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Handcuffs, once used by Chernarus law enforcement before the zombie outbreak, can now be found in abandoned jails and police stations.
They are used to restrict an individuals ability to resist or use objects (by restraining their wrists.) A key is required to unlock the handcuffs once used.
Once an individual's hands have been restrained, he/she can struggle to try and get out of them. The more durability the handcuffs have, the longer it will take to struggle out of them.
To unlock a set of handcuffs, you must use a set of Handcuff Keys.
 How to struggle out
Keep pressing space and you will get out of them eventually.
 Are there other ways to get out?
Yes, use a saw.
 What you can do while cuffed
You can get items in to your hands, and you can change stances and get to your inventory.