BIOMECHANICS
Biomechanics vs. Kinesiology
Kinesiology
• science of movement
• foundations of anatomy, physiology, mechanics
Biomechanics - BIO living
Mechanics – forces and effects
Foundations in physics, anatomy
Biomechanics – a science that examines the internal and external forces acting on the
body and the effects produced by those forces
Our concern Rigid Body Mechanics
• involves the study of the motion of rigid bodies and interrelationships among the
forces acting on these rigid bodies
“A solid body is considered to be a rigid body if the distance between the particles
remains fixes when a force is applied” – from Newton’s Laws of Motion
Fundamental Mechanics Terms
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force – weight
vectors – centre of mass
scales – pressure
mass – inertia
balance
Vector
• mathematical expression representing quantities of both
magnitude and direction
Scales
• mathematical expression representing magnitude only
.15g
Force
• physical quantity which causes (or tends to cause) a change in
motion or shape of a body – push/pull effect
• four characteristics of force
1. magnitude
2. point of application
3. direction
4. line of action
Mass vs Weight
• mass is the measure of quantity of matter
• weight on the other hand is a measure of force that is relative to
the push/pull effect of gravity
Mass = mg
Weight = mass X gravity (9.8m.sec)
Pressure
• the distribution of force over an area =
FORCE = N or N
------------AREA m2 cm2
1 pascal = N/m2 pascal = pa
eg. If a person weighs 400N and the contact for each foot = 100cm2, what
is the pressure exerted on the floor standing on two feet? Standing on one
foot?
Intertia
• resistance of a body or object to motion or a change in motion
• mass effects inertia – more mass, more inertia
• force must be strong enough to overcome this if there is to be
movement
Centre of Mass (Centre of Gravity)
• the centre of mass is the point on the body where everything is
balanced
• the centre of an object’s weight distribution is known as the centre of
mass
• for men approximately 57% of standing height
• for women approximately 56% of standing height
• the approximately height percentage depends upon the morphological
type of the person
• whenever a body of object is projected into the air, it will always rotate
about an axis that goes through the centre of the mass
• the pull of gravity on a mass always acts through the centre of mass
• the total effect of the force of gravity (9.8m/s) acts upon the objects
centre of mass
• by examining the way a body’s centre of mass changes, we can identity
the nature of forces acting upon that body
• when a body is rotating, the axis for rotation is always the centre of
mass
Movements of Force
• causes, or tends to cause, a rotation or turning
Movement of force – the force multiplied by the perpendicular distance
from an axis of rotation
For equilibrium: Where…
M = movement
w = weight
d = distance
Stability
Basic concepts:
Base of Support – more stable when area of base of support is greater
Centre of Mass – more stable when centre of mass is further from edge of
base of support
eg. More stable when line of gravity is within the base of support
Stability
Basic concepts:
• Base of Support – more stable when area of base of support is greater
• Centre of Mass – more stable when centre of mass is further from edge of
base of support
eg. More stable when line of gravity is within the base of support
Stable Balance requires:
• low centre of mass – wide base of support
• centre of mass within base – large mass
Want to be Stable
-football linesmen
-hockey defenseman
VS
Want to be Unstable
-quarterbacks (on balls of feet for quicker
movement
-sprinters (lift hands and topple forward)
Maximal Effort
• involves maximal force, acceleration/velocity
Force – push/pull causing, or tending to cause, motion
Acceleration – rate of change in velocity
- measured in meters per second squared (m/sec2)
Velocity – rate of change of place or position
- basically speed with a directional component
- measured in meters per second (m/sec)
Biomechanics of Sport
• human body is made up of a series of segments linked at joints
• eg. Arm: brachium
forearm
hand
Leg: thigh
calf/shank
foot
• muscles crossing each joint cause segments to rotate about an
axis
• if muscles forces of hip, knee and ankle are properly summed and
timed, then the maximal thrust of a ball kick, jump run, etc is
achieved
• if direction is correct, maximum speed, height or distance is
achieved
• if the force at one joint is not maximum, or is out of sequence,
maximum performance is compromised because “Principle of
Continuity of Joint Forces” rules have been violated
Levers
• every lever requires a centre of rotation (axis, fulcrum, turning
point) a force and a resistive load
1st Class Levers (very versatile)
• occurs when axis is between the force and the resistance
•
crowbar, paddling, teeter totter, hammer removing nail
2nd Class Levers (advantage to user in force)
• resistive load found between the axis and the force
•
nutcracker, wheel barrow, rowing, deltoid
3rd Class Levers
• force between resistance and axis
•
shovelling, fishing (with stomach holster)
• most implements in sporting events (batting)
* most muscoskeletal actions are 3rd class eg. adduction of pectoral girdle
Human Motion
Newton’s Laws of Motion that govern human movement
Newton’s first Law of Motion (Law of Inertia)
• “every body will remain in a state of constant motion or rest unless acted
upon by an outside force”
• eg. if no forces are acting upon a skater, he/she would glide forever
• forces preventing that are air resistance, friction, gravity
Newton’s second Law of Motion (Law of Acceleration)
• states that force is proportional to acceleration, providing that mass is
constant
• acceleration is directly proportional to force and inversely proportional to
mass of object
Newton’s third Law of Motion (Law of Action/Reaction)
• states “for every action force, there is an equal and opposite reaction force”
• eg. direction of applied force should be opposite direction of motion
Skills of Maximum Force
• those skills that involve slower movements at high intensity
• several muscles contract simultaneously to exert maximum force
•
eg. squat in weight lifting, slapshot
Skills of Maximum Velocity
• performed sequentially
• larger joints first, followed by smaller joints
• faster joints (smaller) make contribution when others have
reached peak speed
•
eg. baseball throw, batting
Combination/Intermediate Skills
• combination of both force and velocity
• include overhead pattern skills
•
eg. spike, tennis serve, javelin
• important body movements – sideway trunk rotation, maximum
lateral rotation at shoulder before medial rotation
Linear Motion
• movement of object in a straight line or along a straight pathway
Concepts
Momentum – amount of motion an object/athlete has developed
- linear momentum depends on mass and velocity
- amount calculated by m X v
Impulse – application of force over time to change amount of momentum
- calculated as F x length of time applied
- the greater the applied impulse, the greater the
increase in velocity
Joint R.O.M – amount of movement at each joint
- i.e. whether knee moved at 10 R.O.M. or 90
Streamlining – object or athlete being as smooth as possible to allow water/air to
flow over
-minimize surface area
Types/States of Motion
STATES
1. Stationary Motion
• stable motion
• body in balance, no significant movement
2. Linear Motion
• movement in straight line or path – bobsled down a track
• usually measured in how far/how fast
3. Angular Motion
• movement where body or body part undergoes
rotation/circular
• eg. Elvis Stoiko’s Quad – Triple feat
TYPES
1. Angular/Rotary
• object turning about an axis
• not through centre of gravity, eg. hold a book corner
down and spin it
2. Translatory
• whole object moved from one place to anouther
• can be Rectilinear – weight of body with all parts moving
in the same direction, same distance, same speed, eg.
somersault
• OR
• can be Curvilinear – object travelling about a curve NOT
circular path called parabolic curve, eg. javelin throw
Angular Motion
• always occurs around imaginary “axis of rotation”
• when somersault axis is in contact with the ground and
therefore there is a successive number of axis of rotation
• an airborne body’s axis of rotation is always the centre of gravity
• all balls/throwing implements have axis of rotation
Angular Velocity
• refers to velocity of athlete, body segment, object in angular
motion
• measured in degrees/second, eg 360/sec
Off Centre Forces
• pass through centre of gravity at an angle
• cause torque, ie. wrestler prying on the shoulders of apponent
Projectile Motion
• distance travelled is dependant on velocity of object at release,
height of release and opposing forces
• two of them are : air/wind resistance, :gravity
• optimal angle for release is 45 degrees
•
as air resistance increases the angle is decreased in order to increase the horizontal
component
Magnus Effect
• air travelling around flight of body
ANGULAR MOTION overhead displaying “Magnus Effect”
Velocity
• rate of change in distance over a period of time
V = d/t (m/sec)
Acceleration
• rate of change in velocity
a = change in velocity
change in time
F = final
I = initial
= Vf – Vi
tf – ti
• if a = 0 → constant velocity
a = + (pos) → increasing speed
a = - (neg) → decreasing speed
Momentum
• linear motion
Principles of Linear Momentum
• momentum is conserved unless acted upon by external
forces
• predicts outputs of input situations
• total momentum before collision = total momentum after
collision
momentum = mass X velocity (kg*m/sec)
Impulse
• creation of an impulse creates momentum
• impulse → change of momentum
•
I = Ft
-increase impulse by increasing force or time (theoretically)
good jumpers however, increase impulse with an increase in force and decrease in
time due to the elasticity of the muscles
I = change in momentum
Ft = Mf – Mi
Ft = Mvf – Mvi
F = Mvf – Mvi
--------------t
***important because: large forces need to be absorbed over time during impact or
landing to prevent injuries skills such as: jumping and catching
increase R.O.M., increase time
Force problems and solutions overhead
Problems
• 1. If two wrestlers collide, weights 90kg and
100kg, that are moving from opposite
directions @15m/s and 9m/s respectively,
what is the total momentum before and after
the collision? Which direction would the two
wrestlers move if they remained in contact? At
what final speed?
• 2. A baseball player is running home at a
velocity of 7.5m/s, he slides to rest 0.8
seconds later. What is the rate of change in
velocity (- acceleration)