For someone seated, the popliteal height is the distance from the underside of the foot

Foot

The foot is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made up of one or more segments or bones, generally including claws...

 to the underside of the thigh

Thigh

In humans the thigh is the area between the pelvis and the knee. Anatomically, it is part of the lower limb.The single bone in the thigh is called the femur...

 at the knees. It is sometimes called the "stool height". (The term "sitting height" is reserved for the height to the top of the head when seated.).

This height greatly varies between ethnic groups: For British

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 people the median popliteal height is 400 millimetres for women, and 440 mm for men, while Japanese

Japanese people

The are an ethnic group originating in the Japanese archipelago and are the predominant ethnic group of Japan. Worldwide, approximately 130 million people are of Japanese descent; of these, approximately 127 million are residents of Japan. People of Japanese ancestry who live in other countries...

men have a popliteal height of 400mm and women 360mm.