The plantaris muscle is a small muscle located in the back of the lower leg that forms part of the calf. According to autopsy studies, the muscle can vary in size, thickness, and length, and is thought to be entirely absent in about ten percent of the population.

It is made of a muscle belly and a thin tendon, and ranges from seven centimeters to twelve centimeters in length. Due to its small size in relation to other muscles in the calf and knee, it has often been mistaken for a tendon or a large nerve.

There is relatively little existing research on this muscle, and its role is not fully understood. More research is needed in order better understand its function and also to improve the diagnosis and treatment of injuries to the muscle; currently, accurate diagnosis can only be determined through MRI scans.