Download TSCC 10 The Basics of Biomechanics and Technical

Specialist Certification Program
The Throwing Events
Basics of Biomechanics and
Technical Model Design
Fundamental Biomechanics
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Force. Force is defined as something that tends to cause a change in the state of motion of a
body. Forces may tend to move a body at rest, slow or stop a body that is moving, or accelerate
a body that is already moving.

Linear and Angular Motion. There are two types of motion, linear and angular. Linear motion is
the motion of a body along a straight path. Angular motion involves a circular path, and the
body or system rotates around a line called an axis. Any biomechanical concept has
corresponding linear and angular applications.

The Center of Mass. Mass is the amount of matter a body possesses. The center of mass of a
body is defined as the point at which we can assume all of the mass of that body to be located
when examining that body’s behavior. The center of mass is in effect a balancing point for the
body, and can be thought of as the “average” location of a body. Bodies that are capable of
changing their shape can effectively move the location of the center of mass to some degree. In
humans, the center of mass lies within the body in the vicinity of the hips. Since a human is
capable of changing body position, one can move the location of his center of mass somewhat if
the body is in contact with the ground.
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Linear Kinematic Parameters
o
Displacement. Displacement is the change in position of a body with respect to a
particular starting point and a given direction. We will frequently use this term to
describe the movement that occurs during a certain phase of event technique.
o
Velocity. Velocity is defined as displacement per unit of time. Velocity is always
expressed with respect to a particular specified direction, so any change in direction
results in a change in velocity.
o
Acceleration. Acceleration is defined as the change in velocity per unit of time.
Acceleration is always expressed in respect to a particular direction, so any change in
direction results in acceleration.
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Angular Kinematic Parameters
o
Angular Displacement. Displacement is the change in rotational position of a body with
respect to a particular starting point and a given direction of rotation. It represents the
size of the angle through which a body turns. We will frequently use this term to
describe the movement that occurs during a certain phase of event technique.
o
Angular Velocity. Velocity is defined as angular displacement per unit of time. It
identifies the speed of rotation of a rotating body. Velocity is always expressed with
respect to a particular specified direction, so any change in direction results in a change
in velocity.
o
Angular Acceleration. Angular Acceleration is defined as the change in angular velocity
per unit of time. Acceleration is always expressed in respect to a particular direction, so
any change in direction results in acceleration.
Newton’s Laws
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Newton’s First Law. Newton’s First Law states that an object will retain its state of motion until
it is acted upon by some outside force. Commonly stated, an object at rest tends to stay at rest,
and an object in motion tends to stay in motion. When an object is at rest, we refer to its
tendency to remain so as inertia. When an object is in motion, we refer to its tendency to
remain so as momentum.

Newton’s Second Law. Newton’s Second Law expresses the relationship between force,
acceleration and mass. This law states that a force applied to an object tends to accelerate it,
and that the acceleration caused is proportional to the force applied and in the direction of that
force. Also, the acceleration produced is inversely proportional to the mass of the object in
question.

Newton’s Third Law. Newton’s Third Law is commonly referred to as the Law of Action and
Reaction. Commonly stated, for every action, there is an equal and opposite reaction. More
precisely, Newton’s Third Law states that for every force exerted, there is an equal force exerted
in the opposite direction. Forces occur in pairs, conserving equilibrium in the universe.
Momentum and Impulse

Momentum. Momentum is the quantity of motion demonstrated by a body. It is expressed
mathematically as the product of mass and velocity, so the momentum values of a body are
dependent on its mass and velocity.

Impulse. Impulse is defined as the momentum change produced in a body. Mathematically, it is
expressed as the product of force and time, so the two factors that determine the impulse
generated are the amount of force applied, and the time over which this force is applied.
Impulse generation is of primary importance during acceleration, when momentum buildup is
critical. Impulse development is not critical at high velocities when momentum has already been
established and the time available for force application is less.

Angular Momentum. Rotating systems possess momentum, and angular momentum is the
quantity of motion imparted to a rotating system. Two factors that determine the angular
momentum of a system are its angular velocity and it’s mass. Increasing either results in
increased angular momentum. Also, angular momentum is increased when the mass of a system
is distributed farther from the axis of rotation.

Angular Impulse. Angular impulse is defined as the angular momentum change produced in a
rotating body. Mathematically, it is expressed as the product of angular force (torque) and time,
so the two factors that determine the impulse generated are the amount of torque applied, and
the time over which this torque is applied. Impulse generation is of primary importance during
acceleration, when momentum buildup is critical. Impulse development is not critical at high
velocities when momentum has already been established and the time available for force
application is less.
An Expansion of Newton’s Third Law

Ground Reaction Forces. When we apply force against the ground in any situation, the ground
supplies force back to the athlete. If this force is sufficient, displacement of the body occurs. This
reaction force is called a ground reaction force.

Swinging Segment Usage. We commonly see, as one leg pushes against the ground, the
opposite leg performing a swinging movement. Swinging body segments enhance force
production. During the upward movement, downward reactive force is created which enables
the athlete to produce more force against the ground and receive a greater ground reaction
force.

Principle of Rotational Opposition. Newton’s Third Law applies to rotational forces as well. The
principle of rotational opposition states that when the body is free to rotate, rotation of one
part of the body produces rotation in another part in an opposite direction.

Action/Reaction. Action/reactions situations are readily evident in flight and single support. In
these situations, enough degrees of freedom exist to enable these action/reaction relationships
to show themselves. However, in double support situations, the body does not enjoy the
freedom of movement it does in flight or single support, so most reaction forces produced are
absorbed by the ground.

Commensurate Momentum Values. If stability of a body is to be maintained, body parts
working in action/reaction relationships should possess similar momentum values. In most
situations we are using upper body rotations to counter lower body rotations. Since the arms
have less mass than the legs, more radius and wider movements are needed to counter these
rotations.

Commensurate Flexion and Extension. In single support and flight situations, free limbs on
opposite sides of the body should exhibit similar degrees of flexion or extension if stability and
balance are to be maintained. When cyclic flexion and extension are part of an activity, we
should see a joint in one limb reach maximal extension at the same time the corresponding joint
on the other side of the body reaches maximum flexion, and vice versa.

Symmetry. A moving body in flight or single support must constantly exhibit symmetry with
respect to the center of mass or some axis in order to maintain stability and balance. When in
flight, the body constantly displays symmetry with respect to the center of mass.
Arrested Motion
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The Hinged Moment
o
Hinged Moments. When a body is in motion and one end of the body is stopped, the
other end will continue to move, rotating about the axis formed at the stopped end.
o
Accelerations and Decelerations. When the hinged moment occurs, velocity changes
occur to nearly all points in the body. The lowest point of the body stops and becomes
the axis of rotation. The uppermost points in the body accelerate as the rotation begins,
while the lower points decelerate.
Transfers of Momentum. There are instances where some momentum of a system can be
imparted to a part of that system or vice versa. We call these cases transfer of momentum.
These situations require stopping a part of the system. When a part of that system is stopped,
the momentum of other parts is preserved. For example, when the horizontal movement of the
body is stopped in a throwing event, the momentum of the implement is preserved.
Bodies in Flight

Gravity. When a body is in flight gravity acts as a force to accelerate the body toward the
ground. Gravity acts upon every object in proportion to its mass, therefore all fall at the same
rate.

The Flight Path as a Parabola. The center of mass of any object projected into flight uses some
parabolic curve as a path. The characteristics of this parabola are determined by the velocity of
the body when flight, begins, the height of the body when flight begins, and the angle of
projection into flight.

Predetermined Flight Path and Rotations. When a body is projected into flight, the flight path
of the body’s center of mass is predetermined and unalterable. The angular momentum of a
body in flight is unalterable as well.
Rotating Systems

Angular Velocity and Curvilinear Velocity. All points in a rigid, rotating body exhibit the same
angular velocity. They all rotate through the same angle in a given period of time. However, if
we express the velocity of any point of the system as distance traveled in units of linear measure
per unit of time, different points in a rotating system show different velocities. We refer to this
expression of velocity as curvilinear velocity, and points farther form the axis show greater
curvilinear velocities than points closer to the axis of rotation. To maximize a point’s curvilinear
velocity, we must maximize the distance of that point from the axis of rotation.

Transfers of Angular Momentum. Angular momentum may be transferred. A body
experiencing rotation may slow, stop, or even reverse that rotation by rotating its body parts in
the same direction. These secondary axes of rotation give the body a tool to absorb the rotation
of the entire body.

Tangential and Axial Forces. A body held to a circular path experiences two key forces. A
tangential force continuously acts upon the body at a tangent to the curved path of travel. An
axial force acts upon the body, directed toward the axis of rotation. To preserve continued
rotation, these tangential and axial forces must be kept in balance.

Conservation of Angular Momentum. Rotating systems possess a certain amount of
momentum. In athletics, the two factors that determine the amount of momentum a system
possesses are the radius of the system and the angular velocity (speed of rotation) of the
system. The greater the radius of the system, the more angular momentum it will have. Also the
faster it spins, the greater the angular momentum values. The Law of Conservation of Angular
Momentum states that a rotating system will keep its angular momentum values constant
unless acted upon by an outside force. Any internal alteration of the system will produce
another compensating alteration of the system, in order to keep momentum values constant.
Radius and angular velocity are in constant interplay. If radius decreases, the system will spin
faster in order to compensate and keep momentum values constant. If the radius of the system
increases, the system will spin slower to accomplish the same goal.
Stability
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Stability. A body’s stability is defined as its level of resistance to toppling. Two factors affect the
stability of an object.
o
Height of the Center of Mass. The higher the center of mass is located, the less stability
the object exhibits and the more likely it is to topple over.
o
Center of Mass/Base of Support Relationships. The horizontal distance between the
center of mass and the base of support also determines a body’s stability levels. The
closer the center of mass is to the edge of the base of support, the more likely the
object is to topple. A body cannot remain stable when the center of mass lies outside
the base of support. In athletic competition, the feet provide the base of support.
Single and Double Support
o
Double Support. In some athletic situations, both feet are in contact with the ground.
This condition is called double support. In double support typically the base of support is
fairly wide, and instability issues are not great.
o
Single Support. In some athletic situations, one foot is in contact with the ground. This
condition is called single support. In single support stability becomes a more complex
issue.
Dynamic Stability. At touchdown of any running stride, the body’s center of mass generally lies
over the base of support, so stability is gained at this point. However, if we examine the body’s
position at toeoff from that stride (just before the flight phase), we see that the center of mass
of the body lies well beyond the body’s base of support. This is an inherently unstable position,
and the body experiences this instability until the next stride grounds. Walking, running, and all
of the locomotive movements associated with the throws involve cyclically losing, then
regaining stability, and instability is required to initiate locomotion. This condition is called
dynamic stability. While humans must experience some instability to move, excessive instability
leads to technical breakdowns and errors.
Developing Technical Models

Commonalities. Commonalities are technical features we see in common when we examine
the various events and skills of athletics, or features we see various performers in an event
share. No two performers in an event share identical technical models. However, a search for
commonalities will yield many features that are shared by high-level performers in an event. We
build technical models around these commonalities, rather than around differences.

Commonality Based Teaching. Developing a commonality based philosophy makes teaching
and learning simpler. Fundamental concepts and skills can be taught and brought to various
events in appropriate ways, rather than approaching each event as a separate entity.

Technique and Style. Individual differences displayed by numerous successful performers are
likely to have little effect on performance. We classify these as stylistic differences, and
differentiate them from technique for this reason.

Sports Science Contributions to the Technical Model. When building a technical model for an
event, we draw upon various fields of sports science for reason, insight, and supporting
evidence. These fields of sports science may agree in supporting a particular technique.
However, they may conflict. In this case it is important to identify the conflict and weigh the
positives and negatives.

Evaluation of Technical Model Changes. Technical improvements can result in several benefits.
These include

o
Performance Level. Improvements in performance level may result
o
Consistency. Improvements in consistency and frequency of good performances may
result
o
Injury Prevention. Decreased levels of risk associated with the technique may allow for
injury free performances.
Cost/Benefit Analysis. When deciding a change in technique is needed, a cost/benefit analysis
should be done. These questions should be asked.
o
Benefit. What will be the type and amount of potential benefit?

o
Difficulty. How difficult will this change be?
o
Further Issues. Is the change likely to result in other problems?
o
Scientific Rationale. Do sports science and commonality study support such a change?
The Principle of Biomechanical Efficiency. When we choose and teach a technical model for
any track and field event, there are two primary goals we wish to accomplish with this model.
One is a high level of performance. The other is injury prevention, meaning that the technique is
safe and consistent with the body’s structure and intended operation. The Principle of
Biomechanical Efficiency states that the processes of attaining these two goals do not conflict.
Achieving biomechanical efficiency accomplishes both goals simultaneously by allowing the
body to operate as nature intended. We need not sacrifice one goal to accomplish the other.
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