Finale's Human Playback has never been documented as well as we'd like. You can just start unchecking/checking different parameters like "automatic expression" and/or different settings in the "dynamics/Volume" settings.

Just wondering, what do you mean by orchestral partitions?

Thank you I will try this at first. 

By orchestral I mean with loads of instruments :)

Just wondering, what do you mean by orchestral partitions?

"partition" = is French for "score"

You said you were using VST strings. You did not say Garritan. Human Playback (HP) is, of course, only configured to work with SoftSynth and Garritan sounds.To have it work with additional VST libraries, one may have to add statements to expand HP's capabilities.

Or else a way to exports my partitions in a DAW to record

File > Export > MIDI file, choose MIDI File Type Format 1 - all instruments saved to separate tracks.

Import the MIDI file into your DAW. You need to set up your instrument sounds again, but then you have (depending on your DAW) many more and better possibilities to mix to a sound file.

Thank you all for your answers, I unchecked the automatic expression and it worked, I can now manage everything manually. The midi exoort method is also a good one.

And it works actually very well with other vst libraries than garritan, the thing is... it would be great if this expression option took into account the slurs we write, treating all the scale as one, instead of making this note by note crescendo / decrescendo. 

PS : Sorry Jeffrey, Michael is right, the word was "score", my mistake

This seems to apply to my concern that the playback of my viola ostinato passages sounds too legato, too "soupy."  I, too, will try drilling down a bit into the Human Playback options.  (I favor "Classical" or even "Standard" for most of what I do.)  Thanks to you all.

Rich P