I saw a video of champion compound shooter, Randy Ulmer, saying studies show THE most important physical attribute of accurate archers is strong LEG strength for the reasons cited by Shaun and George. Why? They provide a solid platform from which to shoot. If you're sitting/kneeling, you're relying even on less on your accessory muscles to stabilize, and more on... the GROUND. Makes sense: kind of like using a sand bag for gun shooting.
An easy way to test is... Look through your binoculars from different body positions. Check which position provides you with the steadiest view. I found sitting-shots to offer to most stabilization, followed by kneeling on both knees, followed by standing, followed by kneeling on one-knee.