Primary Actions of the Flexor Digitorum Superficialis: 1. Flexion of the digits at the proximal interphalangeal joints. Agonists: Flexor Digitorum Profundus. Antagonists: Lumbricals, Dorsal Interossei, Palmar Interossei, Extensor Digitorum, Extensor Indicis, Extensor Digiti Minimi.