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The word impact is derived from
the Latin word impingere, “to
press together.” Impact is further defined as force of
contact, violent collision, striking together. Receiving impact
is opposing or resisting in some manner the force with which a moving
body tends to maintain its speed and direction. Impact may be from
one’s own body, as in landing from a jump or fall, or imparted
by external objects, as in catching or spotting.
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Impact of one’s own body is experienced following any
fall through space. Such falling motion, which occurs subsequent
to a jump, a dive, or an accidental fall, has a rapidly increasing
velocity because of the uniform acceleration effect of gravitational
force. When the body lands on a supporting surface, impact has been
said to occur. The impact is felt as the force of contact. Likewise, impact
is experienced in a horizontally moving body when the motion is
stopped as a result of contact with a resisting surface, such as
a wall or another obstacle.
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Examples of receiving impact from external objects are commonly
seen in sports. Baseballs are caught or fielded with the hands,
hockey balls and pucks are received with a stick, soccer balls are trapped
with the feet, and blows from an opponent’s fist are received
by various parts of the body. Examples of receiving impact are also
seen in industry and in daily life. Cartons and tools are tossed
from one person to another, red-hot rivets are caught in tongs,
and victims from a fire are caught in nets or air bags.
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Impact generally has a negative connotation, but there are benefits
to controlled impacts, such as mechanical loading of bone during
walking. Specifically, it is widely accepted that bone will adapt
to the mechanical stresses placed upon it. Intuitively, it is apparent
that the bones of adults and adolescents will not adapt in the same
manner, yet the bones of both groups are altered by mechanical loading
(Ruff et al. 2006).
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Problems and
Concepts
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What are the particular problems involved in these diverse forms
of receiving impact, and what are the principles that enable us
to solve these problems satisfactorily? In the reception of the body’s
own impact, the chief problems seem to be those of avoiding injury and regaining
equilibrium promptly. Three problems appear to be involved
in receiving the impact of external objects: (1) avoiding
injury, (2) maintaining equilibrium, and
(3) receiving the object with accuracy and
control.
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Whether the moving object is one’s own body or an external
object, the basic concepts enabling us to understand and solve these
problems are the same. The first concept is the kinetic
energy–work relationship: When a body or object is “received,” it
has work done on it equal in amount to the change in kinetic energy
of the moving body (see Chapter 12). If, for example, velocity
of an object is reduced to zero, all of its kinetic energy would
be used to do work on the receiver. Because work equals the product
of force and distance, the work done in “receiving” may
consist of any combination of force and distance as long as the
product of the two is equivalent to the kinetic energy lost by the
moving object.
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The second applicable concept is the momentum–impulse
relationship (see Impulse): Any
change in momentum requires a force applied over a period of time
(impulse) and is equal to the product of that force and the time. Again,
the reduction in momentum of any object can be accomplished by any
equivalent product combination of force and time. Both impulse and
kinetic energy are proportional to the mass and velocity (momentum)
of any object and will change if the momentum does.
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The third concept involves pressure and
is especially important with respect to avoiding injury: The pressure that any part of the body must
absorb is inversely proportional to the area over which the force
is applied.
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The abrupt loss of motion resulting from collision with an unyielding
surface is likely to cause an injury in landing after falls and
landings. To avoid injury, it is necessary to find some means of losing
the body’s kinetic energy more gradually. This is achieved
by increasing the distance and time over which the kinetic energy
and momentum are lost. Landing on “giving” surfaces,
such as mats or sand, controlled flexion at the joints of the landing
extremities through eccentric contraction of the antagonist muscles,
and rolling are important contributors to the gradual decrease in
momentum without injury. Landings are often catagorized as “soft” or “stiff” based
on joint involvement. The knee is a major contributor to force dissipation
in most landings. In soft landings, the involvement of both the
ankle and the hip increase. In stiff landings, the contribution
of the ankle is greater while the contribution of the hip decreases.
A similar pattern occurs with landings from different heights. As
the height of landing increases, the involvement of both the hip and
the ankle increase (McNitt-Gray 1993; Zhang et al. 2000)
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Another factor in injury that should not be overlooked is the
relationship of the force of impact to the size of the area that
bears the brunt of the impact. A force of 450 N concentrated on
1 square centimeter of body surface, for instance, is likely to
cause more serious injury than the same amount of force spread over
an area of 50 square centimeters. Hence the solution is to increase
the size of the area that receives the force of the impact. This
is especially important when there is limited opportunity for increasing
the distance over which the kinetic energy is lost.
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Athletes in activities involving landing from substantial heights
adopt a variety of landing strategies based on the requirements
of the sport. Jumpers such as high jumpers and pole-vaulters decrease
the trauma to the body by landing on very deep crash pads. Gymnasts,
on the other hand, land on mats of varying thickness, based on the
event. As mat thickness decreases, the gymnast uses an increased
angle of hip and knee flexion on landing to increase the time over
which kinetic energy is lost (McNitt-Gray et al. 1993).
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The problem of regaining equilibrium in falling and landing is
largely a problem in controlling the placement of the limbs in preparation
for landing because equilibrium is regained when an adequate base
of support is established. This requires sufficient control to place
the feet, or perhaps both the hands and the feet, in a position
that will provide a favorable base of support. The problem of regaining
equilibrium is closely related to that of avoiding injury because
establishing an adequate base depends on the integrity of the bones
and joints that receive the force of impact.
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Various methods of falling are taught in classes in tumbling,
martial arts, and modern dance. Perhaps one of the most effective
measures for the prevention of injury in accidental falls is this
kind of instruction, followed by the practice of a variety of falls
until the techniques have been mastered. Practice helps establish
the right patterns, patterns that will be followed automatically
when accidental falls occur.
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The goal of most protective equipment is to take a blow from
a relatively small point of contact and dissipate or distribute
that force over a greater surface area or to use a compressible
material to dissipate or absorb the shock of the impact. The giving
material will increase the time over which the striking object will
be in contact with the protective equipment and thus will decrease the
peak force that is imparted to the protective equipment. This is
another example of impulse. By increasing the time over which the
force is applied, a smaller force is required to bring about the
same change in momentum.
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Helmets are designed to protect the head and, in particular,
the brain. Helmets are broadly classified as single-impact helmets
(motorcycle or bicycle helmets) and multi-impact helmets (football, hockey,
and lacrosse helmets). Single-impact helmets must be replaced after
every significant impact, whereas multi-impact helmets are designed
to sustain repeated impacts before needing replacement.
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Bicycle helmets are a single-impact helmet. Once a bicycle helmet
has sustained damage, it cannot be used safely again. Bicycle helmets
use interior foam to dissipate the energy of an impact. By crushing
the interior foam, the speed of the skull can be brought to rest
over a slightly longer period of time than if no helmet was worn.
This application of impulse is highlighted by the fact that the
change in momentum of the head will be nearly the same with and
without the helmet (except for the mass of the helmet); however,
by increasing the time over which the head is brought to rest, the
force required to bring about such a change can be diminished.
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Recently, much attention has been paid to the rate of concussion
in American football. All football helmets worn in high school,
college, and professional American football must adhere to the demands
of the National Operating Committee on Standards for Athletic Equipment
(NOCSAE). Fortunately, rule changes and better helmet designs have
decreased the occurrence of concussion. However, due to the devastating
long-term effects of concussion, research continues. The most notable,
recent advance in football helmet design addresses the findings
that concussions are often the result of side head impacts. Manufacturers
have addressed this newly identified injury mechanism by increasing
the hard shell coverage of the helmet and redesigning the interior
padding arrangement to attenuate side shocks with greater effectiveness
(Collins et al. 2006).
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Forces imparted to a head wearing a helmet can be particularly
devastating. Helmets help to attenuate these forces through compression
of the interior foam. The purpose of the interior foam is to increase
the time over which the head is brought to rest. This helps to dissipate
the energy transmitted during the collision. This energy is converted
to heat and is used in the compression of the interior foam.
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A natural assumption would be to suggest that more foam is better.
However, more foam moves the hard shell further away from the skull
and leaves the head and neck susceptible to greater rotational forces.
The foam needs to be soft enough to be comfortable but dense enough
so as not to completely flatten out during an impact.
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Shoulder pads dissipate force over a greater surface area, thus
decreasing the stress on the body beneath. If the surface area over
which that force is transmitted can be increased, the force per unit
area will decrease, thus diminishing the peak force at any one given
contact point. It is important for padding to fit properly with
no significantly higher contact points so that no point will be forced
to withstand a larger portion of the impact force.
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Elbow and knee pads use foam surrounding the joint, thus increasing
the surface area over which the force is dissipated. Energy is absorbed
in deforming the foam. The stiffer the foam, the more energy required
to deform it. The foam should not be so thick and dense as to restrict
movement, yet be thick enough to withstand a direct blow. It should
be pointed out that however hard one hits the ground while diving
for a volleyball or a line drive, the ground is going to hit back
just as hard. The protective padding is being asked to respond to
the loading that comes from inside the material and from the external
impact. Therefore, the outside layer of the protective padding must be
rugged enough to withstand the frictional forces associated with
sliding across the floor or the ground, yet the inside material
must be soft enough to prevent significant damage to the underlying
joint or anatomical structure.
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As in the case of receiving the impact of one’s own
body, avoidance of injury in catching or receiving external forces
is achieved by increasing the distance over which the object’s
kinetic energy is lost. When catching a fast-moving baseball, the
experienced player will not hold the hands rigidly in front of the
body but will give with the ball. By moving the hands toward the body
through a distance of 10 to 20 inches as the ball is received, the
catcher is making it possible for the ball’s kinetic energy
to be lost gradually. This same principle is likewise true for the
player who is reaching for a high ball with one hand. The extended
arm acts as a lever, the force being applied by the impact of the
ball on the palm. The torque is therefore the product of the force
of impact and the perpendicular distance from the shoulder joint
to the ball’s line of flight at the instant it is caught.
If the line of flight is perpendicular to the outstretched arm,
the torque is the product of the force of impact and the length
of the arm. Catching a fast ball with the arm extended can put a
tremendous strain on the shoulder joint, as well as endanger the
bones of the hands. To avoid injury, the player should give by reaching
somewhat forward for the ball and drawing the arm back at the moment
of impact and by rotating the body and stepping back if the force
is sufficiently great. If the elbow flexes slightly, the lever of
the arm will be shortened and the torque reduced.
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Another factor in avoiding injury when catching fast balls is
the position of the hands. Beginners often reach with outstretched
arms and point their fingers toward the approaching ball. This leads to
many a “baseball finger.” The fingers should be
pointed either down or up, according to whether the ball is below
or above waist level. Balls approaching at approximately waist level
can be caught above the waist if the player bends the knees.
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The second problem in receiving the impact of external objects—that
of maintaining equilibrium—is often neglected. The receiver
should prepare for it in advance because a fast ball or a sudden
blow can easily cause anyone caught off balance to lose equilibrium.
The stance is of great importance here. The base of support needs
to be widened in the direction of the ball’s flight, thus making
it possible for the catcher to shift the weight of the body from
the forward to the rear foot at the moment of impact. This action
not only increases the chances of maintaining equilibrium but also
contributes to the gradual reduction of the ball’s motion.
Widening the stance in a direction at right angles to the flight
of the approaching ball does little to increase the catcher’s
stability.
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The third problem—that of receiving the ball or another
object with accuracy and control—is perhaps the one given
the most emphasis in game situations. As in the attempt to avoid
injury, one of the key factors is the gradual loss of the object’s
kinetic energy. This reduces the danger of the ball’s bouncing
off the hands. Accurate vision, judgment, and positioning of the
body are of vital importance. “Keeping the eye on the ball” is
essential to judging its speed and direction, and hence to adjusting
the position of the body. Thus accurate judgment depends on accurate
vision, and accurate adjustment of the body depends on both of these,
as well as on agility and smoothness of neuromuscular response.
Together, these factors make up what is known as “hand–eye” and “foot–eye
coordination.” To a certain extent this is innate, but
it is also developed and improved by practice.
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Intercepting a ball or a puck is another illustration of receiving
impact. Ice hockey, field hockey, soccer, basketball, and football
are all games in which a player tries to intercept a pass. The same principles
that apply to catching apply to intercepting, but with this difference:
Whereas in catching there is usually time to place oneself in a
favorable position and to use one’s arms, hands, or feet
advantageously, in intercepting, one must take advantage of the
opportunity when it comes. There is no time for preparation. The
important principle to observe is to give with the hands, feet, or
stick the moment that contact is made with the ball or puck in order
to keep control of it; otherwise it is likely to bounce off.
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In receiving both the impact of one’s body and that
of external objects, an important factor to be considered is the
subsequent movement one expects to make. It may be the determining
factor in deciding what stance to assume. For instance, if a run
is anticipated, a forward–backward stance will be more
favorable than a lateral one. Furthermore, it will be desirable
to have the weight over the forward foot. If a catch is to be followed
immediately by a throw, the movements used for giving may be blended
into the preparatory movements of the throw. These are fine points
that have much to do with the degree of one’s skill in
an activity.
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A summary of the principles to observe in receiving impact, both
that of one’s own body and that of external objects, is
presented in the next section, together with some representative
applications of these principles.