Download 408 4 Biomechanics for the Speed and Power Events

Sports Science for the Speed
and Power Events
Biomechanics
Biomechanics
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Biomechanics. Biomechanics is the study of how physics and the laws of mechanics
interplay with anatomy and physiology to affect human performance. An understanding
of Biomechanics is important to success in coaching, since biomechanics provides
rationale and reason for the techniques we teach.
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Human Movement. The problems of human performance cannot be approached purely
from the standpoint of the laws of mechanics. The laws of rigid body physics, as they
apply to human performance, are often unrecognizable due to the elastic nature,
anatomical structure, injury prevention mechanisms, reflexes, and motor control
mechanisms of humans. Coaching practice must bridge the gap between mechanical
and physiological aspects of human performance.
Developing Technical Models
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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.
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Intraevent Commonalities. Intraevent commonalities are certain skills or movements
that can be found in repeated places within the execution of the event.
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Interevent Commonalities. Interevent Commonalities are certain skills or movements
that can be found repeated in two or more events.
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Intersport Commonalities. Intersport Commonalities are certain skills or movements
that can be found in two or more sports.
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.
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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.
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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.
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Tradeoffs. Frequently, when we consider the laws of mechanics and their application to sports
technique, we try to maximize the benefit derived with respect to one particular biomechanical
parameter without examining the cost in other areas. Often a tradeoff exists in the benefits
derived from application of differing biomechanical principles. This text will not attempt to
indicate optimal degrees of involvement of these parameters in technique, but will make every
effort to point out the tradeoffs. There are several commonly seen tradeoffs that might explain
diminished performance when examination in light of pure physics would suggest improvement.
Most fall into these categories.
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Physiological Dysfunction. The attempted technical adjustment results in some
physiological or proprioceptive compromise.
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Elastic Dysfunction. The attempted technical adjustment results in diminished elastic
energy production.
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Anatomical Dysfunction. The attempted technical adjustment cannot be efficiently
executed as a result of inherent musculoskeletal alignments.
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Coordinative Dysfunction. The attempted technical adjustment cannot be efficiently
executed as a result of increased coordinative demand.
Evaluation of Technical Model Changes. Technical improvements can result in several benefits.
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Performance Level. Improvements in performance level may result.
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Consistency. Improvements in consistency and frequency of good performances may
result.
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Injury Prevention. Decreased levels of risk associated with the technique may allow for
injury free performances.
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Cost/Benefit Analysis. When deciding a change in technique is needed, a cost/benefit analysis
should be done. These questions should be asked.
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Benefit. What will be the type and amount of potential benefit?
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Difficulty. How difficult will this change be?
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Further Issues. Is the change likely to result in other problems?
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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.
Fundamental Terms
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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.
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Statics and Dynamics. Dynamic is a term used to describe situations where certain aspects of
movement are of concern. There are other instances in athletics where stabilization or absence
of movement in a body or body part is evident. Static is a term we use to describe these
situations. Most athletic endeavors are a combination of static and dynamic situations. As a rule,
when equal forces act in opposite directions, static situations result. When opposing forces are
unbalanced, movement occurs and dynamic situations result.
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Kinematics and Kinetics.
kinetically.
All description of movement can be classified kinematically or
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Kinematics. Kinematics is the study of the appearance of motion. In studying an athletic
effort, factors such as the path, position, and movement of the center of mass and
various body parts, displacements, velocities, accelerations, takeoff or release angles,
etc., are kinematic factors; they describe the visual appearance of the effort.
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Kinetics. Kinetics is the study of the forces acting upon a body, which explain the
appearance of its movement. A study of momentum values, the forces involved,
(including the magnitude and direction of these forces) and reactive forces, would
involve kinetics.
Linear and Angular Motion. In this text we will discuss two types of motion, linear and angular.
In athletics, we seldom see pure cases of linear or angular motion. Most movement in athletics
is a combination of the two. Although they occur together, for the sake of simplicity we usually
study them separately.
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Linear Motion. Linear motion is the motion of a body along a straight path.
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Angular Motion. Angular motion is rotational in nature, meaning the path of the body
in question is circular and the body or system rotates around a certain line. This line is
called an axis.
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Corresponding Linear and Angular Concepts. Most kinematic and kinetic concepts are
similar regardless of the type of motion in question. For this reason every kinematic or
kinetic parameter has corresponding linear and angular applications.
Mass. Mass is defined as the amount of matter a body possesses.
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Mass and Weight. The terms mass and weight are often used interchangeably, yet
there is a difference. Weight is related to mass; it refers to the force gravity exerts upon
a body.
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Units. The unit most commonly used for measuring mass is the gram, and the unit we
use to measure forces (such as gravity) is the Newton. In the British system, we use the
slug to measure mass, and the pound to measure weight.
Center of Mass
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Center of Mass. 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.
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Location of the Center of Mass. Rigid objects have a fixed location of the center of mass.
Bodies that are capable of changing their shape can effectively move the location of the
center of mass to some degree.
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Location of the Human Center of Mass. 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. The center of mass of the body may actually lie outside of the body itself. A
human can move the center of mass outside the body by assuming piked or reverse piked
positions.
Linear Kinematic Parameters
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Displacement
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Displacement. Displacement is the change in position of a body with respect to a
particular starting point and a given direction.
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Directionality. In everyday speech, we may exchange the phrases displacement and
distance traveled, and they are similar concepts. With displacement, a direction in which
movement will be measured must be specified, or at least implied by context. For
example, a body that moves directly north 20 miles has demonstrated a displacement of
20 miles, but only if the specified direction is north. If the specified direction were east,
for example, there would be no displacement in spite of the northward movement,
because there was no eastward travel.
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Coaching Practice. Displacement is a frequently neglected parameter in the coaching
practice. Movement characteristics of the entire system are usually more important
than movements within the system.
Velocity
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Velocity. Velocity is displacement per unit of time. Recalling the definition of
displacement, we can further define velocity as change in position, with respect to a
given direction, per unit of time.
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Directionality. In everyday speech, we exchange the words velocity and speed, and
they are similar concepts. The difference is that with velocity (as with displacement), a
direction in which the speed will be measured must be specified. If there is no motion,
or if the movement seen is not at all in the specified direction, velocity is zero.
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Measuring Velocity
Instantaneous Velocity. An object at any instant in time, even though its
velocity may be changing, exhibits a certain velocity. This constitutes an
instantaneous velocity.
Average Velocity. Velocity can be expressed as an average velocity, expressed
as the displacement achieved over a certain period of time.
Acceleration
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Acceleration. Acceleration is the change in velocity per unit of time.
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Positive and Negative Accelerations. The concept of speeding up is commonly
associated with acceleration. If the direction specified is the same, speeding up does
constitute a positive acceleration. On the other hand, slowing or decelerating (again
assuming the direction is correct) constitutes a negative acceleration. A body moving at
a constant velocity demonstrates an acceleration value of zero.
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Directionality. Acceleration is considered with respect to a particular direction. Thus,
any time a moving object changes direction it has accelerated in some form since
velocities with respect to the chosen direction have changed.
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Measuring Acceleration
Instantaneous Acceleration. An object at any instant in time, even though its
acceleration may be changing, exhibits a certain value of acceleration. This
constitutes an instantaneous acceleration.
Average Acceleration.
Acceleration can be expressed as an average
acceleration, expressed as the change in velocity over a certain period of time.
Units of Measure
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Displacement. Displacement is measured in units of length such as the foot, meter, etc.
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Velocity. Since velocity is defined as displacement per unit of time, it is measured in units
expressing distance/time. Examples include miles/hour, meters/second, etc.
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Acceleration. Since acceleration is defined as the change in velocity per unit of time, it is
then measured in units expressing velocity (distance/time) over time. We commonly
express this mathematical arrangement as distance/time2. Examples include,
meters/second2, miles/hour2, etc.
Computations
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Displacement. Consider the definition of displacement (change in position with respect
to a particular starting point and direction). We can then find displacement by finding the
difference between the initial and final positions. Therefore:
d = sf - si
In the above d represents displacement. The s represents position, therefore sf represents
final position and si represents initial position. In a 100 meter race, if we consider the start
to be the zero mark and the finish to be the 100 meter mark, we can compute the
runner’s displacement as follows:
d = sf - si
d = 100 m – 0 m
d = 100 m
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Velocity. Consider the definition of velocity (displacement per unit of time). We can then
find velocity by dividing displacement by the elapsed time. Therefore:
v= d
t
In the above, v represents velocity, d represents displacement, and t represents elapsed
time. If a runner covers 100 meters in 11 seconds, we can find the average velocity as
follows:
v= d
t
v = 100 m
11 sec
v = 9.09 m/sec
In the above, if we didn’t know d, we could use d = sf - si to find it.
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Acceleration. Finally, we will consider the definition of acceleration (change in velocity
per unit of time). We can find the change in velocity by finding the difference in the final
and initial velocities. We can then find acceleration by dividing the change in velocity by
the elapsed time. Therefore:
a = vf – vi
t
In the above a represents acceleration. The v represents velocity, so v f represents final
velocity and v I represents initial velocity. The t represents elapsed time. If a runner’s race
velocity increases from 5 m/sec to 6.5 m/sec over a 3 second interval, we can find the
average acceleration as follows:
a = vf – vi
t
a = 6.5 m/sec – 5 m/sec
3 sec
a = 1.5 m/sec
3 sec
a = 0.5 m/sec2
Similarly, assume that a runner over the course of 10 seconds slows from a velocity of 8
m/sec to 7 m/sec. Average acceleration can be found as follows:
a = vf – vi
t
a = 6 m/sec – 7 m/sec
10 sec
a = -1 m/sec
10 sec
a = -0.1 m/sec2
In the above, a case of deceleration, note that a is negative (with respect to the direction of
travel).
Angular Kinematic Parameters
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Angular Displacement
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Angular Displacement. Angular displacement is the change in position of a rotating
body. In angular terms, we measure this change in position by the size of the angle or
number of revolutions the body has rotated through.
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Directionality. Values of angular displacement must be defined with respect to some
direction of rotation.
Angular Velocity
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Angular Velocity. Angular velocity is angular displacement per unit of time. It describes
the speed of rotation of a body. In a rigid body, all points rotate in unison. Thus, all parts
of a rigid rotating body have the same angular velocity, since they rotate through the
same angle in a given period of time.
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Directionality. Values of angular velocity must be defined with respect to some
direction of rotation.
Angular Acceleration
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Angular Acceleration. Angular acceleration is defined as the change in angular velocity
per unit of time. It describes the change in speed of rotation of a body. Like
accelerations, these may be positive or negative.
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Directionality.
Values of angular displacement, angular velocity, and angular
acceleration must all be defined with respect to some direction of rotation.
Angular Units of Measure
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Angular Displacement. Since angular displacement is defined as the size of the angle
that the body has rotated through, it is measured in units of circular measure. Examples
are degrees (°), radians (1 radian is approximately 57.3°), or revolutions.
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Angular Velocity. Since angular velocity is defined as angular displacement per unit of
time, it is measured in units of angular displacement over time. Examples are
degrees/second, revolutions/minute, etc.
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Angular Acceleration. Since angular acceleration is defined as change in angular
velocity per unit of time, it is measured in units of angular displacement/time2.
Examples are degrees/second2 or radians/second2.
Angular Computations
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Angular Displacement. Consider the definition of angular displacement (the change in
position of a rotating body). We can then find angular displacement by finding the
difference between the initial and final positions. Therefore:
θ = sf - si
In the above, the Greek letter theta (θ) represents angular displacement, and sf and si
represent final and initial position.
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Angular Velocity. Consider the definition of angular velocity (angular displacement per
unit of time). We can then find angular velocity by dividing angular displacement by
elapsed time. Therefore:
ω=θ
T
In the above, the omega (ω) represents angular velocity, theta (θ) represents angular
displacement, and t represents elapsed time. If a discus thrower who rotates through an
angle of 1.6 radians during 2 seconds, we can calculate the angular velocity as follows:
ω = 1.6 rad
2 sec
ω = 0.8 rad / sec
In the above, if we didn’t know θ, we could use θ = sf - si to find it.
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Angular Acceleration. We will consider the definition of angular acceleration (change
in angular velocity per unit of time). We can find the change in angular velocity by
finding the difference in the final and initial angular velocities. We can then find
acceleration by dividing the change in velocity by the elapsed time. Therefore:
α = ωf - ωi
t
In the above, alpha (α) represents angular acceleration. The ω represents angular
velocity, therefore ωf represents final velocity and ωi represents initial velocity. The t
represents elapsed time. If a discus thrower accelerates from 2.0 rad/sec to 2.5 rad/sec
during 0.8 seconds, we can calculate the angular acceleration as follows:
α = ωf - ωi
t
α = 2.5 rad/sec – 2 rad/sec
0.8 sec
α = 0.5 rad/sec
0.8 sec
α = 0.625 rad /sec2
Newton’s Laws
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Sir Isaac Newton. Sir Isaac Newton postulated three laws in the late 1600’s that remain the
basis for all of the fundamental laws of physics and mechanics that govern movement. This
section will serve as a brief overview of these laws, and we will examine them in more practical
detail later.
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Newton’s Laws
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Newton’s First Law
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.
Inertia. When an object is at rest, we refer to its tendency to remain so as
inertia.
Momentum. When an object is in motion, we refer to its tendency to remain so
as momentum.
Inertia, Momentum, and Mass. Resting objects with more mass are harder to
move than objects with less mass. Mass is then an effective measure of a body’s
inertia. Similarly, an object with more mass will require more force to alter its
movement. Mass affects the amount of momentum a body possesses as well.
Friction. Although Newton’s First Law states that a moving body remains in
motion, all objects eventually stop. This is because in the world we live in,
gravity and friction are inescapable outside forces that prevent perpetual
motion. In a vacuum, away from gravity, Newton’s First Law as it pertains to
moving objects could be demonstrated to be true.
Newton’s Second Law
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.
Mathematical Expression. Mathematically, we can express Newton’s Second Law
as:
f = ma
In the above, f represents the force applied, m the mass of the object, and a the
acceleration.
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Newton’s Third Law
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.
Coaching Practice. Newton’s Third Law has great reaching consequences in the
realm of athletics. For every force we exert in competition, some opposite force
occurs. When we design and teach techniques, we must be concerned with the
reactive consequences of our actions. As coaches, perhaps the most important thing
we do is to recognize what elements of technique are volitional and which elements
are reactive to those. A challenging demand of coaching is that sometimes these
pairs of forces are quite visible, while at other times they aren’t.
Angular Considerations of Newton’s Laws
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Angular Considerations of Newton’s First Law
Angular Inertia. Rotating bodies possess inertia, just as linear ones do. The
linear concept of inertia corresponds to angular inertia. Both are measures of
how difficult it is to create movement. There are two primary characteristics of
a rotating system that determine its inertial value. There are two characteristics
of a rotating system that determine it angular inertia.
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Mass. The greater the mass of the system the greater the angular
inertia of the system. Greater mass makes it more difficult to alter the
angular movement of the rotating system.
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Distribution of Mass. The way the mass is distributed also determines
how much angular inertia the system possesses. The further away from
the axis of rotation the mass is distributed, the greater the angular
inertia.
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Angular Momentum. Rotating bodies can possess momentum as well.
Momentum and angular momentum are both are measures of quantity of
movement and how difficult it is to alter movement. Similarly, mass and its
distribution are key factors that affect how much momentum a moving system
would have.
Coaching Practice. In athletics, the mass of the system is usually a constant.
Athletes don’t change weight significantly during a performance. When we
consider humans in athletics, the distribution of the mass with respect to the
axis of rotation determines the angular inertia value and much of the angular
momentum value of the rotating system.
Angular Considerations for Newton’s Second Law
Newton’s Second Law. A force applied to a body tends to accelerate it. In the
same way, a torque applied to a rotating body tends to produce an angular
acceleration in the direction of the torque. Also, rotating systems with more
angular inertia are more difficult to accelerate angularly. We expressed
Newton’s Second Law as f=ma, so in angular term
T = Iα
In the above, T represents torque, I represents angular inertia, and α represents
angular acceleration.
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Torque. The torque itself is a somewhat complex issue. There are two factors,
which determine the amount of torque applied. One is the size of the force. The
other is the distance from the axis of rotation at which the force is applied. The
further away from the axis of rotation the force operates, the greater the torque.
Angular Considerations for Newton’s Third Law. Newton’s Third Law can be examined in an
angular sense as well. Recalling that every force produces an equal force of an opposite direction,
clockwise forces produce counterclockwise reactions and vice versa.
Momentum and Impulse
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Momentum
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Momentum. Momentum is a term used frequently used with a general idea of its
meaning, yet we often encounter difficulty in defining it. Momentum is the quantity of
motion imparted to a body. The momentum of a body is dependent on its mass and
velocity. Mathematically we can express momentum as the product of mass and
velocity.
M= mv
In the above, M represents momentum, m the body’s mass, and v the body’s velocity.
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Momentum and Velocity. Development of momentum is as important as development
of velocity and is usually prerequisite to it. Momentum and velocity are related, but they
are not the same. Failure to develop momentum properly leads to numerous technical
faults and inefficiencies, and certain techniques are associated with good momentum
development.
Impulse
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Impulse. Impulse is defined as the momentum change produced in a body, and Impulse
generation is the process through which momentum is produced. Impulse is valued as
the product of force and time, and can be represented mathematically as:
J =ft
Where J represents impulse, f represents the force applied to the body, and t represents
the time over which force is applied.
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Momentum Generation. The two factors that determine the impulse generated are the
amount of force applied, and the time over which force is applied. The amount of
momentum developed is directly dependent upon the amount of impulse generated, so
developing momentum involves the same factors.
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Coaching Practice
Momentum Generation Techniques. In many instances in athletics, we are trying
to produce the greatest possible positive changes in momentum. In designing
athletic techniques for these situations we try to produce great forces and apply
them over long periods of time. Since force production capability is inherent and
dependent upon an athlete’s talent and training status, most manipulations of
techniques when momentum development is concerned involve lengthening the
time for force application. This is done by lengthening the distance over which
force is applied.
Event Study. In the initial movements of an athletic event (the start of a throw,
jump approach, or run) velocities are low, and acceleration is taking place. Since
velocities are low at this point, opportunity exists to apply large forces over a
longer period of time to produce impulse and develop momentum that will assist
later in the event. It is during these times that impulse generation and the
resulting momentum gains are most crucial. Later in an event, when maximal
velocities are achieved, the amount of time available for force production is
limited. Impulse and momentum development have already been achieved, and
concern shifts to conserving momentum, rather than generating it. For this
reason, impulse generation and momentum development are of concern when
acceleration is taking place, but not at high velocities. This acceleration can take
the form of a change in direction, such as a high jumper that changes from a
horizontal run to a vertical takeoff. Attempts to create longer force application
times at high velocities are usually inappropriate. They may result in loss of
velocity, and the impulse created is often in the wrong direction.
Technical Considerations. Impulse generation and the resulting development of
momentum are dependent upon developing large forces and applying them over
long periods of time. From a technical standpoint, momentum development is
enhanced by starting a run, throw, or jump with powerful movements, employing
full use of and range of motion of the joints. Velocities then increase as the event
progresses.
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Angular Considerations for Momentum and Impulse
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Angular Momentum
Angular Momentum. Angular momentum is the quantity of motion imparted to
a rotating body. The angular momentum of a rotating body is dependent on its
angular inertia and angular velocity. Mathematically we can express momentum
as the product of angular inertia and angular velocity.
L=Iω
In the above, L represents angular momentum, I the body’s angular inertia, and
ω the body’s angular velocity.
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Angular Momentum and Angular Velocity.
Development of angular
momentum is as important as development of angular velocity and is usually
prerequisite to it. Angular momentum and angular velocity are related, but they
are not the same. Failure to develop momentum properly leads to numerous
technical faults and inefficiencies, and certain techniques are associated with
good momentum development.
Angular Impulse
Angular Impulse. Angular Impulse is the momentum change produced in a
rotating body, and angular impulse generation is the process through which
momentum is produced. Impulse is valued as the product of torque and time,
and can be represented mathematically as:
J=Tt
Where J represents angular impulse, T represents the torque applied to the
body, and t represents the time over which torque is applied.
Angular Momentum Generation. The two factors that determine the angular
impulse generated are the amount of torque applied, and the time over which
force is applied. The amount of angular momentum developed is directly
dependent upon the amount of angular impulse generated, so developing
momentum involves the same factors.
Coaching Practice
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Angular Momentum Generation Techniques. In many instances in
athletics, we are trying to produce the greatest possible positive
changes in angular momentum. In designing athletic techniques for
these situations we try to produce great torques and apply them over
long periods of time. Since force production capability is inherent and
dependent upon an athlete’s talent and training status, most
manipulations of techniques when momentum development is
concerned involve lengthening the time for force application. This is
done by lengthening the angle of rotation through which force is
applied.
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Event Study. In the initial movements of a rotational athletic movement
angular velocities are low, and acceleration is taking place. Since angular
velocities are low at this point, we have an opportunity to apply large
torques over a longer period of time to produce angular impulse and
develop momentum that will assist later in the event. It is during these
times that angular impulse generation and the resulting angular
momentum gains are most crucial. Later in an event, when maximal
velocities are achieved, the amount of time available for force production
is limited. Angular impulse and momentum development have already
been achieved, and concern shifts to conserving momentum, rather than
generating it. For this reason, angular impulse generation and angular
momentum development are of concern when acceleration is taking
place, but not at high velocities. Attempts to create longer force
application times at high velocities are usually inappropriate. They may
result in loss of velocity, and the impulse created is often in the wrong
direction.
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Technical Considerations. Angular impulse generation and the resulting
development of angular momentum are dependent upon developing
large forces and applying them over long periods of time. From a
technical standpoint, momentum development is enhanced by starting a
rotational movement with powerful movements, employing full use of
and range of motion of the joints. Velocities then increase as the event
progresses.
An Expansion of Newton’s Third Law
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The Earth and Action/Reaction
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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. It is a ground reaction
force that we are experiencing when we push against the pedals of a starting block,
against a takeoff board, against the ground on any running stride, or against the surface
in throwing.
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Ground Force Absorption. Forces act in pairs. When we push ourselves into the air, as
in a jump, we actually push the earth away from ourselves as well. However, the mass of
the earth is so great as compared to the mass of the human that this movement is
undetectable. Many of the forces that occur in reaction to human movement are
undetectable because the earth effectively absorbs the force in this way.
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Swinging Segment Usage. Consider a running stride or a jump takeoff. As one leg pushes
against the ground, we see the opposite leg (we will call it the free leg) swinging upward. These
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 thus receive a greater ground reaction force. In a running stride or jump takeoff, the free leg
stops its upward movement just as the body leaves the ground. This stopping of the free leg is
commonly called blocking. When this blocking occurs, the momentum of the limb is imparted to
the system, effectively making the body lighter and aiding in liftoff. Arm actions have a similar
effect.
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Internal Rotations
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Principle of Rotational Opposition. For every force exerted, an opposite force is
exerted. This 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.
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Countering Rotations. When the technique of an event requires us to produce desired
rotation, we must produce rotation in the opposite direction in some form to maintain
stability of the body. This process is called countering.
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Law of the Ends and Middle. A biomechanical principle that is convenient for coaching
purposes is the law of the ends and middle. This law explains the nature of body
movement about the body’s center of mass when freedom of movement in some plane
exists. If a body moves one of its ends in any direction, the other end of the body moves
in the same direction and the middle of the body moves in the opposite direction.
•
Action/Reaction in Flight. Action/reactions situations are readily evident in flight. Muscles
apply forces to the skeleton that supply reactive forces and movements. Since the body is not in
contact with the ground, freedom of movement exists in all planes, and reaction forces can be
exhibited in any plane.
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Action/Reaction in Single Support. Single support is a term used to identify situations in sport
when one only foot is in contact with the ground, such as in a running stride or jump takeoff.
Many of the forces in athletics generated in single support situations result in ground reaction
forces. However the body is free to rotate and move about its center of mass in single support.
In this situation the body is subject to the principle of rotational opposition, and other
movements while in single support result in efforts to maintain symmetry about the body’s
center of mass.
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Action/Reaction in Double Support. Double support is a term used to identify situations in
sport where both feet are in contact with the ground such as in a throw. The reduced freedom
of movement results in a greater production of ground reaction forces, and action/reaction
situations, particularly rotations, result in reaction forces that are absorbed by the ground.
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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, upper body rotations are used to counter lower body rotations. Since the upper body
generally possesses less mass, increased radius (wider movements) is needed to counter these
rotations.
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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.
o
Symmetry in Flight. When in flight, the body constantly displays symmetry with respect
to the center of mass.
o
Symmetry in Support. In support, when continued movement and efficiency is a goal,
symmetry limb movement should show symmetry with respect to the body’s center of
mass and/or certain axes. Recognizing instances where symmetry is applicable can be
helpful diagnostic tool for the coach. Most athletic dysfunction, particularly locomotive
dysfunction, results from attempts to maintain symmetry under duress.
Arrested Motion
•
•
Transfer 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. In athletic techniques we use the principle of
transfer of momentum to impart momentum from a system to a part of the system.
o
Transfers from the System to Parts of the System. Transfer of momentum provides the
rationale behind blocking with the forward leg in the delivery of a throw. The deceleration
of the thrower provides a base from which force can be applied, yet much of the
momentum of the implement is preserved.
o
Transfers from Parts of the System to the System. In certain situations the momentum
of parts of the system is imparted to the entire system. These situations require stopping
a part of the system as well. These transfers of momentum provide the rationale for
swinging segment usage.
The Hinged Moment. Assume a body is in linear motion, and that one end of the body is
stopped. The other end will continue to move, rotating about the axis formed at the stopped end.
This effect is called a hinged moment, and several phenomenon occur in this situation. Hinged
moments are a frequent occurrence in athletics, and may be advantageous or disadvantageous
depending upon the circumstances and values of the acceleration and rotation produced.
o
Stopping. One end of the body is stopped, and the stopped end forms an axis of rotation.
o
Rotation. The body moves into rotation about the axis formed at the stopped end.
o
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Acceleration.
percussion.
Curvilinear velocity increases in points above the body’s point of
Angular Applications of the Hinged Moment. The hinged moment can occur in a rotational sense
as well. If an object is rotating, stopping one end moves the axis of rotation from the body’s
center of mass to the stopped end, increasing the curvilinear velocity of the opposite end. This
phenomenon provides the rationale for the blocking of the nonthrowing arm/side in throwing
events.
Bodies in Flight
•
Gravity
o
Gravity as a Force. Gravity is a force, and according to Newton’s Second Law, forces
produce accelerations. 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 objects (barring special aerodynamic situations) fall at the same rate.
o
g. The rate of gravitational acceleration is labeled g, and is valued at 9.81
meters/second2. g is nearly the same all over the world, varying only slightly because of
differences in elevation.
•
Aerodynamics. Certain objects, because of their shapes and rotations in flight, are able to stay
airborne longer than other objects of comparable mass. This is because they, through the
principles of fluid dynamics, are buoyed by the flow of air around themselves. The javelin and
discus are two such objects. A complete discussion of aerodynamics is beyond the scope of this
text, but suffice it to say that proper rotations and release angles enhance aerodynamic
properties of these implements.
•
The Flight Path as a Parabola. The center of mass of any object projected into flight (barring
aerodynamic situations) 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. A parabola is a curve derived from cutting a cone,
and can be described by a mathematical equation.
•
•
Predetermined Flight Characteristics
o
Flight Path. The parabolic flight path of a body’s center of mass in flight is predetermined
and unalterable. Nothing can be done once a body is airborne to alter the established
flight path of its center of mass.
o
Flight Rotations. Any rotations of the body (desired or undesired) present in flight are
predetermined as well. Forces produce these rotations prior to flight, and the angular
momentum values of these rotations are unalterable in flight.
o
Coaching Practice. Obviously, all of the flight characteristics of an implement are
established during the throw. In a jump, all of the flight characteristics (distance, height,
rotation, etc.) are established during approach and takeoff. For this reason the majority of
coaching time should be devoted to movements that occur while still in support.
Body Positioning in Flight. The path of the center of mass in flight is predetermined, so the
location of the center of mass in flight is fixed at any instant. Yet, changing body positions can
change the relative location of the body’s center of mass. Therefore, in flight, the body can be
rearranged into different positions about that fixed location. Humans can use their ability to
manipulate body parts about the center of mass to achieve better landing and bar clearance
positions, yet nothing can be done to alter the flight path itself.
Special Considerations for Rotating Systems
•
•
Axes of Rotation
o
Rotation in Support. A rotation occurs about some line called an axis. If rotation is to be
stable and continued, the axis of rotation must pass though the body’s center of mass. If
this is not the case, the rotation deteriorates and movement is disrupted.
o
Rotation in Flight. When the body is experiencing flight and rotation simultaneously, the
axis of rotation always passes through the center of mass. This is true regardless of how
many planes the rotation may be occurring in.
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, all
points on a rotating body do not exhibit the same velocity. Velocity is measured as distance
covered per unit of time, and the points further away from the axis actually cover more distance
in a given period of time than points closer to the axis. Their curvilinear velocity is higher.
•
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 serve as a tool to absorb the rotation of the entire
body.
•
Rotational Accelerations.
Systems in rotation constantly exhibit tangential and axial
accelerations. These forces must remain in balance for rotation to continue in a stable manner.
•
o
Tangential Forces. All points in a rotating system experience a force acting in an outward
direction away from the axis tangential force. We will call this this force is a tangential
force, because any part of the system that is released into flight will travel in a straight
path that marks a tangent to its prior curved path.
o
Axial Forces. To maintain the stability of a rotation, a force must be applied toward the
axis of rotation. This force is called an axial force.
Angular Momentum
o
Angular Momentum. Rotating systems possess a certain amount of momentum. The two
factors that determine the amount of momentum a system possesses are the angular
inertia values of the system (determined by mass and the distribution of the mass with
respect to the axis) radius and the angular velocity (speed of rotation) of the system.
Since mass in athletic situations is constant, 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.
o
Conservation of Angular Momentum. 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. Thus, in athletic situations, 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. Changes in angular velocity will affect the radius of
the system. This law applies to rotating systems in flight or on the ground.
o
Coaching Practice. Coaching practice involves teaching athletes to use extended body
positions when slowing rotation is necessary, and using flexion of the limbs and shortened
body positions when rotations must be accelerated. Also, the observed lengthening or
shortening of effective body length is a useful diagnostic tool.
Stability
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Stability. A body’s stability is defined as its level of resistance to toppling over.
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Stability Determining Factors. There are two factors that affect the stability of an object.
o
Height of the Center of Mass. The first is the height of the object’s 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
Location of the Base of Support. The second factor that determines stability of an object
is the horizontal distance between the center of mass and the base of support. 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 be stable when the center of mass lies outside the base of support.
References and Suggested Readings
•
Dyson, Geoffrey H.G. (1977). The Mechanics of Athletics. Holmes and Meier: New York, New
York. (Applications of biomechanics to track and field).
•
Enoka, Roger (1990). Neuromechanical Basis of Kinesiology. Human Kinetics: Champaign, Illinois.
(Advanced neurophysiology and basic biomechanics)
•
Hay, James G. (1985). The Biomechanics of Sports Techniques. Prentice Hall: Englewood Cliffs,
New Jersey. (Basic biomechanics and their application to various sports, including track and field)
•
Hay, James G. & Reid, J. Gavin (1988). Anatomy, Mechanics, and Human Motion. Prentice Hall:
Englewood Cliffs, New Jersey. (Basic anatomy and biomechanics)
The Biomechanics of Athletic Movement. Sportverlag: Berlin
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Hochmuth, Gerhard (1984).
(Advanced biomechanics)
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Payne, Howard (1985). Athletes in Action. Pelham Books: London. (Technical analysis and
photosequences of all track and field events)
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