Serratus Posterior Superior and Serratus Posterior Inferior The video tutorial below covers the Serratus Posterior Superior (SPS) and Serratus Posterior Inferior (SPI) muscles as they relate to trigger points. The SPS and SPI are two different muscles, but they are mirror images of each other.          About Serratus Posterior Superior and Inferior The name comes from the Latin, Serratus meaning saw-like and posterior meaning “coming after” as in the back side of something. With superior and inferior meaning above and below. Winglike in shape, the superior is situated at the upper part of the posterior thorax, while the...