BackgroundThe genicular or knee joint angles of terrestrial mammals remain constant during the stance phase of walking; however, the angles differ among taxa. The knee joint angle is known to correlate with taxa and body mass among extant mammals, yet several extinct mammals, such as desmostylians, do not have closely related descendants. Furthermore, fossils lose their soft tissues by the time they are unearthed, making body mass estimates difficult. These factors cause significant problems when reconstructing the proper postures of extinct mammals. Terrestrial mammals use potential and kinetic energy for locomotion; particularly, an inverted pendulum mechanism is used for walking. This mechanism requires maintaining the rod length constant, therefore, terrestrial mammals maintain their joint angle in a small range. A muscle reaction referred to as co-contraction is known to increase joint stiffness; both the agonist and antagonist muscles work simultaneously on the same joint at the same time. Themusculus semimembranosusflexes the knee joint and acts as an antagonist to muscles that extend it.MethodsTwenty-one species of terrestrial mammals were examined to identify the elements that constitute the angle between them.semimembranosusand the tibia based on the period between the hindlimb touching down and taking off from the ground. Measurements were captured from videos in high-speed mode (420 fps), selecting 13 pictures from the first 75% of each video while the animals were walking. The angles between the main force line of them. semimembranosusand the tibia, which were defined as θsm−t, were measured.ResultsThe maximum and minimum angles between them. semimembranosusand the tibia (θsm−t) of the stance instance (SI) were successfully determined for more than 80% of the target animals (17 out of 21 species) during SI-1 to SI-13 within ±10° from the mean. The difference between each successive SI was small and, therefore, the θsm−ttransition was smooth. According to the results of the total stance differences among the target animals, θsm−twas relatively constant during a stance and, therefore, average θsm−t(θave) can represent each animal. Only Carnivora had a significant difference in the correlation between body mass and θave. In addition, there were significant differences in θavebetween plantigrade and unguligrade locomotion.ConclusionOur measurements show that θavewas 100 ± 10° regardless taxon, body mass, and locomotor mode. Thus, only three points on skeletons need to be measured to determine θave. This offers a new approximation approach for understanding hindlimb posture that could be applied to the study of the hindlimbs of extinct mammals with no closely related extant descendants.