You have probably noticed that it is easier to sing while standing rather than sitting down. For some reason, your voice does not feel supported while sitting down.
Why is That?
When we are sitting down, our posture is likely to be out of alignment. A good posture for singing is a straight line running down your ear, shoulder, hip, knee and ankle (when standing). While sitting, aim for a straight line from ear, shoulder to hip. So, if that straight line is broken, your posture is bad.
A bad posture makes it hard for you to keep your rib cage expanded. That means you will struggle to get enough breath in. Also, your breath pressure will also increase. This causes your tone to sound pressed/forced.
We use our core muscles when we breathe in and out. The abdominal wall (or the abs) need to relax when we breathe in. Sitting down makes that difficult. As a result, we cannot take quick and deep breaths.
There are times when sitting down to sing is unavoidable.
During a choir rehearsal or singing in a musical theatre scene where you have to sit down, there are ways to make the best of the situation.
First, you need to sit at the edge of the chair. It is easier to maintain a good posture that way.
Try to lift your sternum and keep it lifted when you are singing. This helps to keep your rib cage expanded.
Sternum down Sternum lifted
Try to let your belly/abs rebound and relax when breathing in. Sitting at the edge of your chair will make this easier.
Be aware that you are not pushing your breath too hard when you are singing sitting down. Using appoggio (a breathing technique that helps you to increase breath capacity) would be quite effective for a steady airflow.