system
Given the following post, respond as if you're commenting on Reddit:
human
If our fingers are operated by tendons in our forearms, why is it hard to move your fingers when they get cold?

Seems like they are two different issues.
gpt
The problem arises from the joints of the fingers. Each joint has cartilage and on the surface of that cartilage is a protein called lubricin. This lubricates your joints and allows them to move smoothly. Like most liquids, when the lubricin gets cold the viscosity goes up. This increases the friction within the joint and thus makes it more difficult to move.