What is the exact meaning of "crunching" and "knuckles" in this sentence?

Crunching numbers is usually taken as doing a lot of calculations in order to find an answer.

But I have no idea why the knuckles will hurt. It sounds like the man was literally "crunching on" the numbers with his knuckles.

One definition of "crunch" is:

Process large amounts of information or perform operations of great complexity, especially by computer. [Lexico]

so "crunching numbers" essentially means:

Performing operations with numbers.

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The part about "knuckles" isn't as simple.

The author seems to be using "crunching" metaphorically (in addition to the definition above).

The author urges the reader to imagine someone literally crunching numbers (a physical representation of numbers) with their fists to the point where their knuckles hurt.

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Edit: As BoldBen pointed out, their knuckles could be hurting because they have been typing on a keyboard for a prolonged period of time (while crunching numbers). This would not be metaphorical and is the more likely situation because "crunching numbers" is usually done on a computer.

The basic meaning of crunch is to put something under pressure, especially to chew something crunchy with you teeth, like potato chips; or to crunch leaves (or snow) on a path underfoot as you walk; or crunch a piece of paper in your hand into a ball before you throw it away. And crunches are like sit-ups, the exercise you do for your abs. So crunching numbers is an analogy to these physical activities. hard work. thus sore knuckles.