Humans evolved to an upright stance with straightened knees (unlike the apes that can't straighten knees). The shift in orientation of the "axial skeleton" or trunk, from the horizontal to the vertical with respect to gravity necessitated some changes to our gait pattern.
Instead of major side bending trunk action for locomotion, humans evolved to use a counter rotation of the spine. Not forgetting some remnants of side bending (reptiles and fish) and also trunk flexion and extension (whales, dolphins and quadripeds).
To experience trunk rotation used in gait, try walking on your "butt". Imagine having no legs and your feet are attached to the pelvis. This is how you would locomote. Now add legs but retain the action of the trunk. This rotation and side bend from the pelvis is a key to the power and coordination of human locomotion.
We don't walk only with legs, we walk with legs and body. One major body action for gait is rotation. Counter rotation because as the pelvis rotates in one direction the shoulders go opposite.
The other major action is a side bending of the pelvis and trunk. The pelvis drops on the side of the swinging leg. That whole side lengthens like a spring (recall the newt trunk and leg action!).
Both of these actions extend the swing of the leg, increase energy efficiency and lessen strain at the hip and knee.
Like the cheetah, the head in human locomotion deviates little in the horizontal and frontal planes. To have a head that glides forward unimpeded in perceiving the environment while the powerful and legs propel the human animal, the thorax or chest is flexibile enough to accomodate this demand. Without this flexibility something else must accomodate the simultaneous demand for a steady head position while the pelvis and legs power propulsion.
Interestingly, one researcher proposed that the arch in human spine is critical to locomotion. Any structure with an arch that is then bent to one side will rotate. Gracovetsky (1986)* proposed this as the source of intitiation and power of human locomotion. So as well as muscles being a source of power, proper organization of the skeleton will allow muscular power to excel. Gracovetsky's ideas fit well with a Feldenkrais view of gait as being powered and inititiated from the pelivs. Many other authors attritbute power to the calf or hip flexor muscles.