A stationary bike.

A bike would be perfect. It focused almost entirely on her lower body, and could test both her strength and endurance. Plus, there would be plenty of jiggling for her to enjoy.

Bouncing to the beat, she sauntered over to the closest bike and straddled the seat. Her ass was almost too big for it, and her flesh spilled over the edges as it sank into her rear. Getting comfortable, she gave the digital console an ocular pat-down. It wasn't too complex, and all the standard functions were there.

She cranked the resistance setting as high as it could go. She was feeling confident in her body, and she was going to prove just how strong she was. She slowly started peddling, feeling out the pressure she needed, until she was at a moderate pace.

"This is easy!" she exclaimed. She wasn't breaking a sweat, at all. She could barely feel the resistance in the bike, her legs were just so damn powerful.

Determined to push herself and the bike to the limit, Samus started accelerating. Faster and faster her legs pumped. The faintest hint of muscle could be seen tensing under the soft padding of her legs. Sweat finally started dripping down her skin, running down her curves as her body heated up from the exertion. She lifted her rear off the seat to get better leverage, straightening her legs out and using her lower body weight to her advantage. Had anyone been behind her, they would have gotten an amazing view of her ample butt.

As she pedaled more rapidly, her ass shook and wobbled like it never had before. She couldn't believe how good it felt to have it bouncing around so much. "I could really get used to having this much mass," she thought to herself.

And then she heard it. Clap, clap, clap, clap. Her cheeks were moving so much, and were so large, they were rhythmically clapping along with her pedaling. Clap, clap, clap, clap. The realization made her warm in her crotch, as her mind slipped further into derriere-based debauchery. Clap, clap, clap, clap. Each clap resounded in her ears, bringing her utter bliss, spurring her to pump faster. Clap clap clap clap. Faster and faster she pushed herself. The bike's axle started smoking from the friction. Clapclapclapclapclap. Her heart was pounding. Her suit was soaked with sweat. But she didn't even notice. All she cared about was the movement of her booty, to feel each ass cheek jiggle, to relish as they collided together to make that wonderful noise.

And then, suddenly, interrupting the clapping, she heard a loud SNAP! as all resistance in the bike suddenly disappeared. The bike's console lit up and incessantly beeped as the pedals spun out of control under her feet. Unable to maintain her balance, Samus tumbled off with a soft thud as her dump-truck butt cushioned the fall.

"Ow..." groaned the bottom-heavy woman. Samus stood up, leaving a large, round stain of sweat on the floor, as if her butt had given it a stamp of approval. "Damn bike couldn't take it, I guess."

She rubbed her behind to sooth non-existent pain, looking for any excuse to grab her own butt. The upgrade had stayed true to its purpose and took all the shock, of course.

The music in the room stopped as the AI interrupted it. "Samus, I have something to tell you."