Transcript Shoes Under Pressure

Shoes Under Pressure
The Engineering Components to Making Shoes
Objectives:
• Describe how force and pressure are related.
• Calculate force, pressure, impact force and impulse.
• Identify the different parts of the walking gait.
• Explain how pressures on different parts of the foot increase and
decrease while walking or running.
• Explain the difference between over pronation and under
pronation, and how to fix the misalignments with orthotics.
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Why….
• Of the many different types of engineers, some design shoes!
• Walking and running are both complex series of movements, shoes are
designed to provide support to specific foot areas to prevent injury.
• Shoes must withstand a multitude of forces, pressures and impacts on a
daily basis and for the life of the shoe.
• Designing a heeled shoe produces a different set of challenges from an
athletic shoe, both for the designer and the wearer.
• High-heeled designs feature a storable heel to help alleviate the problems
associated with high heels, including driving and long-term discomfort
leading to injury.
Pre-Lesson Assessment
• Complete Force and Pressure Quiz
Vocabulary
force:
Pushes or pulls; anything that causes an object to accelerate or change
direction.
impulse:
Average force x change in time or change in momentum. A measure of
how "hard" a shoe hits the ground.
orthotic:
An insert placed inside a shoe to correct either overpronation or
supination.
Over pronation: Excessive rolling inward movement of the foot when walking or running.
Predisposes lower extremity injuries (such as knee injuries). Causes heavier
wear on shoes on the inner margin. Collapsing arches while walking.
Vocabulary
pressure:
Force per area.
stiletto heel:
A very high heel on a woman's shoe, tapering to a very narrow tip. Also
called a spike heel.
A rotation of the foot and leg in which the foot rolls outward with an
elevated arch so that in walking the foot tends to come down on its outer
edge. Leads to shoes wearing on the outer edge, and knee injuries. High
arches. The opposite of pronation. Same as under pronation.
supination:
Force
• Force is anything that causes an object to undergo acceleration.
• Gravity exerts a downwards force on a shoe and the ground exerts an
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upwards force on a shoe. When standing, these forces are equal and
opposite, causing the person not to move.
A scale measures a person's weight. The force s/he exerts on the
ground can be calculated using Newton's second law.
F=m*a
F = force (Newtons)
m = mass (kilograms)
a = acceleration (meters/second2) [gravity, in our case]
Force
• Pushing off the ground while walking, s/he exerts a force upwards on the
shoe greater than the downward force of gravity so the shoe (and foot)
accelerate upwards to start the step.
• Foot hits the ground, the ground exerts an upward force on the foot, causing
it to decelerate to a stop as the foot hits the ground.
• The faster the foot comes to a stop, the larger the deceleration and the larger
the impact force.
• Running on sand, the deceleration is significantly less than when running on
concrete, so the impact force that the foot feels every step is less for sand
than for concrete.
Pressure
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Pressure is the force per area applied to an object.
P = F/A
P = pressure (Pascals)
F = force perpendicular to the area (Newtons)
A = area (square meters)
Pressure
• The same force applied over a small area creates a higher pressure than if it
were applied over a large area.
• For example, the pressure caused by the force of a person applied over the
sole of an entire shoe is much less than the pressure due to the force of a
person applied over the area of a stiletto heel.
• If a large force is applied to an area, it creates a greater pressure than if a
small force is applied over the same area. Thus, when a heavier person walks,
s/he exerts a greater pressure on the ground than a smaller person with the
same shoe size and shape.
Impulse
• Impulse is defined as change in momentum of an object or the integral of
force with respect to time.
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I = m Δv = F Δt
I = impulse (Newton x second)
m = mass (kilograms)
dv = change in velocity (meters per second)
F = force (Newton)
dt = change in time (seconds)
Impulse
• Impulse is the common physical measure of how hard the foot hits the
ground.
• Using the above set of equations, the force felt by the foot can be calculated
by knowing how fast the foot hits the ground and making an estimation of
how long it takes the foot to decelerate from that velocity.
• A softer surface causes the deceleration to take longer than a hard surface,
meaning that a foot feels a greater force landing on a hard surface than a soft
one.
Basic Foot Anatomy
• Made of 26 bones and 100 muscles and tendons.
• Divided into three different parts:
• the forefoot, which is compromised of the toes and the ball of the foot
• the midfoot, which is made up of the arch
• the rearfoot, which consists of the heel.
• Major tendon is the plantar fasciitis: stretches from the ball of the foot to the
heel.
• When the foot first impacts the ground during the stride, the plantar fascitis
acts as a shock absorber and then tightens during the take-off phase of the
stride, causing the foot to act as a lever.
Walking Mechanics
• Walking gait is divided into two parts:
• the stance phase: when the foot is on the ground
• when the foot is in the air.
• On average, the foot spends 60% of the time in the stance phase with each full cycle of a step
taking approximately one second.
Walking Mechanics
• The stance phase is divided three distinct sections:
• Heel strike: the outside of the heel hits the ground first and the foot begins to pronate
inwards, the arch of the foot drops and the ankle turns inward, the outside of the heel
impacts first, the outer edge of the heel of a shoe tends to wear faster than the rest of the
shoe. This is the part of the gait when the foot experiences the highest impact forces and
pressures.
• Midstance: weight is evenly distributed over the foot, the plantar fascitis acts as a shock
absorber. The foot is maximally pronated during this phase and the pressures experienced by
the sole of the foot are at a minimum.
• Heel lift: weight is shifted to the ball of the foot and the foot supinates (rotates outwards)
as the toes bend, and the plantar fascitis is elongated. This causes the foot to transition from
a soft, shock absorber to a rigid lever necessary for propulsion. The pressure under the ball
of the foot increases again, but the force is still less than what the heel experiences during
the heel strike phase.
Walking Energetics
• The walking stride can be approximated as a swinging pendulum with
both legs straight, like a wheel. In this approximation, the energy used
to lift the body each stride is not recovered, leading to the low
efficiencies found in walking. Using the natural period of a pendulum,
the walking cadence can be approximated as:
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T = 2* π [2L / 3g]1/2
T = period (seconds)
L = length of leg (meters)
g = gravity (9.8 meters per second squared)
v = velocity (meters per second)
Walking Energetics
• Further calculations determine that the power needed to walk can be
approximated as:
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Pw = (mg / p) [3gL / 2]1/2{1 - [1 – π*2v2 / 6gL]1/2}
P = power (joules)
m = mass (kilograms)
g = gravity (9.8 meters per second squared)
L = length of leg (meters)
v = velocity (meters per second)
Running Mechanics
• Running is divided into the stance phase and the airborne phase.
• 40% of the time is spent in the stance phase and each full cycle takes approximately 0.6
seconds.
• Two distinct types of runners exist: heel strikers, and midfoot strikers.
• More than 80% of the population are heel strikers, meaning that the heel is the first part of the foot
to impact the ground during the running gait. The other
• 20% of the population are midfood strikers; either landing on their midfoot, or even their forefoot,
while running. Most top runners fall into the midfoot strikers category.
• Unlike walking, the maximum forces felt by the foot are during the lift-off phase rather than
the heelstrike phase. This distribution is true regardless of the running surface, but the
absolute value of the force varies depending on whether the surface is hard or soft. Because
of this, running on a hard surface, such as concrete, creates higher stresses on the foot than
running on a soft surface, such as sand or grass.
Running Energetics
• By making similar assumptions to the walking pendulum approximation, the energetics of
running can be calculated. Once again, the legs are considered straight and treated like a
wheel, the energy used to flex the leg through the airborne phase is negligible, and the
energy used to raise and lower the body each step is not recovered. In addition, the body is
propelled forward at a 45° angle each step to maximize the stride length. Using this, the
power needed to run can be approximated as:
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P = mgv / 4
P = power (joules)
m = mass (kilograms)
g = gravity (9.8 meters per second squared)
v = velocity (meters per second)
Heels
• Human use of high-heeled shoes can be traced back to the ancient Egyptians, with the
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precursor to the stiletto heel found in tombs dating to 1000 BC. It is believed that they
were considered a symbol of social status. Since then, they have become a prominent
form of footwear.
High-heeled shoes cause a variety of injuries because they alter the natural alignment of
the ankle, knee and hip.
Wearing a high heel shifts a person's center of gravity forward, causing a shortening of
the airborne phase of the stride and increasing his/her cadence.
A three-inch heel increases the pressure on the forefoot three to six times, leading to a
variety of foot injuries, such as bunions, shortened Achilles tendons, trapped nerves and
toe deformities.
Women receive more than 90% of the 800,000 foot surgeries performed each year, many
due to their use of high heels.
Athletic Shoes
• Modern athletic shoes are most commonly constructed with the 22-12 padding
paradigm, meaning that they are designed with 22 mm of padding under the heel,
and 12 mm under the forefoot.
• This configuration favors heel-to-toe runners.
• Recently, some shoe manufacturers have moved away from heavily-padded shoes,
following the trend of barefoot running, which is believed to promote forefoot
striking running and prevent many common running injuries.
Gait Misalignments
• Over pronation, or flat feet, is one of the most common gait misalignments
• occurs when the foot pronates too far inwards during the midstride of
the stance phase
• due to the arches of the foot collapsing too far.
• May be caused by genetic factors that lead to flexible feet, overuse, or
improperly fit shoes.
• Overpronation can lead to knee, ankle and hip problems, as well as
plantar fascitis, bunions, tendonitis, and heel spurs.
• It can be corrected by orthotics that provide additional support under
the arch.
Gait Misalignments
• Underpronation, or supination, is the other common gait misalignment.
occurs when the arches do not collapse enough during the midstride, causing
the foot to roll outwards.
• Supination is easily diagnosed by observing excessive wear on the outer edge
of a shoe.
• Supination can cause all of the problems associated with over pronation and
is fixed by adding extra padding to the sole of the shoe.
Assignments
• Static Forces Worksheet
• Kinetic Movement Worksheet