system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do some prosthetic legs tend to hand a backwards curve when human legs are straight?
gpt
Prosthetic legs aren't designed to simulate human bones, but rather to support the weight as close to how an entire leg would as possible. This means they need to simulate not just the support of the leg, but the flexibility and cushioning of the foot and ankle.

That flexibility isn't usually possible from a straight structure, unless it's soft, but prosthetics need to be durable because they aren't made of constantly regenerating cells.