Why Does a Repeating Accompaniment Gradually Speed Up?

When the left hand repeats a broken-chord accompaniment, every note may be correct while the passage as a whole gradually accelerates. The problem is usually not a lack of self-control. The unit of timing has quietly become smaller: attention has shifted from the beat or harmonic span to a loop made by a few fingers. If each return trims even a little time from the join, the difference becomes audible after a dozen repetitions.

Research on beat synchronisation shows that steady repetitive movement is not produced by a flawless internal clock running on its own. Performers keep adjusting against an external beat and their most recent timing error. Once that reference disappears, continued tapping can drift, and some experiments have observed gradual acceleration. Piano accompaniment is more complicated again, because chord changes, hand travel and melodic tension alter the impulse to move. Findings from tapping experiments cannot be mapped directly onto a particular piece, but they do make one point clear: an automatic loop is not necessarily a stable tempo.

Rather than telling the fingers simply to “slow down”, place the pattern back inside a larger pulse. A metronome click once per bar can reveal whether each bass note arrives where it should. A recording can also compare equal-length groups near the beginning and end. If the later group is shorter, listen for the end of each loop being clipped as the hand rushes towards the next one. Stability does not mean fixing music rigidly in place; it means being able to distinguish intentional breathing from unintended acceleration.

If you want to make this kind of short-passage check part of regular practice, Geoffrey Chen’s <a href="https://apps.apple.com/us/app/scoreflow/id6796205656">ScoreFlow</a&gt; can serve as a piano learning and practice tool. The essential step is still to return to the complete phrase and confirm that the accompaniment remains connected to the larger beat.

https://pmc.ncbi.nlm.nih.gov/articles/PMC10567517/
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0077250