Surface rhythm – what is it?

Surface rhythm or surface rhythmic activity is the rhythm of the actual notes. This is to distinguish the underlying harmonic rhythm or harmonic rate of change.

Increasing surface rhythmic activity means you are literally adding more note onsets in a measure, like using 8ths instead of quarter notes, which tends to increase tension. You may have 5 measures of 1 chord, but if the surface rhythmic activity increases it will feel like it is moving somewhere.