Many Systems At Once

Exercising to music puts several brain regions to work together. The auditory cortex tracks the beat. The motor cortex times your movements to it. Meanwhile the thinking part of your brain pulls attention toward the music and the emotional part responds to how the song makes you feel.

That shared workload is the setup for the next idea.

Here is the payoff: your brain only has so much attention to spend. When music grabs a chunk of it, there is less left over to register pain and fatigue from your muscles.

The work is just as hard. You simply notice it less. That is why a good song can carry you through the rep or the mile that would otherwise feel brutal.