Editing Walking (section)

==Biomechanics==

[[File:Muybridge human male walking animated.gif|alt=An 1887 stop-motion of a human walking.|thumb|upright=.65|[[Eadweard Muybridge|Human walking cycle]]]]
[[File:CGI Human Walk.jpg|thumb|Computer simulation of a human walk cycle. In this model the head keeps the same level at all times, whereas the hip follows a sine curve.]]
Human walking is accomplished with a strategy called the [[double pendulum]]. During forward motion, the leg that leaves the ground swings forward from the hip. This sweep is the first pendulum. Then the leg strikes the ground with the heel and rolls through to the toe in a motion described as an inverted pendulum. The motion of the two legs is coordinated so that one foot or the other is always in contact with the ground. While walking, the muscles of the calf contract, raising the body's center of mass, while this muscle is contracted, [[potential energy]] is stored. Then [[gravity]] pulls the body forward and down onto the other leg and the potential energy is then transformed into [[kinetic energy]]. The process of human walking can save approximately sixty-five percent of the energy used by utilizing gravity in forward motion.<ref name=":23"/>

Walking differs from a [[running]] [[gait]] in a number of ways. The most obvious is that during walking one leg always stays on the ground while the other is swinging. In running there is typically a [[Ballistics|ballistic]] phase where the runner is airborne with both feet in the air (for bipedals).

Another difference concerns the movement of the [[centre of mass]] of the body. In walking the body "vaults" over the leg on the ground, raising the centre of mass to its highest point as the leg passes the vertical, and dropping it to the lowest as the legs are spread apart. Essentially [[kinetic energy]] of forward motion is constantly being traded for a rise in [[potential energy]]. This is reversed in running where the centre of mass is at its lowest as the leg is vertical. This is because the impact of landing from the ballistic phase is absorbed by bending the leg and consequently storing energy in [[muscles]] and [[tendons]]. In running there is a conversion between kinetic, potential, and [[elastic energy]].

There is an absolute limit on an individual's speed of walking (without special techniques such as those employed in [[speed walking]]) due to the upwards acceleration of the centre of mass during a stride – if it is greater than the acceleration due to gravity the person will become airborne as they vault over the leg on the ground. Typically, however, animals switch to a run at a lower speed than this due to energy efficiencies.

Based on the 2D inverted pendulum model of walking, there are at least five physical constraints that place fundamental limits on walking like an inverted pendulum.<ref>{{Cite journal|last=Patnaik|first=Lalit |display-authors=etal |date=October 2015|title=Physical constraints, fundamental limits, and optimal locus of operating points for an inverted pendulum based actuated dynamic walker|journal=Bioinspiration & Biomimetics|volume=10|issue=6 |pages=064001|doi=10.1088/1748-3190/10/6/064001 |pmid=26502096 |s2cid=206102181 }}</ref> These constraints are: take-off constraint, sliding constraint, fall-back constraint, steady-state constraint, high step-frequency constraint.