WordNet records no synonym and no antonym for serratus posterior inferior. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a thin quadrilateral muscle at the junction of the thoracic and lumbar regions; acts to counteract the pull of the diaphragm on the ribs to which it is attached
Synonyms, antonyms and the broader and narrower relations for serratus posterior inferior come from Princeton University’s WordNet. Full sources and licences.