“Is your back getting stiff?”

You're having a picnic with a group of people and have been sitting on the ground for a while. The person next to you keeps changing his position and looks uncomfortable. You ask him:

Is your back getting stiff?

a stiff back

"Stiff" is the opposite of "flexible". It describes when something doesn't bend easily. "Stiff" can be used to describe body parts that aren't flexible because of injury, posture, or not enough exercise. Some body parts that often become stiff are:

• back
• shoulders
• legs
• knees
• neck

be getting (adjective)

You use this phrase to talk about a process that started in the past, has a certain goal or end point, but hasn't finished yet.

For example, when you sit on the ground for a long time, your back will eventually get very stiff. When you ask someone, "Is your back getting stiff?", you're asking whether they are starting to have a stiff back. You're assuming that their back isn't totally stiff yet.