the arms are actually supposed to be as open as possible during the drive phase. if you kept your elbows at 90 for acceleration, there would be absolutely no power coming from the arms. if you look at maurice greene, ato boldon, justin gatlin, etc. during the first 20m, their arms are flying straight back and forth, the elbow joint completely straightening out during the backswing. during upright running though is when the elbows are always locked at 90 degrees.