Most orchestral playing is done sitting down. When sitting, feet should be positioned shoulder width apart in a similar manner to standing posture. A player’s weight should be balanced on their “sitz bones” rather than leaning backwards onto the sacrum. Balanced posture will permit weight transfer between the feet. One general posture principle when sitting: students should be able to stand up without shifting their weight forward. A game you can play is to ask them to stand up when you randomly give a signal, like snapping your fingers or saying “fire drill,” to assess if your students are balanced when sitting.
Positive remarks and modeling work better than negative comments when trying to improve sitting posture in the string class. Saying “don't slouch” to a student gives them no information about what you actually want them to do. Solution: Model an incorrect posture for students and get them to imitate you. Then model the transition to correct posture and ask them to do the same.
Young students will become tired if required to hold their instruments in playing position for too long. Solution: Ask beginning students to return instruments to rest position when they are finished playing.
If your students sit down when they play, build some standing into the routine. My experience suggests that the longer students sit, the worse their posture becomes. Including movement activities and stretching into the routine will help students maintain good posture throughout rehearsal.
A common problem for violinists and violists playing in orchestra is reading the music and seeing the conductor while maintaining good posture and instrument position. Inside and outside players must make adjustments to position themselves correctly. Remind students that they should adjust the height of the music stand so that they can see both the music and the conductor’s baton without needing to adjust their posture or instrument position.