A paradigm to study the learning of muscle activity patterns outside of the natural repertoire.

The acquisition of novel muscle activity patterns is a key aspect of motor skill learning, which can be seen, for example, when beginner musicians learn new guitar or piano chords. To study this process, we introduce here a new paradigm that requires learning new patterns of flexion and extension of multiple fingers. First, participants practiced all the 242 possible combinations of isometric finger flexion and extension around the metacarpophalangeal joint (i.e., chords). We found that some chords were initially extremely challenging, but with practice, participants could eventually achieve them quickly and synchronously, showing that the initial difficulty did not reflect hard biomechanical constraints imposed by the interaction of tendons and ligaments. In a second experiment, we found that chord learning was largely chord-specific and did not generalize to untrained chords. Finally, we explored which factors made it difficult to produce some chords quickly and synchronously. Both variables were well predicted by the muscle activity pattern required by the chord. Specifically, chords that required muscle activity patterns that were smaller and more similar to muscle activity patterns required by everyday hand use could be produced more synchronously. Together, our results suggest that our new paradigm provides a valuable tool to study the neural processes underlying the acquisition of novel muscle activity patterns in the human motor system.NEW & NOTEWORTHY In this study, we introduce a paradigm to study the learning of novel muscle activation patterns that deviate from those we are used to producing in everyday activities. Participants learned to produce different combinations of concurrent flexion and extension of 1-5 fingers of the right hand. We found that the ability to produce muscle activation patterns quickly and synchronously depended on how far they were from everyday hand activities.