* Literal usage: He cracked his knuckles loudly, a habit that annoyed everyone in the office.
* Figurative usage: She knuckled down and finished the project ahead of schedule.
* Informal usage: "You're going to have to knuckle under to the boss's demands," he said, sighing.
* Descriptive usage: The old leather gloves had worn thin, revealing the knuckles of the farmer's calloused hands.
* Idiom usage: She was determined to win, and she was ready to knuckle down and work hard.
Let me know if you'd like more examples!