The Mat Companion series provides a methodical approach for using science to balance and connect the body and mind in yoga. The fourth book concludes the series with arm balances and inversions—two pose categories that clearly integrate opposites within the body. In our bipedal form, the hips and lower extremities are the weight-bearing construct. The more mobile shoulders and upper extremities allow us to interact with the world. In arm balances, we invert this construct, thereby strengthening the muscles, bones, and ligaments of the upper extremities. From an energetic perspective, arm balances stimulate the nerve plexuses associated with the fourth and fifth chakras. Practicing these poses with precision moves nerve impulses upwards through the subtle body. This opens the way for the unimpeded flow of energy from the lower and more primordial regions of the sacral plexus to the higher-functioning levels of the brain. A similar balancing of opposites takes place with inverted poses. Consider that most of our time is spent with the head above the heart and the lower extremities below it. Inversions reverse this, bringing the head below the heart and the lower extremities above it. Potential benefits of this body position include lowered blood pressure and heart rate and improved circulation of endorphins in the brain. All of this prepares the body for deep relaxation. Arm balances invigorate and stimulate the mind. Inversions bring it to rest.