Final answer:
An isotonic exercise should be done 2-3 times a week, with at least one day of rest between sessions.
Step-by-step explanation:
An isotonic exercise focuses on 1-2 body parts. The frequency of doing an isotonic exercise a week can vary depending on the individual's fitness level and goals. Generally, it is recommended to perform isotonic exercises 2-3 times a week, allowing at least one day of rest between sessions to give the muscles time to recover and adapt.