Download Introduction to Biomechanics

Biomechanics of Human Motion
Introduction to Biomechanics
Background for the study of
Biomechanics
As a new discipline but early root back to
many centuries
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Aristotle
Archimedes
Leonardo De Vinci
Alfonso Borelli
Eadweard Muybridge: to settle of hour runs with
four legs off the group with pictures (fig.)
Eadweard Muybridge’s studies
The Analysis of Human Motion
Biomechanics is the
science of physical
principles applied to
biology systems
– Kinematics
A description of the
temporal and spatial
components of movement
– Kinetics
A study of the forces
acting on an object
Kinematic of Human Movement
Motion refers to an object changing its position in
space over a period of time
In most of activities, success depends on getting an
object to travel to a certain location as quickly as
possible
Speed and Velocity
Speed refers to how fast an object travels
Velocity refers to how fast an object moves in
a particular direction
Speed = d / t; where d=distance, t=time
Acceleration occurs when an object changes
its velocity over a particular time interval
Acceleration= Vf -Vi / Δt ; read as acceleration
change in velocity over a change in time
Newton’s Law of Motion
are three physical laws that form the basis for
classical mechanics. They describe the
relationship between the forces acting on a
body and its motion due to those forces.
were first compiled by Sir Isaac Newton in his
work Philosophiæ Naturalis Principia
Mathematica, first published on July 5, 1687.
Newton used them to explain and investigate
the motion of many physical objects and
systems.
Law of inertia: every object in a state of
uniform motion tends to remain in that state of
motion unless an external force is applied to it
Law of acceleration: the relationship between
an object's mass m, its acceleration a, and the
applied force F is F = ma. Acceleration and
force are vectors; in this law the direction of
the force vector is the same as the direction of
the acceleration vector.
Law of Action-Reaction: For every action there
is an equal and opposite reaction
Relationship of Force and Motion
Using gravity, ground reaction forces, muscle
forces, friction and other forces, will clarify
the concept of force and show how it is related
to the change in motion of an object
The impulse-Momentum relationship
Impulse = Ft = m(Vf -Vi ); read the product of
force and the duration of time that the force is
applied
Momentum = mv ; represent a force applied to
an object over a known amount of time causes a
change in the momentum of that object
The motion of a body represented by its
momentum, is changed by the impulse applied to
the body, or force applied over time; Ft= Δmv
Gravity
Is a force that created as the attractive pull
between any two objects
The size of the pull is affected by both the
mass of the two objects and the distance
between them
Center of gravity: the weigh concentrated at
single point
The Fosbury Flop is a style used in the
athletics event of high jump. It was
popularized and perfected by American athlete
Dick Fosbury, whose gold medal in the 1968
Summer Olympics brought it to the world's
attention. Over the next few years the flop
became the dominant style of the event and
remains so today
Contact Force
When two bodies physically touch each other,
contact forces, contact force are created.
– Ground reaction force (GRF)
The force exerted on the performer by the ground
GRF affects the vertical, anteroposterior, and
mediolateral motion of the object
Important consideration in injury prevention
– Demonstrating ground reaction force
Measuring Ground Reaction Force
(GRF)
An instrument called a
force platform is used to
measure the magnitude
and direction of the
ground reaction
Typically this interfaced
with a computer to
record the GRF and
facilitate analysis
Friction
is the force resisting the relative motion of solid
surfaces, fluid layers, and/or material elements sliding
against each other. There are several types of friction:
– Dry friction resists relative lateral motion of two solid
surfaces in contact. Dry friction is subdivided into static
friction between non-moving surfaces, and kinetic friction
between moving surfaces.
– Fluid friction describes the friction between layers within a
viscous fluid that are moving relative to each other.
– Lubricated friction is a case of fluid friction where a fluid
separates two solid surfaces.
– Skin friction is a component of drag, the force resisting the
motion of a solid body through a fluid.
– Internal friction is the force resisting motion between the
elements making up a solid material while it undergoes
deformation
Friction
The relationship among
maximum limiting friction,
the nature of the surfaces in
contact, and the normal
force
Friction ≦ μN
Where
– μ (Greek letter “mu”);
coefficient of friction
– N; normal force
Friction
Propelling force
Contact Surface as a Friction Element
Shoes’ sole match the
conflicting needs for grip
(increased friction) and
slide (decreased friction)
The different patterns and
materials used on this part
of the sole
– Rotation of a pivot
Hard and smooth in the ball
and metatarsal area
– Grip (turn, push off, stop)
Soft and ripple in the lateral
side
For a classical ski that kick wax will be applied to
the grip zone.
No kick wax in the glide zones and no glide wax in
the grip zone
The high-friction wax increased friction is
necessary to develop a forward propelling push
during the kick
Low-friction wax is applied at both tips of the ski
camber
KICK ZONE
Fluid Force
Refers to the forces imposed on an object
when it moves through a fluid such as air of
water
Is created as the object disrupts the fluid when
passing through it
The greater the disruption of the fluid the
greater the fluid forces developed
How much the fluid gets disrupted depends on
factors related to both the fluid and the object
Fluid factors
– Influence the size of the force exerted on an object
moving through it include
Density; the distribution of mass throughout a volume
of space
Viscosity; a fluid’s resistance to flow
– The more dense or more viscous a fluid is, the
more it is disturbed as an object passes though it
Fluid Resistance
Body factors
– Aerodynamic and hydrodynamic are used to
describe the features of the object that affect the
size and direction of the fluid resistance
Cross-section Area
– A larger cross-sectional area increases the size of
the fluid resistance force
Nature of the Surface
– A smooth surface produces less disruption in the
fluid as it flows over the body and reduces
resistance, and roughened surface vice versa
Velocity
– The faster velocity, the greater fluid resistance
– This factor related to fluid resistance magnitude is very
critical in the performance of many skills
– Relative motion of the body and the fluid
– With a tail wind, the relative motion of the air over the ball
is reduced because the wind is pushing the air forward
The force on a moving object due to a fluid is:
Biomechanics of Muscle Force
Muscle Force-Velocity
Muscles produce more force at some velocity than at
others
The speed at which a muscle changes length (usually
regulated by external forces, such as load or other
muscles) also affects the force it can generate.
Force declines in a hyperbolic fashion relative to the
isometric force as the shortening velocity increases,
eventually reaching zero at some maximum velocity.
The reverse holds true for when the muscle is
stretched – force increases above isometric maximum,
until finally reaching an absolute maximum.
Muscle contraction occurs when individual myosin heads attach themselves to the actin filament,
drawing the actin and myosin filaments in opposite directions past one another
(Sliding Filament Theory)
This has strong implications for the rate at
which muscles can perform mechanical work
(power). Since power is equal to force times
velocity, the muscle generates no power at
either isometric force (due to zero velocity) or
maximal velocity (due to zero force). Instead,
the optimal shortening velocity for power
generation is approximately one-third of
maximum shortening velocity
Force–velocity
relationship: right of the
vertical axis concentric
contractions (the muscle
is shortening), left of the
axis eccentric
contractions (the muscle
is lengthened under
load); power developed
by the muscle in red
Muscle produces the
greatest force when there is
optimal overlap thick and
thin filament in the
sarcomere (b and c)
No muscle force is produced
at very long (a) or very
short (e) muscle lengths.
Less force is produced at
shorter and longer sacomere
lengths (d)
Muscle length versus
isometric force during
active and passive
isometric contraction
Who Use Biomechanics
Sport scientists
– Improving wind resistance in skiing, cycling, and sailing
Physical therapists
– Analyze gait defects
– Muscle function
Ergonomist
– Assembly line injury
Rehabilitation
– Design new prosthetic devices
– Artificial hearts
Summary Points
Discussion
What’s the simplistic way to explain baseball
pitcher’s kinetics and kinematics.
How could you devise a test to quantify long
distance runners’ running economy (movement
efficiency) that has been considered as such an
important factor in athletic performance.
How you scheme to apply a force plate to
investigate the effects of balance (equilibrium)
on athletic sport.
How you explain the pitcher’s forkball
underlying Newton law of motion.