The Violin I plays the Soprano parts, the Violin II plays the Alto parts, the Viola plays the Tenor parts, and the Cello plays the Bass parts. The two most important parts of a string quartet are the Violin I and Cello. The Violin I and the Cello the melody plays the counterpoint, while the Violin II and Viola parts are filler parts that are there to make the piece sound nicer.
Abdominal muscles (more specifically transversus abdominis, internal and external oblique muscles) play a very important role in breathing for singing.
