3 points to consider whether leading a meeting, giving a presentation, talking on a panel, or interviewing for a job ...
You may feel more energized or comfortable when standing but as a leader or good communicator, you need to be able to bring your best self whether standing OR sitting.
Rather than warming up during your talk, come in ready from the start.
Being a good speaker/leader doesn't mean you always have to have a response at the ready. Take a breath and allow a moment to gather your thoughts. We all need that extra pause.