Wheels-in-wheels: Use of gravity in human locomotion.

Although a wheel is an ideal method for transportation and the invention of the spoke wheel made a wheel lighter and swifter, a wheel cannot function well on slanted or rough surfaces; these are common in the natural environment. Further, the load support of the wheel is limited to a point of the whole wheel in contact with the ground. Then, we humans may be using the legs as a part of spoke wheel and place our legs and feet on the ground alternatively to support the body weight while the gravitational torque makes the center of mass (COM) rotate around the hip joint when proper stiffness and balance is made. Through a pulley-like action involving the hamstrings and a lever-like action of back muscles via the psoas muscle, the energy expenditure for locomotion can be reduced to the energy for lifting the swing leg to maintain the proper position of the COM. Further, the stabilizing action of the psoas muscle to the spinal column can be achieved between the stance leg and the swing leg by the weight of the lifted swing leg during the forward movement. This lifting action during swing phase can assist an energy-efficient eccentric contraction of the stance leg. The passive tension generated by gravity (own weight and the carried load) can be the reason for the energy efficiency of both head-carrying and the Nepalese porter method. Using this passive gravitational force via actively synchronized neuromuscular action may be universal for animal locomotion.