Abstract: Apelin receptor modulators can improve physical performance, slow progression of age-related frailty, and can reduce age-related muscle weakness in human patients. This disclosure provides methods for treating muscle conditions using a particular class of apelin receptor modulators (e.g., agonists). The muscle condition can be an age-related muscle condition. Also provided is a method for maintaining and/or increasing muscle mass, muscle function, and/or muscle strength in an elderly subject by administration of the apelin receptor modulator. In some embodiments, the apelin receptor modulator (e.g., agonist) is BGE-105, or a pharmaceutically acceptable salt thereof.