Leverage is a funny thing. Even not thinking about what one would or wouldn't do, let's look at what happened.

Okay, she pulled her knees back. That's good. In turn, it at least suggests an anchoring of weight to her heels while the knee joints did the work. But it's also not a particularly wide stance, thus subject to stability issues.

And now we're moving. Neither of them are exactly hambeasts, so I'd put her maybe around 110lbs while he's perhaps similar, maybe a bit more. Okay, so we then have to entertain the possibility the knee joints can handle 200+ pounds of person combined. Not unreasonable since one person can weigh that much, and even then people would like to carry things. But I'm also iffy on her lower legs basically being vertical at this point. The moment her upper body leaves the ground, gravity should be trying to pull her back, leaving her teetering on the back of her heels and toward Kai's lower body. Even if the joints were strong enough to hold them in that position, they should just kind of plop since the weight of one end would be greater than another. Had her knees been bent more toward her toes, like she was trying to sit back on her ankles, I'd offer a bit more leeway. Though, as we can see, that's not the case.

I would propose the crunch isn't the result of the machine itself, but rather his weight against her. However, that also doesn't add up because...

...he goes flying away from her, not rolling toward her head and landing behind her. He was basically at or below her center of gravity (hips/upper thighs), making the weight required to snap her back at that awkward angle highly questionable.
Later perspective puts him on the sidewalk, maybe 8-10 feet away. We're somehow supposed to believe that these little lightweight legs not only defied physics, but generated hundreds of pounds of force, presumably at the hip joint, in an instant where an otherwise slow and deliberate pace had been established prior.
Christ, why not sprint off the cliff at that point instead of slow-mo death march? It's just some heavy-handed authoring before we even step into the realms of sensibly preventing it. I'd even question the likelihood of the legs having geographical awareness to know that both a cliff was there and to keep going for it. Was she being remote controlled, then? Maybe we'll see. Just a potentially powerful moment ruined by some inconsistencies. Doubly so if it was meant to take out Kai in the process.
And at that age, all I really did for exercise was bike ride. He does similar. Yet, I was also capable of carrying 200+ pounds on my back or shoulder. Either I'm a freak of nature or, again, writing convenience.