Effects of a Digital Functional Exercise Program on the Disease Activities and Physical Capabilities of Patients With Ankylosing Spondylitis: Protocol for a Randomized Controlled Trial - Xingkang Liu; Xiaojian Ji; Lidong Hu; Tianyu Jiang; Ang Min; Lulu Zeng; Yiwen Wang; Meihui Zhao; Jian Zhu; Feng Huang; Changshui Weng; Zheng Zhao (JMIR Research Protocols)