system
Given the following post, respond as if you're commenting on Reddit:
human
[Assassin's Creed Series] How do the Assassins parkour across cities without ever: slipping, twisting their ankle, getting splinters, having things they jump on break, pulling a muscle, etc?

Especially for Assassin's Creed III. How do those skinny tree branches never break under his weight and equipment?
gpt
Ok, firstly, there's one thing you need to remember: **The Assassin feats we witness are (almost) all seen through the Animus.**  The Animus is a device that allows its users to read the genetic memories stored in a subject's DNA.  However, these genetic memories are stored in a way that means the only way to view them is to "relive" them as your ancestor.  When the person reliving the genetic memories performs a series of actions, the Animus compares the results with the genetic memories stored in the DNA, and produces a value that represents how closely the user's actions matched the actions recorded in the genetic memories.  This value is commonly known as "synchronization".  

Memories, however, are a funny thing.  There are many things that occur in a day that you won't remember, and thus won't be stored in your genetic memories.  For example, how many steps did you take when you walked from your bed to your bathroom sink this morning?  How many people did you pass in the street as you went to work?  How many times did you trip over your own feet over the last twelve months?  Do you remember any of these things?  

Consequently, if these irrelevant minutiae aren't remembered by the ancestor, they won't be stored in the descendant's genetic memories, and the Animus won't read them.  For the purposes of determining synchronization, these minor details don't matter.  So let's say the Assassin ancestor ran across a rooftop, then jumped to the next rooftop.  Maybe he stumbled slightly at the beginning of his run, but was able to regain his balance.  Maybe he suffered a slight cut on his hand when he jumped and grabbed the adjoining rooftop.  Maybe a bird pooped on his head during his run.  Ultimately these details did not matter, and so they were not remembered.  The important thing is that the ancestor ran and jumped to the next rooftop, so that's the result that will be used for comparisons when determining synchronization.  

For the issue of injuries, the idea is similar.  For small, niggling injuries, such as a sprained ankle or a jammed finger, they didn't matter much so the ancestor didn't remember them.  For injuries that did matter, such as Ezio getting wounded after the attack on Monteriggioni or Ratonhnhaké:ton being almost too hurt to stand when he confronts Charles Lee, the ancestor remembered them, and thus they were also present in the genetic memories.  

**tl;dr:** Assassins may very well have hurt themselves over the course of their adventures. However, if the injuries didn't matter then they wouldn't remember them, and there would be no corresponding genetic memories for us to see via the Animus.