Download 1 - Introduction Biomechanics Sport – technique development, injury

-1Introduction
Biomechanics
Sport – technique development, injury reduction
Clinical – rehabilitation, gait
Occupational – ergonomics, kinanthropometry (growth)
Learning path for skill – tactical ! technical ! physical ! mental.
Technique – past experience as player or coach, current world trends, the individual flair
of the athlete. An understanding of the mechanics of the movement.
Kinematics – Motion characteristics (describes the motion) – examines motion from
spatial and temporal perspective without reference to the forces causing the motion.
Position, velocity and acceleration. Describes the spatial and temporal components of
motion. Description involves position, velocity, and acceleration of a body with no
concern for the forces causing the motion – quantitative or qualitative analysis.
Kinetics – Examines the forces acting on a system – the causes/forces involved in the
motion (linear + angular). Defines forces causing movement.
- Position and movement are described using a Cartesian system – a coordinate system of
x, y, z.
- The planes of motion are the ‘camera’ angle. Ie sagittal looks from the left (sees L + R).
Linear kinematics
- description of motion in a straight line
- When doing calculations – create data board 1st
Linear – movement refers to a point
Angular – movement refers to segment/implement.
Displacement (r/s) =
Velocity (v) = s/t m/s – with direction
- Instantaneous velocity – E.g. V @ foot strike as apposed to average for one cycle. The
horizontal velocity decreases immediately at foot contact, velocity increases as leg
extends.
Acceleration (linear = a) = a = (v2-v1)/t ms(-2)
- Time rate of change of velocity (Horizontal component).
Relationship b/w linear displacement, velocity and acceleration
- When a = 0, velocity is at peak/trough, and s is at max gradient.
Newton’s law (3 – Vertical component) – a = g = gravity = (-)9.8ms(-2)
- Vf = Vi + at
- Vf2 = Vi2 + 2as
- s = (Vi x t) + (0.5)at2
Vectors – magnitude (size), direction, sense (orientation).
Trigonometry
SOH CAH TOA
! Hypotenuse is the longest side
-2- Sine ! = opp C/ Hyp A (vertical)
- Tangent ! = opp C / Adj B
- Cosine ! = adj B/Hyp A (horizontal)
Linear
m
v (s/t)
a
Angular
!
" (!/t)
x
Angular motion – occurs when all parts of a body move through the same angle but do
not undergo the same linear displacement.
Angular kinematics – describes angular motion without regard to causes of the motion.
Degree – portion of a 360’ circle
Revolution – the number of 360’ rotations
Radian – the measure of a single angle at the centre of a circle described by an arc equal
to the length of the radius of the circle.
One radian is equivalent of 57.3’. To convert angle from degrees ! radians. Divide the
angle by 57.3. (To convert radians ! degrees, multiply angle by 57.3).
- Angular measurement sin radians are often determined in multiples of pi. There are 2#
radians in a complete circle, 180’ may be represented as # radians, 90’ as #/2 radians etc.
! ! = s/r = 1 radian. (s = arc of length r, r = radius of circle).
Angular displacement – Difference b/w the initial and final positions of the rotating
object. Counterclockwise rotation is +ve, clockwise rotation is –ve.
Angular velocity – ($ – omega). = % ! /%t – shown as degrees per second (‘/s), or radians
per second (rad/s) preferably.
- Linear velocity = Angular velocity x radius of rotation
! use radians not degrees (! degrees/57.3)
Instantaneous angular velocity – The slope of a tangent to an angular position-time graph
and calculated with a limit.
! limit $ = d !/dt
- Angular velocity is therefore the 1st derivative of angular position.
- 1st central difference method – calculates the angular velocity at the same instant at
which data for angular position are available
! $i = (!i + 1 – !i – 1) / ((ti + 1 – ti – 1)
Angular acceleration – rate of change of angular velocity with respect to time – Alpha &.
' = %$ / %t. Represented in degrees per second squared, or preferably (rad/s2).
Instantaneous angular acceleration = limit & = d$/dt
- &i = ($i + 1 – $i – 1) / ((ti + 1 – ti – 1)
- The sign/polarity of angular acceleration does not indicate the direction of rotation
(inc/dec in positive or negative direction).
Analysis of movement
Subjective analysis (spectator) – preparation ! observation ! evaluation/diagnosis !
intervention
-3- Pre-analysis – skill level, aspirations, general technique(s) used, consistency of
performance, age of group.
VARIOUS MODELS IN LECTURE.
e.g. tennis serve –
Phasic assessment - preliminary movements ! backswing ! forward-swing
(impact/release) ! follow-through.
Body area – head and trunk (line of sight, shoulder and pelvic alignment rotations,
position of head to COG), shoulder (position of limbs to trunk, grip, elbow angle at
impact/release). Lower limbs (knee flexion, alignment of feet, alignment of hips).
OUTCOMES (i.e exam stuff) – constructing a mechanical model for a performance of
your choice and selecting 2 mechanical variables that are critical to successful
performance. Provide a range of acceptability values for each variable. Explain how
objective analysis would be of assistance in sport of choice.
Rowing – as an outcome model
Length of stroke –
- Length of trunk (posture), upper and lower limbs.
- Oar/boat rigging
- Flexibility
Stroke rate –
- Speed on drive – oar/boat rigging, length, muscle power, boat speed. Components of
drive. Wind/drag/current.
- Speed on recovery – hands, arms, trunk, legs forward speed – sequencing order.
Objective analysis – force/pressure results, electromyography (sequencing of muscle),
angles of body segments/boat implements (oar).
Projectile motion
! Velocity at take-off, angle at take-off, height of release compared to landing.
- V cos ! (horizontal)
- V sine ! (vertical)
General principles – throwing + hitting
- Release height = landing height ! = 45’
- Release height > landing height ! < 45’
- Release height < Landing height ! > 45’
- Long jump ~20’ (lower than expected)
! Relationship b/w take off angle and take off speed.
The science of motion capture
Motion capture – the recording of motion for immediate or delayed analysis and
playback.
- Sports biomechanists, clinical biomechanists (cerebral palsy, ACL injuries,
osteoarthritis, amputees). ! Pre- + post-surgery analysis.
-42D limitations – human movement occurs in 3 planes – should not make decisions about
motion from one view.
VICON motion analysis system – 2 or more cameras, 3 markers per segment.
Kinetics – Newton’s laws of motion
Laws of motion
1. Law of Inertia – “Every body continues in its state of rest, or uniform motion in a
straight line, unless it is compelled to change that state by forces impressed on it”.
- Internal or external forces
Inertia – describes resistance to motion and is related to mass of object. To overcome the
inertia of objects, requires a net external force greater than the inertia of the object – if
force is greater, will either positively or negatively accelerate.
2. Law of acceleration – “The change of motion is proportional to the force impressed
and is made in the direction of the straight line in which that force is impressed. F = ma
! F = m x (v/t).
- Energy creation and energy absorption
- Injury – the time rate of momentum is proportional to the applie4d force and takes place
in the line of action of the force.
- Change in time of impact – different peak forces, impulse is the same
Momentum – quantity of motion of an object = product of mass and velocity.
= p = mass x velocity = kgm/s
!F = dp/dt (force is equal to the time rate of change of momentum).
3. Law of action-reaction – “To every action there is always opposed an equal reaction:
or, the mutual actions of 2 bodies upon each other are always equal and directed to
contrary parts”. Forces always occur in pairs, never in isolation – equal in magnitude,
opposite in direction.
- How to optimise performance, and how to minimise potential for injury.
Summation of velocity
Velocity tasks – Max number of segments. Segments occur in sequence (when the
velocity at one segment is at max velocity ! recruit next segment to add)
Accuracy tasks – Minimum number of segments. All segments moving as one unit.
Lecture 9 – Speed/power development in sport
- Increase use of elastic energy
- Increased distance of swing – more time to accelerate implement.
- Increased number of segments in stroke
- Increase linking of linear and angular motion
- Use appropriate equipment
- Muscle strength/power
Elastic energy/Pre-stretch – Ability to store elastic energy and prepare muscle for
concentric contraction is affected by: Pause time (minimal), magnitude of stretch (until
tension), and speed of stretch (relatively fast). Can store energy in muscle and tissue
when eccentrically contracted. Get more muscle force (~20%) when loaded eccentrically.
Takes 4s to lose all energy benefit.
-5Separation angle – angle b/w to body segments – creates stretch (elastic energy) in
muscle groups and increases distance.
Muscle strength/power – needs to be specific.
- Angular motions of joints to create linear motion of a segment.
Flattening the arch of the swing.
Linear kinetics
Momentum – quantity of motion of an object = product of mass and velocity.
= p = mass x velocity = kgm/s
Force applied over a time period –
- F x dt = d(m x v)
- F x dt = mvfinal – mvinitial
! Equation derived from F = M (vf-vi)/t
Impulse = product of F x dt (Ns – Newton seconds) – measure of what is required to
change the motion of an object. Can be represented graphically under a force-time curve.
- The RHS of the equation describes the change in momentum.
!Impulse-momentum relationship – when a force is applied to an object over time, the
momentum of the object changes.
P = F/A
Energy – in movement: store and return energy
- Anything capable of doing work possesses energy.
Sports equipment/surfaces – return energy at right time and location. Minimise loss of
energy. Reduce friction, drag and vibrations.
Work – equal to the product of the magnitude of a force applied against an object and the
distance the object moves in the direction of the force while the force is applied to the
object. Work is only done when the object is moving and its motion is influenced by the
applied force.
- W = F x (cos!) x s.
- cos! – the angle b/w the force vector and the line of displacement.
- More work is done if the force is applied parallel to the direction of motion, than if the
force is applied at an angle.
! 1Nm = 1J
Energy – Capacity to do work. Mechanical – joule. 2 forms – kinetic + potential.
Kinetic energy – energy resulting from motion. Object possesses KE when it is in motion
(has velocity).
KE = ( mv2
Potential energy – The capacity to do work because of position or form.
PE = mgh
- A deformed object may have PE – elastic forces. The resitance to deformation increases
as the object is stretched = Strain energy. Elastic energy is PE due to form of object.
Stress – force applied per unit area = Pressure
Strain – deformation
- Area under stress-strain = bones ability to absorb energy
Conservation of momentum – (Newton’s 1st law) – If 2 bodies (or more) interact, the
momentum after the impact is equal to the momentum before the impact.
-6Types of collisions –
Perfectly elastic – Kinetic energy is perfectly maintained – no energy is stored (billiard)
Perfectly inelastic – Some potential energy stored. Loos in velocity is gained in mass
Imperfectly elastic – Some potential elastic energy is stored – objects have a coefficient
of restitution = e = )(Bounce height/drop height)
Spin on ball – changes angle of ground force vector.
Tissue mechanics
Irritability – respond to stimulation – motor neuron (chemical neurotransmitter).
Contractility – ability to shorten.
Extensibility – ability to lengthen beyond resting length.
Elasticity – Ability to return to resting length once stretch is removed. Determined by the
CT in the muscle rather than the fibres.
- Ability to support loads – absorption ie compressive and tensile forces.
- Musculotendinous units increase the load bearing capacity of the skeleton.
- Skeletal muscle is one of the most adaptable tissues of the human body
Adaptation to stretch – Synthesis of new sarcomeres, maintenance of sarcomere length,
changes in length-tension relationship
Tension and irritability – contractile component
Elasticity and extensibility – Parallel elastic component. Series elastic component.
Fibre organisations – arrangement determines whether for force or length of movement.
Fusiform – Parallel muscle fibres and fascicles. Muscle pull force in same direction as
musculature. Large amounts of shortening + high velocity movements. Shortening
distance is dependent on muscle:tendon ratio. Larger ROM.
Penniform – Fibres run diagonally with respect to a central tendon running the length of
the muscle. Force generated by each fibre is in a different direction to the muscle force
direction. Create slower movements through a smaller range of motion – can produce
more strength.
Tension and irritability – Contractile components (cross-bridges)
Elasticity and extensibility – Protective mechanism to prevent over-contraction/stretch of
muscle.
Parallel elastic component – Prevents over-stretch. The elastic CT around myosin get
closer together when muscle is stretched. The squashing of the CT prevents further
stretching of the muscle. Role when muscle is longest.
Series elastic component – Prevents over contraction. When the muscle contracts,
stretches the tendon, creating a restricting form of tension. Role when muscle is shortest.
Isometric – Muscle is active, developing tension without any visible or external change in
joint position. F = L.
Concentric – Muscle visibly shortens while generating tension actively. Generates the
lowest force out of contraction types. F>L
-7Eccentric – Muscle is subjected to an external torque greater than the torque generated by
the muscle – the muscle lengthens. Greatest force output in eccentric contraction –
sarcomeres are stimulated under stretch. F<L.
Length-tension relationship – Max when muscle is activated at length slightly longer than
resting length (contractile components are optimally producing tension, and the passive
components are storing elastic energy and adding to the total tension in the unit).
Filament overlapping. The tension-developing characteristics of active components of
muscle fibres diminish with elongation – tension in total muscle increases because of the
contribution of the passive elements in the muscle.
- Long fibres (small cross sectional area). Long ROM, smaller force
- short fibres – large CSA
- Spastic muscle – overactive muscle, difficult to relax (need to stretch to relax. Less
control of irritability.
Force-velocity relationship – Muscles create an active force to match the load in
shortening, and the active force continually adjusts to the speed at which the contractile
system moves. When the load is low, the active force is adjusted by increasing the speed
of contraction, with greater loads, muscle reduces speed of contraction.
Concentric – Force production decreases as velocity increases (less time for cross-bridges
to hold and exert effect). Muscle is weaker concentrically than eccentrically.
Eccentric – force production increases as velocity increases. PEC elastic slows down the
lengthening.
Power – product of force and velocity. P =F x V
Strengthening muscle
Training for strength – increasing CSA (Cross section area) ! hypertrophy
Strength – maximum amount of forced produced by a muscle/group at site of attachment
on the skeleton. Mechanically – strength = maximum isometric torque at a specific angle.
Variables influencing strength – Muscle action/modality (eccentric, concentric,
isometric), speed of limb movement. Length-tension, force-angle, force-time
characteristics – strength varies throughout the range of motion.
Hypertrophy – greater cross-section – increase in size of muscle fibre + more capillaries.
Increase in tension per unit reflet neural influence on strength development.
Genetic predisposition – distribution of fibre type, body type, musculoskeletal
anthropometrics.
Training specificity – training principle – same movements in sport/activity. Establishes
neural coordination. Muscle is trained at the speed it is strengthened at. SAID (specific
adaptation to be improved demands).
- Once strength base is established, power is obtained with high-intensity loads, low reps.
Intensity – strength gains related to tension produced on muscle. Muscle must be
overloaded to a particular threshold before it will respond + adapt to the training. It is the
amount of tension in muscle rather than repetitions for strength stimulation. Maximum
strength gain achieved when muscle is worked near its max tension before fatigue state.
Rest – As skeletal muscle fatigues, tension-development capability deteriorates – muscle
does not operate at optimal overload.
Volume – reps x load/weight lifted.
-8EIMD (Exercise induced muscle damage - Protects muscles) – DOMS (delayed onset
muscle soreness) – linked to eccentric actions.
Isotonic – same load through ROM
Isokinetic – same velocity – varying load.
Injury prevention –
Cause – intense exercise, long duration or eccentric = fatigue
Result – pain, soreness, swelling, possible anatomical deformity, athletic dysfunction.
- The muscle fibre may be the site of damage, but the immediate source of soreness is
CT. – In muscle sheaths, epimysium, perimysium, endomysium or tendon/ligament.
Common site is muscle-tendon junction – high tensions transmitted. – or within muscle
fibre
Greatest risk – muscles at 2-joints, muscles limiting ROM, and muscles used
eccentrically.
- injury chance increases with muscular fatigue as neuromuscular system loses ability to
control forces imposed on system ! alteration in mechanics of movement, and a shifting
of shock-absorbing load responsibilities.
- Muscle strain at onset of practice if muscles are weak from recent usage. Need
recovery.
- If un/trained individuals perform a unique task for the first time ! pain, swelling, and
loss of ROM after performance.
- Injured are susceptible to recurrence of injury or develop injury elsewhere from
compensatory mechanisms.
Injury Prevention
Conditioning of CT in muscle can reduce injury incidence. CT responds to loading by
becoming stronger. The rate of CT strengthening lags behind the rate of muscle
strengthening. Therefore, base work with low load, high reps needed to begin
strengthening of CT before muscle strength is increased.
Warm-up/stretching before exercise.
Endurance – increase the size + tensile strength of ligaments + tendons
Sprint training – improves ligament weight and thickness
Heavy loading – Strengthens muscle sheaths by stimulating production of more collagen.
Centre of gravity and stability
COG – the resultant force of gravitational attraction acts on the body in a vertically
downward direction toward the centre of the earth (weight) through a point at the centre
of the body’s mass.
Centre of gravity – The point of origin of weight vector. The point about which all the
particles of the body are evenly distributed.
- Centre of gravity refers only to the vertical direction because that is the direction in
which gravity acts, whereas centre of mass does not depend on vertical orientation.
- COG depends on the sum of rotational effects of each segment’s mass centre.
- The COM/=COG – remains ~ same % body height through growth.
- The COM does not need to be inside the body – represents the point of rotation.
-9- Generally lower % of total height for females = 55%, compared to 55% for males. Is
very subject specific. Approx 2cm anterior to S2. COM dependent on body structure,
proportions and weight distribution.
Stability – The resistance to both linear and angular acceleration.
- Enduring stability – (static) COG low, central base, feet in line with impact
- Momentary stability – COG low (flexed knees), positioned close to edge of base (balls
of feet), feet in line with intended direction of travel.
Balance – The ability of an individual to assume and maintain a stable position.
- Dependent on area of base of support + position of the COG with respect to base of
support (forward-back, side-side).
Stable equilibrium – If the object is displaced as a result of work done by a force and
returns to its original position
Unstable equilibrium – The object is displaced and tends to increase its displacement.
Neutral equilibrium – the object is displaced by a force and returns to the position from
which it was displaced.
Determinants of stability
- Line of gravity with respect to base of support – will be most stable if the line of gravity
is in the geometric centre of the base of support. Increasing the area of base of support
generally increases stability.
- The stability of an object is inversely proportional to the height of the COM because the
line of gravity of the centre of mass of the object will move outside the limit of the base
of support than a shorter object.
- The mass of the object. The greater the mass, the greater the stability.
Angular kinetics in sport
Eccentric forces – force applied off centre of COM – creates rotation.
- If force on COG – causes translation
- If force off-centre – causes translation + rotation.
Centre/area of percussion – ‘sweet spot’ – Creates: minimal vibration and maximal
rebound velocity.
Rotation and leverage –
Lever is a rigid rod that is rotated about a fixed point or axis = fulcrum.
- Resistance force, effort force, a bar-like structure, and a fulcrum
- There are 2 moment/lever arms = effort arm, resistance arm.
Effort arm – the perpendicular distance from the line of action of the effort force to the
fulcrum.
Resistance arm – the distance from the line of action of the resistance force to the
fulcrum.
- Both resistance and effort arms produce torques around fulcrum.
Mechanical advantage – the ratio of the effort arm to the resistance arm (Force
arm/resistance arm)
- RA>FA for speed, >RA for force.
- MA = effort arm/resistance arm
- MA>1 – the effort arm >resistance arm – the greater effort arm magnifies the torque
created by the effort force – the lever magnifies the effort force