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: Civil Code of People's Republic of China (PRC).

Article 555 A party may, with the consent of the other party, assign its rights and obligations under the contract to a third party.

Article 556 Where the rights and obligations under a contract are transferred at the same time, the relevant provisions on the transfer of creditor's rights and debts shall apply.