Musician's Muscle Memory

When playing any instrument, the muscle memory has an important role in mastering this instrument. Basically, the muscle memory kicks in when you practice a passage long enough that you can reproduce it without thinking about it. It just flows out of your fingers because it has been engrained so deeply into your memory that you can recall it at once.

The more you practice an instrument, the more musical phrases and passages will be stored like that and you can recall them at once. For professional musicians, relying on their muscle memory is a huge factor. Especially quick passages benefit greatly from this as it effectively is not possible to think about every single note in such a quick passage. So the way how for instance passages with quick runs are performed is to process them in larger chunks and fill the notes in between from muscle memory.

For us as composers and orchestrators that means that we can use this muscle memory to our advantage. When not taking this into consideration, it is possible to write incredibly tricky passages that trip even the most professional players.

Professional musicians have spent a lot of time practicing all sorts of runs, arpeggios, scales and idiomatic passages so they are all somewhere stored in their muscle memory. So writing a run up an octave in C major causes them to source that from their muscle memory like a building block that they have ready to use in their vocabulary. So basically, any common scale, arpeggio, run etc. can easily be recalled and in spite of looking very impressive, is actually just them playing on auto pilot. In most cases, you will get instantly good results with those, especially the more common the shapes are.

However, you can also very quickly cause chaos. If you for instance insert a chromatic note somewhere in that C major scale or you write another oddity into a common looking shape, you can be very sure that this will trip your players. Suddenly, they can't recall that from muscle memory anymore but need to actively think about that. Single instances of such a writing with a specific musical purpose are not much of a problem as they can quickly rehearse these passages for themselves a few times in order to get them into "their system" but constantly ignoring common shapes, especially in technically demanding passages (like for instance extended runs up and down with several non expectable notes) can create very problematic results compared to runs that follow a common scale.

Being aware of these things helps alot to avoid problems and write well for your players and especially in situations without rehearsal time (like almost all scoring sessions) you should rely as much as possible on these things. This shouldn't prevent you from sacrificing an important musical idea but asking for a "non standard shape" should have a plausible musical reason and not happen by accident or lazyness.

Of course, if there is time for the players to rehearse or practice things beforehand you can be a bit more adventurous in requesting "non standard shapes" but anything that needs to be sight read benefits greatly from sticking to these shapes. And just to be clear here, these muscle memory instances work relatively granular. So If you for instance have virtuosic runs over a common scale like going 5 notes up, 4 down, 6 up etc. these are all not too tricky as the player will just chain several "musle memory vocabularies" after each other.