Can I use the social security card (medical insurance money) to apply for a fitness card?

It depends on your local social security policy. There are allowed places, but not all places are allowed.

The funds in the medical insurance account in the social security card are social security funds, which shall not be occupied or misappropriated, and shall not be used for cash withdrawal or transfer. Can only be used for outpatient service, hospitalization and purchase of basic medical insurance drugs. In some places, such as Changzhou, Zhangjiagang, Xuzhou, Lianyungang and other cities, the business of purchasing fitness services with social security cards has been opened.