Principles of biomechanics


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Principles of biomechanics

  1. 1. Moment of inertiaFrom Wikipedia, the free encyclopediaThis article is about the moment of inertia of a rotating object, also termed the mass moment of inertia. For the moment ofinertia dealing with the bending of a beam, also termed the area moment of inertia, see second moment of area.In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment ofinertia of mass, or the angular mass, (SI units kg·m²) is a measure of an objects resistance to changes to its rotation. It isthe inertia of a rotating body with respect to its rotation. The moment of inertia plays much the same role in rotationaldynamics as mass does in linear dynamics, describing the relationship between angular momentum and angularvelocity, torque and angular acceleration, and several other quantities. The symbolsI and sometimes J are usually used torefer to the moment of inertia or polar moment of inertia.While a simple scalar treatment of the moment of inertia suffices for many situations, a more advanced tensor treatmentallows the analysis of such complicated systems as spinning tops and gyroscopic motion.The concept was introduced by Leonhard Euler in his book Theoria motus corporum solidorum seu rigidorum in 1765.[1] Inthis book, he discussed the moment of inertia and many related concepts, such as the principal axis of inertia. Contents [hide]1 Overview2 Scalar moment of inertia for single body3 Scalar moment of inertia for many bodies o 3.1 Moment of inertia about a point o 3.2 Moment of inertia theorems o 3.3 Properties o 3.4 Energy, angular momentum, torque o 3.5 Examples4 Moment of inertia tensor o 4.1 Definition o 4.2 Derivation of the tensor components o 4.3 Reduction to scalar5 Principal axes of inertia o 5.1 Parallel axis theorem o 5.2 Rotational symmetry o 5.3 Comparison with covariance matrix6 See also7 Notes8 References
  2. 2. 9 External links[edit]OverviewThe moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about thataxis. Therefore, it encompasses not just how much mass the object has overall, but how far each bit of mass is from theaxis. The further out the objects mass is, the more rotational inertia the object has, and the more torque (force* distancefrom axis of rotation) is required to change its rotation rate. For example, consider two hoops, A and B, made of the samematerial and of equal mass. Hoop A is larger in diameter but thinner than B. It requires more effort to accelerate hoop A(change its angular velocity) because its mass is distributed farther from its axis of rotation: mass that is farther out from thataxis must, for a given angular velocity, move more quickly than mass closer in. So in this case, hoop A has a larger momentof inertia than hoop B.Divers reducing their moments of inertia to increase their rates of rotationThe moment of inertia of an object can change if its shape changes. Figure skaters who begin a spin with arms outstretchedprovide a striking example. By pulling in their arms, they reduce their moment of inertia, causing them to spin faster (by theconservation of angular momentum).The moment of inertia has two forms, a scalar form, I, (used when the axis of rotation is specified) and a moregeneral tensor form that does not require the axis of rotation to be specified. The scalar moment of inertia, I, (often calledsimply the "moment of inertia") allows a succinct analysis of many simple problems in rotational dynamics, such as objectsrolling down inclines and the behavior of pulleys. For instance, while a block of any shape will slide down a frictionlessdecline at the same rate, rolling objects may descend at different rates, depending on their moments of inertia. A hoop willdescend more slowly than a solid disk of equal mass and radius because more of its mass is located far from the axis ofrotation. However, for (more complicated) problems in which the axis of rotation can change, the scalar treatment isinadequate, and the tensor treatment must be used (although shortcuts are possible in special situations). Examplesrequiring such a treatment include gyroscopes, tops, and even satellites, all objects whose alignment can change.The moment of inertia is also called the mass moment of inertia (especially by mechanical engineers) to avoid confusionwith the second moment of area, which is sometimes called the area moment of inertia (especially by structural engineers).The easiest way to differentiate these quantities is through their units (kg·m² as opposed to m4). In addition, moment ofinertia should not be confused with polar moment of inertia (more specifically, polar moment of inertia of area), which isa measure of an objects ability to resist torsion(twisting) only, although, mathematically, they are similar: if the solid forwhich the moment of inertia is being calculated has uniform thickness in the direction of the rotating axis, and also hasuniform mass density, the difference between the two types of moments of inertia is a factor of mass per unit area.
  3. 3. InertiaInertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resistany change in its motion. The principle of inertia is one of the fundamental principles of classical physics which are used todescribe the motion of matter and how it is affected by applied forces. Inertia comes from the Latin word, iners, meaningidle, or lazy. Isaac Newton defined inertia as his first law in his Philosophiæ Naturalis Principia Mathematica, which states:[1]The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours topreserve its present state, whether it be of rest or of moving uniformly forward in a straight line.In common usage the term "inertia" may refer to an objects "amount of resistance to change in velocity" (which is quantifiedby its mass), or sometimes to its momentum, depending on the context. The term "inertia" is more properly understood asshorthand for "the principle of inertia" as described by Newton in his First Law of Motion; that an object not subject to any netexternal force moves at a constant velocity. Thus an object will continue moving at its current velocityuntil some forcecauses its speed or direction to change.On the surface of the Earth inertia is often masked by the effects of friction and gravity, both of which tend to decrease thespeed of moving objects (commonly to the point of rest). This misled classical theorists such as Aristotle, who believed thatobjects would move only as long as force was applied to them.
  4. 4. 2A/2B BIOMECHANICS©PE STUDIES REVISION SEMINARSCONTENTIntroduction to Biomechanics•What is it?•Benefits of BiomechanicsTypes of motion in Physical Activity•Linear•Angular•GeneralCoordination of linear motion•Types of forces•Kinematic Chain•Simultaneous force summation•Sequential force summationStability and Balance•Balance•Stability•Centre of Gravity (COG)•Base of Support•Factors affecting balance and stability2CONTENTNewton’s laws of motion•Force production•Newton’s 1stLaw of motion•Inertia•Newton’s 2ndLaw of motion•Momentum
  5. 5. •Conservation of momentum•Impulse•Flattening the arc•Newton’s 3rdLaw of motionProjectile motion•Trajectory of a projectile•Factors affecting flight of a projectile•Angle of release•Height of release•Velocity at take off•Gravity•Air resistance•Spin 32. SEQUENTIALLY• Where body parts are moved in sequence to produce a force.• Generally used to produce maximal force in whole body actions suchas throwing, kicking and striking• E.g. A baseball pitcher, striking in golf, kicking in rugbyCO ORDINATION CONTINUUM.FORCE SUMMATIONFOR MAXIMAL OR SUBMAXIMAL©PE STUDIES REVISION SEMINARSHomeSUCCESSFUL SUMMATION OF FORCE/MOMENTUM•Body parts move in a sequence to generate the largest force oracceleration possible.•To sequentially produce maximal force effectively, the followingprinciples need to be applied:1. The stronger and larger muscles of the thighs and trunk are moved firstfollowed by the smaller and faster muscles2. Sequentially accelerate each body part so that optimum momentum passesfrom one body part to the next.3. Each body part should be stable so that the next body part acceleratesaround a stable base to transfer momentum
  6. 6. 4. Use as many body parts as possible, so force can be applied over themaximum possible time5. Follow through is important to prevent deceleration of last segment andsafe dissipation of force.6. Ensure all forces are directed towards the target©PE STUDIES REVISION SEMINARS 5HomeSEQUENTIAL SUMMATION OF FORCES - THROWINGBig body parts of legs andtrunk initiate movementWide base provides stable basefor acceleration of eachsegmentMaximise number of segmentsusedFollow through towards thetarget to prevent decelerationof final segment and maximisemomentum towards the target©PE STUDIES REVISION SEMINARS 6HomeDETERMINING THE CENTRE OF GRAVITY• To determine ones COG, simply draw a box around the objectsouter extremities• Then draw diagonal lines through the box, with the point ofintersection determining the objects approximate©PE STUDIES REVISION SEMINARSHomeApproximateCOGFACTORS AFFECTING BALANCE & STABILITYLow COG =↑ stabilityHigh COG =↓ stability8
  7. 7. ©PE STUDIES REVISION SEMINARSHome3. THE HEIGHT OF THE COG ABOVE THE BASE OF SUPPORT• The line of gravity or pull of gravity will always pass vertically through thecentre of an object’s mass.• The higher the centre of gravity above the base of support, the less stablethe object is. Athletes often lower their centre of gravity by bending theknees in order to increase their stabilityMore stable Less stableLow COG Higher COGWide base of support – 4 point contact Small base of support – 2 point contactLine of gravity in middle of support Similar line of gravitySTABILITY VARIES WITH BODY POSITION©PE STUDIES REVISION SEMINARS 9HomeNEWTON’S 1STLAW OF MOTIONNewton’s First Law of Motion - InertiaThe size of the force required to change the state of motion of an objectdepends on the mass of the object. The greater the mass of the object,the greater the force needed to move it.The 8kg medicine ball has agreater inertia because of itsgreater mass and thereforerequires a greater force to move itThe golf ball on the left willremain stationary on the tee untila force (applied by the club) isapplied to it10©PE STUDIES REVISION SEMINARSHome“A body continues in its state of rest or state of motion unless acted upon bya force”. Newton’s Second Law of Motion – acceleration / momentumThe greater the force applied to an object, the faster the acceleration will be.Acceleration is directly proportional to the force applied.
  8. 8. A small force applied to a ball usinga putter results in slow accelerationA large force applied to a ball using adriver results in faster accelerationNEWTON’S 2NDLAW OF©PE STUDIES REVISION SEMINARSHome“The rate of change of acceleration to a body is proportional to the forceapplied to it”.• If all other factors are constant (i.e. Speed of release, height of release,spin, air resistance);©PE STUDIES REVISION SEMINARS 121. ANGLE OF RELEASEHome•An angle of less than 45⁰ results in shorter horizontal distances,shorter vertical distances and shorter flight times•This might be useful in the following sports;•Throwing in softball, cricket etc, stab pass in AFL•An angle of greater than 45⁰ results in shorter horizontaldistances , greater vertical distances and longer flight times.•This might be useful in the following sports;•High Jump, Pole Vault, punting in American©PE STUDIES REVISION SEMINARS 13VERTICAL MOTIONHORIZONTAL MOTION1. ANGLE OF RELEASEHomeAngle of release = 45⁰•Vertical and horizontalvelocity is equal•Max horizontal distance
  9. 9. attainedAngle of release > 45⁰•Vertical velocity is greaterthan horizontal•↑ height and flight time•↓horizontal distanceAngle of release < 45⁰•Horizontal velocity isgreater than vertical•↓ height and flight time•↓horizontal distance
  10. 10. Q4E Case Study 10 - Cricket Batting 1 Batting is a side-on game – or at least it used t Proposed Subject usage: Sports Science A level & 1st/2nd yr Degre ECB Cricket Coaches Level 1 - 4 GCSE / A-level Sports Science 2008 - Crick National Curriculum 2008-2009 (Key Stage 3. Evaluating and improving performance. Judge how good a performance it AQA GCSE PE 2009 Specification 6.2 Analysis of performance The specification will assess a candidate‟s ability to analyse performance s Determine its strengths and weaknesses Improve its quality and effectiveness AQA A Level PE 2009 Specification At AS, candidates are required to observe, analyse and evaluate performan 21.2 Observe the chosen performer in relation to the competent performa techniques for a chosen activityCricket Batting 1:The demands for International cricket batsmen have changed considerably over the past ten ymatches and one day internationals is testament to this. There is no doubt in my mind that 20increasing the range of flamboyant strokes we see today, the reverse sweep or flick over fine lbowler. Nobody seems to play with a straight bat anymore? Batsmen are intent on working thepick up a quick single with a bat path that only coincides with the ball for a split second… effecjustify the risks taken?Given that batting is such an important part of the game, it is surprising that little biomechanicthe sport. The biomechanics of the off-drive and on-drive have been found to be very similar, wexecution. Grip force patterns of top & bottom hands along with kinematic analysis of selectedhowever, the underlying theme from the biomechanics literature is that batting has many diffecompare the flowing and rhythmical drives of Sachin Tendulkar, Brian Lara & Sunil Gavaskar, tHayden & Adam Gilchrist, or the skill and determination of an Aravinda de Silva, Ricky Pontingbecoming more apparent from the research is the degree of variation the same individual has wlook at players when they first arrive at the crease and then after scoring 50+ runs, why is this
  11. 11. A question which I have been asked on many occasions, most notably by the late Bob Woolmevery much of the opinion that, apart from teaching a few basic fundamentals, ones batting abiprimarily by the players natural flare, athletic instinct and desire to succeed. In Bob‟s opinion,be related back to one or more of his key fundamentals. After numerous debates with Bob on tfundamentals must be consistent throughout all batsmen: and in coaching terminology, we mucricket ball. So, what are these key fundamentals? Well put simply, they are made up of Dynamthe Bat and most importantly the Path of the Bat during the stroke…Some aspects of batting cannot be supported biomechanically, however many elite players aretechniques. The objective of this article is for all coaches to ask themselves „How do you optimthe act of striking a cricket ball as simple as possible for the player?‟. I strongly believe that bioof every batsmen, it is the responsibility of coaches to ensure that the biomechanical informatiwithin an appropriate time frame... The key fundamentals are discussed below;‘Path of the Bat & Angular Momentum’ : Many of today‟s players utilize their bottom handduring the backswing. This is a minor modification on the traditional method when batsmen withe top of the backlift, the face being slightly open towards second slip.By utilizing the bottom hand as a lever, the wrists will remain close to the centre of mass, thusthe base of support (effectively making the bat lighter). If the centre of mass of the bat can begenerated is minimal, therefore, the force required by the muscles in the forearms is reduced aaction will also decrease the rotational inertia of the system. As a result the bat will travel in adynamic and executed faster.The definition for ‘Rotational Inertia’ is as follows:A rotating rigid body (for example the bat) maintains its state of uniform rotation -unless an external TORQUE is applied or otherwise the conservation of angular mommomentum that an object has depends on two physical quantities:the MASS and the VELOCITY of the moving object...Consequently, when translated into coaching terminology, this means that the cricket bat will bcricketer‟s centre of mass. The path of the bat on the backswing and position at the top of theaction has a chain reaction in kinematics, if the arms leave the body on the backswing, this wilcounteract this action. The correct position of the hands and bat at the top of the backswing wmovement to be produced with a greater angular acceleration, it will also enable the path to bestraight bat!Many batsmen have a distinct loop in their backswing & downswing. As a result the bat must ufrom the forearms) in order to set it onto the required plane to make contact with a straight bacompensations may go unnoticed on a good wicket, what happens if the ball nips back off thebat are at complete odds with each other, with only a small „contact zone‟ where the two can cpath, have to wait until after the commencement of their downswing to begin to try and redirelooking to increase stroke accuracy the longer the path of the bat is in line with the ball, the mFigure 1 is a six-image photo sequence of Michael Vaughan (MV), Mark Ramprakash (MR), Matmatch footage is taken from the Second test, West Indies vs. England – Headingley, Leeds 25tNational Cricket Academy at Loughbrough, March 06. The red line highlights the path of the toapproximately 80mph in release speed.Each of the six-image sequences is made up of the following events: Frame 1: The bowler is in the pre-delivery position. Note: All four of the players‟ hand Frame 2: Trigger Movement, un-weighting of the front foot - Back foot landing of the Frame 3: Bowler – Front Foot Contact
  12. 12. Frame 4: Point of Release (POR) Frame 5: Ball – approximately half way down the wicket - Top of Backswing Frame 6: Ball Impact. Figure 1: Six-image photo sequence – Quintic v14 video anaTraditional coaching principles generally suggest that the bat should not be taken back outsidesecond slip. The first 2 players in the sequence (MV & MR) illustrate during frames 1-5, how ththroughout the stroke. The position of the hands directly under their shoulders enables their wthe mass of the bat remains close to the base of support. This will decrease the „ROTATIONALbat also remains close to the body, allowing the mass of the bat to remain close to the base oflighter.
  13. 13. It can be seen from the trace of the toe of the bat, that the backswing & downswing are very sof the backswing, thus no need to re-align the bath at the start of the downswing. This also haswing is based on the most current ball flight information available. A controlled bat path at thevelocity alone.The image sequences of MP & AF also highlight in frames 1-5, their wrists remaining close to tAlthough the positions of their hands are directly under their shoulders, the position and path oout towards second slip. As a result the mass of the bat goes towards the OFF-side (Frames 2will increase the „Rotational Inertia‟ of the system. This will have the effect of making the bat fetoe of the bat, that the backswing & downswing are two distinct traces. Both players have a loalign the bat & shoulders at the start of the downswing. In each case their shoulders re-align tin a continuous rhythmical manner, as in the example of Matthew Prior, demonstrates a very licommonly described as hitting „in to out‟… The additional forces required in the forearms, uppeinertia and re-align the bat with the path of the ball are significant, especially with today‟s heavalign early in the downswing and bring the bat down on a good plane; however, there still is ancomplicate matters on a seaming wicket.Golf is another sport where similar mechanics applies; think of the path of the club on the backcan only think of two exceptions, Jim Furyk and Eamonn Darcy, that have a considerable looptransition. At least with golf, the ball is stationary and the path of the club through impact areahowever (as with Andrew Flower & Matthew Prior) unnecessary movements of the clubhead dupressure and as a result be inconsistent... Figure 2 highlights the trajectory of the clubhead drinote the backswing & downswing are on a very similar path. The slight changes at the top of tduring the swing. Figure 2: Padraig Harrington x6 image photo sequ Quintic v14 video analysis softwareDynamic Balance – Position of Readiness? : For every action there is a reaction. If the futhe required chaining effects of movement will not occur efficiently and effectively. The stanceabout to face a delivery. It is the base to play all your shots, as the majority of coaching materrelaxed at stance”. What is the correct stance & optimal width?Coaching textbooks would recommend the feet approximately a foot length apart either side oA wider base may indicate a desire to optimise stability by increasing the base of support. Thepotential decline in mobility so a trade-off situation exists. In my opinion, the centre of mass ofmidpoint between the feet indicating that the weight is evenly distributed on both the left & rigevenly distributed on their heels & toes. The weight should be through your instep, or arches iwiggle your toes, without having to change your balance. Batting is an athletic, explosive movethe best possible starting position – that of dynamic balance...Where should the weight be positioned?This is a great question to ask every batsman – they should be able to answer. However whatthey do! Technology is needed here to give you the correct answer, force or pressure platformaccurate measurements. Traditional coaching literature would suggest the weight on the balls
  14. 14. reason being, that you can transfer quickly to either front or back foot depending on the lengththe toes, this does in fact limit movement to the leg side; a classic example is people falling ovdeliveries…As long as the batsmen is comfortable in their preferred starting position and any pre deliveryThe most critical point during batting in terms of stability & balance is the position at which thethe ‘Position of Readiness’.The batsmen does not know where the ball is going at POR, short, full, off side, straight or dowbowlers‟ delivery action and even previous deliveries may influence the final outcome, yet untilWith this in mind the batsmen needs to be in the optimal position to move and react to wherebe still & eyes level at this point, but their posture needs to be alive and athletic. The easiest wequal, both: 50% Right & 50% Left, and equally distributed by heels & toes - 50% Balls & 50%POR, it is easy for the player to come forward, but you would require a double movement to goweight is predominately on the front foot at POR – it‟s hard to come further forward, a doubleA simple yet extremely affective way of ascertaining where a player‟s weight is at POR is to doget comfortable by playing a number of different shots at various line & length, then simply onwatch carefully where is their body weight, are they moving forward, backwards, or falling to tdynamically balanced at the point you fake to release the ball, they must be ready to react to wputting themselves at a distinct disadvantage.A recent football study undertaken by Quintic involved analysing English Premiership goalkeepeThe outcome has very similar connotations with batting. The goalkeepers, when saving a penathe ball is struck, as they don‟t know where the ball will be targeted… If, for example, the goalright-handed people do!) then, they were fine when diving to the right, but if they needed to dright leg, move their centre of gravity over to the left before finally pushing off the left leg… byeffectively the distance they could cover on the left in the same time frame was significantly re Figure 3 highlights the position of the four batsmen at the point of release. (Andrew Flower isWhere do you think the weight is distributed for each of them? Are they balanced at the pointright with the same amount of effort, or do they favour one particular movement? Are they in Figure 3: Point of ReleaseIn two of the above examples (MP & MV) the weight distribution is predominately on the backthe opposite having the majority of weight on the front foot, only Andrew Flower has an evenMV the next frame on the video has the left foot airborne. From looking at the above example,
  15. 15. the heels at POR, a reaction to the toe of the bat going away from the body? Chicken or egg, wIf a player has a pre delivery trigger movement there is an opportunity to move into a more dyreleases the ball. This increase in momentum can be later utilised during the stroke. You are mstart in a balanced position. However, the movement must enable the batsmen to arrive at a bthe trigger movements. Too many players are in a poor position at POR as a result of poor timtrigger movements (they do differ during the stages of an innings), different movements to difinconsistency. Have a pre-delivery movement by all means, but ensure it is consistent and youA stable base or a position of dynamic balance at POR would ensure: Increased resistance to work the body levers against other body parts – summation o momentum to the lighter, faster moving body parts… Body energy transferred efficiently to the bat Full force generationFinally food for thought, if you increase your stability during the position of readiness, what effconsistency?In summary, some aspects of elite player‟s technique can not be supported biomechanically. Tcoaches to ask themselves „How do you optimise the art and science of batting whilst keepingpossible for the player?‟Biomechanical analysis is the „why‟ something happens, it is down to the skill of the coach andthe „cause and effect‟ of the any movement they observe...Dr Paul Hurrion(Ph.D Sports Biomechanics)Quintic Consultancy LtdECB Level IV – Biomechanics TutorICC Bowling Review Group – Biomechanics AdvisorDownloads Written Case Study ~0.8 MB