widened considerably as she felt the turd make one final push. With one more quick spike of pain, the thickest part of it finally pushed through.

"O-oh my gosh, this is actually happening." Rainbow chuckled anxiously as she felt the log start moving faster on its own as she passed the point of no return. "I-im actually shitting m-myself..."

The tight fabric of her compression shorts was immediately thrust outwards as a large bulge started to form. The head of the log was easily as big around as grapefruit, and it showed as the immense log started to curl up inside her shorts. The bulge bounced as she continued to run, the sensation sending more strange signals to her brain.

*Frplplpt!*

With the dense and solid boulder gone, her bowels were free to release everything she'd been forced to hold from the past few days. A very wet and bubbly sound suddenly exploded out of her ass, pushing multiple long tubes of creamy shit out into her shorts from one quick push. The bulge doubled in size almost immediately as next a rush of thick and mushy poop filled her crack and rushed up through the gusset of her shorts.

But even through all of this, Rainbow never stopped running, panting fast and hard, but not from exertion, but from the immense waves of relief that she was receiving. It was finally over, she had finally rid herself of the massive boulder, which now rested in the seat of her pants.

Once the flood of diarrhea finally stopped, Rainbow reached up and pressed the off button on her treadmill. She gripped the hand-holds on either side of her as she panted breathlessly. Her constant movements had spread shit all throughout her shorts, leaving nothing undirtied. Looking back, she cringed as she saw the massive sagging bulge that hung down behind her.

"I can't believe I just did that." She gasped, still overwhelmed as adrenaline pumped through her veins. Her shorts were thoroughly stained through, no doubt completely ruined beyond any hope of repair. But she couldn't care less about that, it was finally over!

Stepping down from the treadmill, she was suddenly assaulted by the intense smell of her own shit.

"Oh my god, that's bad!" She coughed, chuckling a little at the ridiculousness of it all.

Looking back into one of the full-body mirrors she had set up in the workout area of her living room, Rainbow gasped as she was able to see the full brunt of the damage. Miraculously, none of it had managed to escape and make a bigger mess, but the same couldn't be said about her compression shorts. The bulge in the seat of her pants was huge, and due to the heavy brown stains that covered most of it, there would be no doubt as to what she'd done.

Rainbow Dash, the top athlete of the whole school, holder of an incredibly powerful magic geode, and a girl who hated showing any signs of weakness, had just purposefully pooped herself. Rainbow Smirked, shaking her rear and watching as the bulge swayed side to side.

She had liked doing it quite a bit...

Author's Note

Hope you enjoyed this one, this is roughly as long as these things will typically get for future reference. Another is coming along so expect that sometime soon.