WordNet records no synonym and no antonym for isotonic exercise. 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.
exercise in which opposing muscles contract and there is controlled movement (tension is constant while the lengths of the muscles change)
“the classic isotonic exercise is lifting free weights”
Synonyms, antonyms and the broader and narrower relations for isotonic exercise come from Princeton University’s WordNet. Full sources and licences.