# College of Nursing and Health Sciences at umass by sanmelody

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```									Biomechanics
Biomechanics -- Defined
   Bio - life; living organism

   Mechanics - the branch of physics concerned with the analysis
of the action of forces on matter or material systems

   Biomechanics – the study of forces and their effects on living
systems

   Exercise & Sport Biomechanics – the study of forces
and their effects on humans in exercise and sport

   Applied or “Functional” Biomechanics – (the focus of
this class); the examination of the application of biomechanics in
the exercise and sports field
Human Biomechanics
   Applications of biomechanics (human biomechanics)
 Purpose of the science – understand, protect and enhance
human function
 Role in sport – ultimately, to improve performance

 Role in therapy – rehabilitate, re-educate

 Role in product design – to design products that

optimally support human function
 Role in injury prevention – to minimize adverse stress and
strain on the body through movement analysis, technique design
and product development
 Role in the workplace – Ergonomics - to maximize
productivity by minimizing worker fatigue and discomfort
 Who uses biomechanics?
Mechanics - analysis of the action of
forces on matter or material systems

Mechanics

Deformable       Fluid
Rigid Body                                 Relativistic   Quantum
Body            Mechanics
Mechanics                                  Mechanics      Mechanics
Mechanics

Rigid Body – objects are assumed to be perfectly rigid
Deformable Body – objects can be deformed by a force
Fluid – Gas or fluid
Humans – Rigid or Deformable?
   Biological tissue, including the human body, is by
nature, deformable. It can absorb forces, it can stretch,
bend, compress.
   With regards to gross human movement, these
deformations are relatively small, and for the sake of
simplicity, Applied or Functional Biomechanics largely
ignores these properties.
   Each segment of the body is considered a rigid body
   In reality, repeated plastic deformation of biological tissue
will result in injury.
Stress – Strain Curve
   Import curve here or prolonged stress at
Repetitive
this strain % will eventually
result in microdamage
(i.e. stress fracture)
Bone Stress-Strain Curve

Bone is relatively rigid – note the rapid strain

Boney body segments determine human rigidity in biomechanical terms
Branches of Rigid Body Mechanics
Rigid Body
Mechanics

Statics                           Dynamics

Statics – mechanics of objects          Kinematics                 Kinetics
at rest, or at constant velocity
Dynamics – mechanics of objects in accelerated motion
Kinematics – describes the motion of a body without regard to the forces or
torques that may produce the motion
Kinetics – describes the effect of forces on the body; i.e.. muscular force,
gravitational force, external resistance force, ground reaction force, etc.
Basic Dimensions and Units of Measurement
Used in Mechanics & Biomechanics
   Biomechanics is a quantifiable science,
measurable, and can be expressed in numbers
   Systeme-Internationale d’Unites (SI Units)
• Length – measured in meters (m)
• Time – measured in seconds (s)
• Mass – measured in kilograms (kg), the
measure of inertia, or resistance to a change in
motion of an object
Mass vs. Weight
Mass is the measure of inertia, whereas Weight is
the measure of the force of gravity acting on an
object.
Additional Dimensions & Units of Measure

   Length – millimeter (mm), centimeter (cm), kilometer
(km), etc. are all based on the meter (m)

   Time – Minutes, hours, days, weeks, months, years, etc.
can all be derived from the second (s)

   Mass – milligram (mg), gram (g), etc. are all based on
the kilogram (kg)
Forces & Torques
   Force – a push or pull; exerted by one object on
another; come in pairs (Newton’s 3rd Law); creates
acceleration or deformation (Newton’s 2nd Law); causes
an object to start, stop, change direction, speed up or
slow down (Newton’s 1st Law)
   SI Unit of Force is the Newton (N) = force required to
accelerate a 1 kg mass 1/m/s/s
   Force is described by its size (magnitude) and direction
   The angular equivalent of F is Torque (T); a Torque
rotates an object about an A of R
   T = F x moment arm
   Resultant Force – the summation of all forces acting on
a body; determines the direction of the body
Forces (cont.)
   Internal Forces and Torques – forces or torques that
act within the studied object; i.e. the human body, or
the object being manipulated by the human; pole vault,
soccer ball, etc. Internal forces can cause movement of
body segments at a joint but cannot produce a change
in the motion of a body’s C of M. Muscular force is the
primary internal force examined in biomechanics. As
the overwhelming majority of motion in the human
body is angular, torque forces are more applicable in
biomechanics.
(The terms Force and Torque will be used interchangeably
throughout this course. Essentially, if the term “Force” is used
to describe angular motion, "Torque” is implied.)
Forces (cont.)
   External Forces – forces that act on an object as a result
of its interaction with the environment surrounding it
• Most External Forces are contact forces, requiring
interaction w/ another object, body or fluid
• Some External Forces are non-contact forces; including
gravitational, magnetic and electrical forces
• The science of biomechanics largely deals with contact
forces and gravity (weight), which accelerates objects at
9.8 m/s
• Contact forces can be sub-divided into normal reaction
force and friction
Contact Forces
Normal Reaction Force –
line of action of the force is
perpendicular to the surfaces in
contact

Friction Force – line of action
of the force is parallel to the
surfaces in contact
Reaction & Friction Forces
Newton’s Laws of Motion
   Newton’s Laws help to explain the relationship
between forces and their impact on individual
joints, as well as on total body motion.
   Knowledge of these concepts can help one
understand athletic movement, improve athletic
function, understand mechanisms of injury, treat
and prevent injury
Newton’s Laws (cont.)
   Newton’s 1st Law – Law of Inertia
• A body remains at rest or in motion except
when compelled by an external force to
change its state. A force is required to start,
stop, or alter motion
• Inertia – the tendency of a body to remain at
rest or resist a change in velocity
• Inertia is directly proportional to its mass
• The angular equivalent is Mass Moment of
Inertia
Mass Moment of Inertia
   Mass Moment of Inertia (I)– The resistance to
change in a body’s angular velocity
   Dependent on both the objects mass and on the
distribution of mass about it’s axis of rotation
   Radius of Gyration – the average distance between
the A of R and the C of M of a body (p)
   I = mass of the object multiplied by the square
of the R of G
• I = m x p2
Law of Inertia – Biomechanical Application
   How can an athlete control their
Mass Moment of Inertia? In other
words how can they manipulate
the resistance to change in angular
velocity to attain a goal?
Newton’s Laws (cont.)
   Newton’s 2nd Law – Law of Acceleration
• The acceleration of a body is directly proportional to
the F causing it, takes place in the same direction in
which the F acts, and is inversely proportional to the
mass of the body
• A = velocity / time
• F = ma (Force = mass x acceleration) (linear)
• Angular equivalent of F is Torque (T)
• T = F x moment arm (rotational force applied to the A
of R, through a moment arm)
• T has the same relationship with direction and mass
moment of inertia as F has with direction and mass
 As I (moment of Inertia) increases (due to increased
R of G or increased mass), Acceleration decreases
Newton’s Laws (cont.)
   Newton’s 2nd (cont.)
• Impulse-Momentum Relationship – from F=ma, we can derive
Momentum (p) and Impulse
• Impulse = Force x time (Ft)
• Momentum = mass x velocity (mv)
• Ft = mv (impulse = momentum)
• If Ft increases, mv increases
• Mass is considered constant
within biomechanics, therefore,
an increase in impulse implies an
increase in velocity

• How are the principles of
Impulse and Momentum
used in the design of sports
equipment?
Newton’s 2nd (cont.) Impulse-Momentum
   Because Mass is constant, and because external forces are largely
non-modifiable, in the world of sports and exercise, the duration
of force application is the most modifiable
   If the Force is not constant, impulse is the avg. force times the
duration of that average force
   Essentially, calculating force as average force holds that force as
a constant, however it is the peak force that we need to minimize
   If the application of Force is prolonged (increased time), in order
to maintain the same magnitude of impulse (Ft), the Force
magnitude (average and peak) must be lowered
   Conversely, if the application
of Force happens more rapidly
(decreased time), there will be a
higher Force (avg. & peak) in
order to maintain impulse
Newton’s 2nd (cont.) Impulse-Momentum
Newton’s 2nd Law (cont.)
Impulse-Momentum
Newton’s 2nd Law (cont.)
Impulse-Momentum
Newton’s Laws (cont.)
   Newton’s 2nd Law (cont.)
• Work-Energy Relationship -- from F=ma,
we can also derive Work (W)
• Work = Force x Distance
(W = FD) (linear)
• Angular equivalent = Torque x Angular displacement (T
x degrees)
• Measured in Newton meters (Nm)
Work is a measure of strength,
measured by the extent to
which a force moves a body over
a distance without regard to time
Newton’s Laws (cont.)
   Newton’s 2nd (cont.)
• Power (P) – the rate of work; W/time
• W/t = F x D/t or F x Velocity (W=FV)

• Training power in an athlete requires doing
work quickly, or explosively

• How is Power measured and trained in
sport and exercise?
Measuring and Training Power in the Athlete
Power in Sport
Newton’s Laws (cont.)
   Newton’s 3rd Law – Law of Action-Reaction
   For every action, there is an equal and opposite
reaction
   The two bodies react
simultaneously, according
to F=ma ; each body
experiences a different
acceleration effect which
is dependent on its mass
References
   Neumann, D.A. (2002). Kinesiology of the
Musculoskeletal System. St. Louis, Missouri. Mosby.
   McGinnis, P.M. (2005). Biomechanics of Sport and
Exercise 2nd ed. Champaign, IL. Human Kinetics.

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