Final answer:
In Upward Facing Dog, the knees are extended, and the quadriceps are engaged to support the pose.
Step-by-step explanation:
In Upward Facing Dog, a popular yoga pose, our knees are off the ground (or 'extended') and our quads are engaged or contracted. This pose involves the action of straightening the knees away from the buttocks and engaging the muscles of the thighs to provide support and stability while keeping the legs active. It's a backbend that strengthens and stretches the spine, wrists, arms, and shoulders, while also stretching the chest, lungs, abdomen, and shoulders. Proper alignment and muscle engagement are critical in this pose to prevent injury and to get the most benefit from the stretch.