Much research has been geared towards the mechanics of how bipedal walking has evolved in the genus Homo. However, little research has been conducted to examine how the specific adaptations for running emerged, and how they influenced human evolution.
The bit of research that has focused on human running provides much evidence for bodily function and structures that improve running only, and are not used in walking. This suggests that running was an adaptation, not that it came about as a byproduct of walking.
Running and walking incorporated different biomechanisms. Walking requires an "inverted pendulum" where the body's center of mass is shifted over the extended leg, to exchange potential and kinetic energy with each step. Running involves a "mass spring" mechanism to exchange potential and kinetic energy, with the use of tendons and ligaments. Tendons and ligaments are elastic tissues that store energy. They are stretched and then release energy as they recoil. This mass spring mechanism becomes less energetically costly at faster speeds and is therefore more efficient than the inverted pendulum of walking mechanics when traveling at greater speeds. Tendons and ligaments, however, do not provide these benefits in walking.
Although the mass spring mechanism can be more energetically favorable at higher speeds, it also results in an increase in ground reaction forces and is less stable because there is more movement and pitching of the limbs and core of the body. Ground forces and body pitching movement is less of an issue in the walking gait, where the position of the body's center of mass varies less, making walking an inherently more stable gait. In response to the destabilization of the running gait, the human body appears to have evolved adaptations to increase stabilization, as well as for the mass-spring mechanism in general. These adaptations, described below, are all evidence for selection for endurance running.
Many researchers compare the skeletal structures of early hominins such as Australopithecus to those of Homo in order to identify structural differences that may be significant to endurance running.