Legal analysis: illegal. According to the relevant provisions of the Civil Code, the creditor-debtor relationship between consumers and fitness centers has been formally established since the service contract was concluded. Consumers have the right to use the venue of the fitness center and get professional guidance, and the fitness center has the obligation to provide fitness-related services to consumers.

Legal basis: Article 546 of the Civil Code of People's Republic of China (PRC). Where the creditor transfers its rights, it shall notify the debtor. Without notice, the assignment is invalid to the debtor.