InnerBody explains that the flexor digitorum profundus muscle flexes the fingers. This muscle, located in the forearm, is classified as a hand muscle since it provides hand functionality. Healthline shows that it starts from the upper part of the ulna and connects to the fingertips with its long tendons.
The Virtual Medical Centre states that both proximal interphalangeal joints (second knuckles) and distal interphalangeal joints (third knuckles) are used in the linear movement of fingers. PIP joints are connected to two muscles, the flexor digitorum superficialis and the flexor digitorum profundus, while flexing of DIP joints is regulated only by the flexor digitorium profundus, according to the Virtual Medical Centre.