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Bipedal Locomotion
Everyday we practice many forms of physics without giving them any second thought.
Perhaps the most used everyday form of physics used by people is ambulation, also
known as bipedal locomotion, or walking. The act of walking consists of numerous
physical concepts, including rotational inertia, acceleration, energy conservation, friction,
and center of gravity.
The law of conservation of energy states that energy cannot be created or destroyed. It is
transformed from one form into another, but the total amount of energy does not change.
Energy in motion is defined as either kinetic; energy able to provide work, or potential;
stored energy held in readiness. A device defining energy conservation is a pendulum.
As the bob on a pendulum swings through the bottom of the arc, the velocity and kinetic
energy reach it’s maximum, while the potential energy is at it’s most at the top of the arc.
The motion of the human gait can be compared to the motion of a pendulum.
The walking human body pivots around the foot that is on the ground, which is providing
the propelling force. The center of mass in the body, located in the belly area, describes
the arc a pendulum makes. As the other foot is placed on the ground in front of the body,
the ground exerts a force back up the leg that slows the body down, and it continues
slowing as the body rises up on that foot to the top of the arc. The kinetic energy is now
at a minimum, while the potential energy is at it’s maximum, defining the laws of the
conservation of energy. As we fall forward into our next step, the stored potential energy
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is now converted back into kinetic energy, providing acceleration. While a perfect
pendulum converts energy at nearly 100%, the human body’s conversion is only 65%.
35% of energy, therefore, is consumed in calories by the body for fuel.1
Just as Newton’s first law of motion states that inertia is the resistance to change in
motion, rotational inertia is the resistance to change of rotation. The more rotational
inertia an object has, the less it responds to being spun. An object rotating about an axis
tends to remain rotating at the same rate unless there is interferrence by an external force
or influence. When walking, people normally bend their knees to lower the rotational
inertia of their legs. This rotation, along with the rotation of the pelvis in the frontal
plane, reduce the vertical movement of the body. The human gait, therefore, is a
somewhat smooth, horizontal movement rather than a bouncy, up/down, horizontal
movement that a stiff-legged gait would provide. The shorter the persons’ legs are, the
less rotational inertia than those with longer legs. This is because between the center of
mass and the axis, the greather the rotational inertia.
Center of mass is the average position of all the mass in a given object. The center of
gravity is used to express the center of mass, but it is the average position of weight
distribution. As a person walks, the center of gravity shifts higher and lower as the legs
move back and forth, widening and shortening the base. Located approximately in the
middle of the abdomen, the center of gravity reaches it’s highest vertical position at about
the middle of the stance phase, or when the legs are farthest apart and the base is wide.2
1
2
http://discovermagazine.com/2001/jul/featphysics
http://jeb.biologists.org/cgi/reprint/201/21/2935.pdf
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Acceleration is how quickly the change of speed and/or direction of an object occurs. So
how does a person accelerate from a standing position to a walking position? This is
where friction and ankle rotation come in to play.
The direction of the friction force opposes the direction of motion of an object. When a
leg swings forward, the weight of the body “falls” forward and the back foot pushes
against the floor, the support force, as the floor pushes back against the foot. Friction
acting against the foot is keeping the foot from sliding back. As the foot is providing a
force against the floor in a horizontal direction opposite the body, friction is pushing back
on the foot in a forward direction toward the body. If no friction existed between the
floor and the foot, such as on ice or a slick, wet floor, the foot would slip all the way back
when a horizontal force was applied by the foot and the person would fall, not able to
gain traction.
The rotation of the ankle permits acceleration by altering support and propulsive
movements in an upwards and downwards motion, acting as a hinge. The ankle joint
allows people to bend their foot and raise up on their toes. The back foot pushing on the
floor and propelling the body forward is bent at the ankle while the front of the foot, toe
region and ball of the foot, provides the pushing force against the floor.
While the back foot is doing the propelling work, expending kinetic energy, the foot
swinging forward provides potential energy up until the time it makes contact with the
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floor. Once the foot lands on the ground, the weight of the body and center of gravity is
shifted to the front foot momentairly while the back foot raises off the ground. This is
when the shift between kinetic and potential energy occurs. Now the front foot is
expending kinetic energy while the back foot is being lifted and moved forward,
providing potential energy.
All these physics concepts work together at the same time to provide what we call
ambulation, or walking. If one or more of the concepts were absent or altered, human
mobility wouldn’t be as successful as it currently is among the majority of people.
http://moon.ouhsc.edu/dthompso/gait/knmatics/gait.htm#cog
http://www.journals.elsevierhealth.com/periodicals/jjbe/article/PIIS1350453303000729/a
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