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Biomechanics - Exam 2 Notes.doc

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Western University
Kinesiology 2241A/B
Daniel Bechard

Lecture 1: November 2 nd Nature of Skills: - Pattern: A movement patter (ex: run, jump, throw, push, etc) - Skill: A pattern adapted to a task (ex: volleyball serve)  a volleyball serve is derived from the ‘push’ pattern - Technique: Variations of a skill (ex: a top spin volleyball serve) - Style: Unique timing / special moves (ex: each serving technique has a different style depending on person) - Constraints: Human (ex: age, strength, power, etc), or Event (ex: boundaries, rules, equipment) Classifications of Skills: - Closed: Skills performed in a predictable environment (ex: tennis serve, swim race, high jump) - Incorporate various biomechanical principles as opposed to open skills - Open: Skills performed in an unpredictable environment (ex: baseball hit, forehand return in tennis) - Execution of these skills is usually not perfect - Discrete: Skills where there is a definite beginning and end - Ex: Shooting a Free Throw - Continuous: Skills where there is no definitive beginning or end (a continuous repetitive motion) - Ex: Run, Swim, Row, Bike, Skate, etc Skill Phases: Occur predominantly in discrete skills - Preparation: The portion of skill that prepares the person for contact (ex: wind-up in tennis forehand) - Execution: The most important phase that determines the outcome of the skill (ex: hitting the ball) - Follow-Through: The final phase where the movement continues and finally stops (ex: arm over shoulder) Overall Performance Objective: - Certain skills can use more than 1 O.P.O. at once (ex: throwing a baseball uses horizontal / accuracy) 1) Projecting for Max Distance Horizontally: - For maximum distance, there must be a kinetic link (based on the conservation of angular momentum) - The conservation of angular momentum involves using largest body parts to smallest - Therefore, using body parts sequentially from largest to smallest will allow maximum distance - In order to achieve maximum distance, one must have a high release point far from the axis of rotation, while moving the object at a high velocity (V = rΩ, velocity = radius x speed of rotation) 2) Projecting for Max Distance Vertically: - Requires a closed kinetic link - This means that all body parts must be used simultaneously (ex: jumping high) 3) Projecting for Accuracy: - The most accurate movements occur when you release the object when it is in front of you - For power, the object is released beside your head - Most movements involving accuracy require a ‘push pattern’, where all movements are simultaneous 4) Projecting for Accuracy with Speed: - A combination of the above two points 5) Manipulating a Resistance: - When tossing someone, bending your knees is essential - By bending your knees, the fulcrum (your ass) is lower on your partners body - Therefore, less resistance (or weight) is found below the fulcrum, which reduced resistance 6) Moving Over a Timed Distance: - There must be a balance between stride/stroke length and stride/stroke frequency to be effective 7) Moving Body in an Ideal / Model Pattern: - To spin at a higher rate, the amount of mass must be brought close to the axis being spun around - In sports such as dancing, both Ideal / Model Pattern and Max Distance Vertically are used 8) Interact with Environment: Moving body with intention of interacting with the natural environment - A sailor adjusts his mast so that it faces the wind Analysis Process: 1) Discrete Parts: Dividing the skill into separate portions (ex: for a tennis serve: toss  wind-up  hit) 2) Mechanical Purpose (MP): Each discrete part has a purpose meant to improve the entire performance, or to enhance the MP of the next discrete part 3) Biomechanical Factors: Factors that influence the achievement of the Mechanical Purpose - Ex: Somersault Dive: one must achieve a maximum vertical height, and an optimal angle of takeoff 4) Biomechanical Principles: Incorporate Biomechanical Factors into specific principles - Ex: Somersault Dive: The greater the action force  the greater board reaction force  greater height - Other examples: momentum, factors affecting muscle force, kinetic link, angle of projection, etc 5) Critical Features: Specific body actions observed by the coach that help the performer achieve the MP - Ex: Somersault Dive: Extend arms high at takeoff (increases the C of G  you will jump higher) Lecture 2: November 6 th Factors Influencing Movement: 1) Magnitude of Net Torque: Increased torque leads to increased motion 2) Inertial Characteristics: Rotational Inertia (I), Friction Factors - Rotational Inertia: Bringing C of G closer to the axis of rotation increases rotary motion 3) Pathway Available: Rotary Motion: - Is represented by a straight line starting from the axis of rotation, and ending at a selected point of the body or segment being examined - Both points 1 and 2 have the same angle, and thus the same angular distance 2 - However, #2 has a greater linear distance, and thus a greater linear velocity 1 Kinematics: - Linear: v = d/t or v = ∆d/∆t, a = v2 –v1/t or a = ∆v/∆t, d=vt (displacement, in m) - Rotary: Occur due to the addition of a net torque - Angular Velocity: The speed at which an object changes in angular position (displacement / time) - Omega (w) = ø/t or ∆ø/∆t  expressed in rad/sec - Angular Acceleration: The acceleration of an object rotating (velocity / time) - Alpha (a) = w2 – w1/t or a = ∆w/∆t  expressed in rad/sec - Angular Displacement: The changing position of an object during a rotation (speed x time) - Theta (ø) = wt  expressed in degrees or radians Angular Velocity / Speed (w): - The speed at which a rotating body changes in angular position - Is measured in degrees or radians per second - 1 radian = 57.3 degrees - Radian: Distance of the radius from the axis of rotation to a body point, expressed on circumference of a circle - w direction is either ccw (+) or cw (-), thus is a vector quantity Linear Velocity of a Point on a Rotating Body: - Is an important concept to understand for kicking, throwing, and striking - Linear Distance: d=rø (radius x angular displacement)  how far a point on the rotating path traveled - Linear Velocity: v = rw (radius x angular velocity)  how fast the point on the rotating path traveled - r = the radius of rotation (distance from the axis of rotation to the point rotating about the axis) - w = angular velocity (speed at which the point is rotating) - A point that is located further from the axis of rotation will have a greater linear distance and linear velocity - This is attributed to the fact that it has a larger radius of rotation Angular Acceleration: - The rate at which a body’s angular speed or direction is changed only when a torque is being applied - Is rare in humans to have a constant (w) as most motions have a continual acceleration (a) or deceleration - Angular Acceleration (a) = change in angular velocity (w) / change in time (t) =∆w/∆t - Is large when there is a large change in (w) over a short period of (t) - Is small when there is a small change in (w) over a long period of (t) - For example, if a fast pitcher’s arm has a ∆w of 20 radians, and a ∆t of 0.2 seconds, the angular acceleration can be expressed as a = 20/.2 = 100 radians/second Average Velocity vs. Instantaneous Velocity: - Average w: Examines the time it takes for a body segment to complete a motion - Is used in qualitative analysis of movement (ø/t) - Instantaneous w: Examines the angular velocity (w) at a particular point of the ROM - v = rw determines instantaneous v Rotational Inertia (pg. 319 – 323): - Resistance to change in angular acceleration (a) - I = mr or I = mk 2 - How massive the object is, and more specifically how far away the object is from the axis of rotation determines its rotational inertia - Doubling the mass of a body doubles its rotational inertia, doubling the radius quadruples its rotational inertia - Angular Acceleration (a) = T (torque)/I (rotational inertia) OR T (torque) = I (rotational inertia) x a (angular acceleration) - When a person is spinning on a turntable, and brings their arms in closer to their C of G, the r-value decreases, resulting in a decreased I value - Angular Acceleration is inversely proportional to the rotational inertia of a body (Newton’s Second Law) - A decreased I value in turn leads to a greater angular acceleration - However, if the I value increases due to an increased r value, more torque (T) will be needed to produce the same amount of angular acceleration (a) as a body with a smaller r value Rotation and Gyration: - r of Rotation: Occurs in symmetrical system where the radius is expressed as distance from the axis of rotation to a point on the rotating system (v = rw) - r of Gyration: Occurs in asymmetrical systems where the radius is expressed as the distance from the axis of 2 rotation to a point where all of the mass is concentrated (I = mr ) - A rotating body whose mass is spread far from the axis has a large radius or gyration, and vice versa Lecture 3: November 9 th Open vs. Closed Kinetic Links: - Open: A process using all muscles from large to small sequentially - Proximal segments are in front of the projectile, and distal segments are behind the projectile - Is used to create an increased velocity in a throw pattern - Uses a wheel-axle motion to project something in a curvi-linear path - Closed: A process using all muscles simultaneously - All segments are behind the projectile - Is used to create an increased force in a push pattern - Uses mainly lever motions to project something in a recti-linear path L: Angular Momentum: - L = Iw  Angular Momentum = Rotational Inertia x Angular Velocity 2 - I is calculated in the same way as explained before (I = mk ) - Ex: Spinning about the longitudinal axis - Once angular momentum is established (ex: the push off), it remains constant - When you bring your arms closer to your body, you decrease the k value, which in turn decreases the I - However, when you decrease the I value, you increase the w (angular velocity) - As a result, when spinning you are increasing or decreasing angular velocity not angular momentum Tt: Angular Impulse: - Tt = Torque x Time of Torque Application - The magnitude of Angular Impulse determines the amount of Angular Momentum Created - To create torque, the action line of the applied force must fall eccentric to the C of G - Ex: Doing a back flip off a diving board - The torque is only applied while standing on the board - You cannot create excess torque once you are in mid-air - The best divers will have a lesser lean (smaller forcar), but will have instead a larger force - When they leave the board, they tend to extend their arms fully (increasing I) - However, the force that they create compensates for this, allowing them to rotate faster (increased w) - Once in the air, the diver can manipulate I by bringing the mass closer or further away from the axis of rotation, thus speeding or slowing the movement’s rotation Conservation of L: The L of a system will stay the same as long as no external torques are placed on it - Consider a diver doing a front flip - At first, when they initially leave the diving board, their body is uniformly about the axis of rotation - As a result, I and w are about equal - They then move into a tuck position - As a result, I decreases and w increases - Prior to entering the water, they extend from the tuck position - As a result I increases and w decreases - * This allows you to see that angular momentum stays constant; I will increase proportionally to the decrease in w, and vice versa - * It is the change in k that is responsible for the change in I, and as a result the change in w - * The flight-path of the C of G is also determined following the takeoff - One cannot change the flight-path of the C of G once they are rotating in the air - Twists are about the Longitudinal Axis (ex: figure skater spin) - Spins are about the A-P Axis (ex: cart-wheel) - Sommies are about the M-L Axis (ex: somersault) Lecture 4: November 11 th Throw Pattern vs. Push Pattern: - Throw Pattern: Involves a sequential movement usually in the wheel-axle type movement - Proximal segments located in front of the projectile, distal segments found behind the projectile - Is used to increase velocity - Has a curvilinear path - Considered an ‘open’ kinetic link - Push Pattern: Involves a simultaneous movement usually in the lever type movement - All segments begin behind the projectile, pushing it in a specific direction - Is used to increase force or accuracy - Has a rectilinear path - Considered a ‘closed’ kinetic link Constraints to the Throw / Push Pattern: - Mass of projectile (increased mass = increased push pattern) - Volume / size of projectile (increased size = increased push pattern) - Shape / profile of projectile (increased aerodynamic = increased throw pattern) - Target area for projectile - Strength or power of the person - Skill of the person Open vs. Closed Kinetic Chains: - Open: A throw or kick where the end segment is free to move - Involves sequential movements of body parts - Closed: A jump, push, or pull where the end segment is restrained - Involves a simultaneous movement of the body segments Throw-like Patterns (review slide on segments a/b/c): Goal is to achieve high end-point velocity - Proximal portions of the body move first (ex: when kicking, the trunk begins to rotate first) - Distal parts lag behind (ex: when kicking, after the trunk rotates the legs will begin to extend) - Achieve either: maximum distance (horizontally or vertically) OR maximum velocity - Angular Momentum is created when an initial segment (massive and proximal segment) of a body rotates - As initial body part decelerates, the next body part (less massive / distal) involved in the movement accelerates - This continues the same Angular Momentum, but increases w Reducing or Stopping Angular Momentum: - Consider someone throwing a baseball in mid-air - First, the trunk rotates, and passes the created momentum to smaller body parts (shoulders  arms  hand) - In mid-air less momentum is created  less velocity - To counter this, person will flex their knees / hips to decrease r  faster rotation  more momentum - To prevent over rotation of the trunk, the contralateral arm moves towards the ipsilateral side (throwing arm)  allows for greater transfer of L to the arm by slowing the L in the shoulders/hips - As a result, more velocity is created - Ex: when one kicks in soccer, their upper body rotates in the opposite way of their lower body v = rw: - The final (linear) velocity of the hand at its release point determines the projectile velocity - The smaller the object, the greater the r (greater distance from axis of rotation to release point) - In early execution you want to decrease the r (allows for faster rotation) - As the body gains L, increase r just prior to release of the object - This increases rotational inertia, but the large end-point velocity more than compensates for this Sequential Motions: 1) Proximal / Massive segment moves first creating L 2) External torque decelerates proximal segment 3) Next segment (less massive) accelerates faster about a new axis with a smaller k (conserving L) 4) Each successive segment increases w more than the previous segment due to decreased m and k values Lever vs. Wheel - Axle: - Lever Motions: Force creating motions such as flexion, extension, abduction, adduction, etc - Wheel-Axle Motions: Velocity creating motions such as medial/lateral rotation, pronation/supination, etc Wheel – Axle Motions (review diagram): - A muscle torque rotates a bone, bone acts as the axle (ex: pronation in a tennis forehand  arm bones = axles) - The wheel is the adjacent segment placed at an angle to the axle (ex: pronation in forehand  hand = wheel) - The wheel is modified vie flexion/extension OR abduction/adduction (ex: if wrist is extended, it will have a larger wheel than if it is held straight) Throwing / Pushing for Speed and Accuracy (review diagram): - Throw patterns have a curvilinear path of the hand, so release points for 3 different targets must be close together  ball is released at the top of the curvilinear path - Push patterns have a rectilinear path of the hand, so the release points for 3 different targets can be spaced out - A higher target will have an earlier release point, and a lower target will have a later release point - *Gyroscopic Stability: The greater a body’s angular momentum about an axis, the more torque necessary to change the direction of its axis of rotation (ex: a football spinning very fast will be more resistant to wind friction than one that is spinning slower) - * During free flight, gravity does not cause a change in L, as it causes no external torque - * Golfers use sequential motions to hit a ball far - This explains why some smaller people can hit a ball further than people who are taller than them - *When momentum stops in a body part (ex: hips) it is then transferred to an upper / smaller body part (ex: shoulders), but the shoulders must then accelerate to conserve this momentum th Lecture 5: November 16 Throw Pattern: Preparation Phase of a Baseball Pitch: - Serves to create the original torque by rotating backwards - Rotation occurs so that contralateral shoulder faces the target - Pitcher then steps forward with his contralateral leg - Pitchers therefore must have strong legs Execution Phase of a Baseball Pitch: - Hips rotate forwards, while the upper torso and arm lag behind - Hips are stopped, while the upper torso and shoulders rotate forwards - The upper torso and shoulders are stopped once they are aligned with the rotated hips / lower torso - The stopping of the shoulder rotation is due to the contralateral (non-throwing) arm moving towards the ipsilateral side  slows the shoulder rotation  angular momentum is transferred to throwing arm Execution Phase Cont: - Ipsilateral Elbow Extension does not increase the force - It increases the radius, thus increasing linear velocity as v = rw - The release point is to the side of the torso rather than above or in front (increases r) - However, if you increase the radius too early, I increases, and the throw will not be as effective - The medial rotation of the shoulder along with pronation serves as a wheel-axle motions that increases v Mechanical Purposes of Push Patterns: 1) Achieve Max Force 2) Achieve Max Power (P = Fv) 3) Achieve Max Accuracy - Push patterns are also used to manipulate a resistance Push Patterns for Force Activities: Moving a resistance slowly - Maximum strength movements require simultaneous segmental rotations that move in a rectilinear path - One must not accelerate during maximum strength movements to comply with the Force / Velocity Principle (increased velocity = decreased force created), and to prevent injury Push Patterns for Power Activities: Moving a resistance fast - Power movements require force and velocity - Most effective resistance moves in a linear fashion with little or no deviation horizontally from vertical path - Moving a resistance fast requires a higher maximum force and rapid acceleration over a short period of time - 2 Types of Power: Strength Dominant (ex: shot-put) and Speed Dominant (ex: high jump) Power in Jumping: - Can either attain maximum vertical distance or maximum horizontal distance - Max Distance Vert: Requires the C of G to have High Vertical Velocity and Moderate Horizontal Velocity - Max Distance Horizont: Requires the C of G to have High Vertical Velocity and High Horizontal Velocity Motion Sequence of Jumping: - Large body segments compose ‘open’ end of the chain - Small body segments (ex: feet) compose ‘closed’ end of the chain - The ideal direction of the applied force is through the bodies C of G - Initial shoulder flexion exerts a downward force - Stopping the shoulder flexion initiates trunk extension - Shoulder flexion and trunk extension loads the legs - Loading of the leg muscles increases ROM and elastic recoil in hip and knee extensors Punch / Strike: Power / Accuracy: - Force, Velocity, and Accuracy are all important in striking - Velocity is increased using a curvilinear path rather than rectilinear - Ex: a curving punch accelerates faster due to decreased I - Force and accuracy is achieved by changing to a rectilinear path at the end of the ROM, prior to contact Accuracy and Velocity: - Consistency to a movement is a key to accuracy - There must be a straight-line movement prior to, during, and just after the release or impact - Rectilinear paths are used for projecting over a short distance - Curvilinear paths with a ‘flat space’ (rectilinear path) just prior and during release/impact are used for projecting for accuracy over a long distance th Lecture 6: November 18 Overarm Throw Pattern: 1) Step forward (make sure contralateral shoulder faces target, ipsilateral shoulder lags behind at first) 2) Pelvis / trunk forward 3) Pelvis rotation 4) Trunk Rotation 5) Shoulder Protraction 6) Shoulder Transverse Adduction 7) Shoulder Medial Rotation (large factor) 8) Elbow Extension 9) Forearm Pronation 10)Wrist/Hand Flexion Transfer L to Arm by Reducing / Stopping L in Shoulders: - The contralateral arm is used to stop the shoulders from rotating - The stopping of movements in the larger body parts transfers their angular momentum to smaller body parts - Next event (movement of smaller parts) begins as previous event (movement of larger parts) is still occurring - Therefore, there is overlap in movements - To maintain the angular momentum created by the larger body parts, smaller parts must increase their velocity Elbow Extension and Margin of Error: - Consider a tennis serve - The higher the tennis racquet hits the ball, the increased margin of error - This means that there are more angles in which one can hit the ball into the service box successfully Motion Analysis: - Segments Involved - Movements Involved - Sequence of Movements - Muscles Causing Movements General Principles: - Smaller / lighter projectiles allow more segments to contribute to the performance - Smaller / lighter projectiles allow for a greater k - Smaller / lighter projectiles allow smaller segments to give the greatest contribution (ex: badminton vs. tennis) - Larger / heavier project get the greatest contribution from larger body segments Performance Errors: Kinetic Link: 1) Segmental Positioning: - Beginners position more distal segment in front of a proximal segment (ex: elbow in front of shoulder) - Occurs prior to release 2) Sequence of Movements / Timing: - A beginner may move segments in blocks, rather than overlapping each with the adjacent segment - This establishes a simultaneous movement pattern rather than a sequential movement 3) Lack of Power / Strength: - Someone lacking enough power or strength pushes the ball closer to their body, rather than using a throw pattern  less velocity is created on the ball Book Notes: - As the movements progress to the distal axes, rotational inertia of the remaining rotating systems decreases - As a result, the distal portions of the system tend to move faster than the larger proximal portions th Lecture 7: November 20 OPO’s in Projecting: 1) Maximum Horizontal Distance (decreased angle = open kinetic link) 2) Maximum Vertical Distance (increased angle = closed kinetic link) - Horizontal and Vertical Distance are affected by the angle of projection and more importantly the velocity of the projectile at release 3) Maximum Accuracy 4) Maximum Accuracy with Speed - Ex: By flexing your wrists in a golf swing, you decrease r, increasing angular acceleration - You then increase your r before contact, increasing linear velocity Projection Angle for Max V Horizontal: - The resultant vector angle is less than 45 degrees (ex: long jump / long throw) - A resultant angle of 20 degrees has a horizontal velocity 3x greater than vertical velocity Projection Angle for Max V Vertically: - The resultant vector angle is greater than 45 degrees (ex: volley ball jump / high jump) - A resultant angle of 60 degrees has a vertical velocity more than 2 times greater than the horizontal velocity - An angle of projection of only 45 degrees is beneficial for when the release height of the projectile is equal to the landing height of the projectile - The angle of projection for a golf ball is less than 45, but the angle of the golf club puts a backspin on the ball making it climb vertically Vector Composition: - Finding the resultant (ex: resultant velocity) by using the horizontal and vertical vectors - Ex: Horiz = 4, Vert = 3, Therefore the resultant = square root of 25 = 5mps Forces Affecting Projectiles: - Gravity: Decelerates an ascent, accelerates a descent - Drag: Results from airflow past a projectile - Comprised of: Profile / Form Drag + Skin Friction / Surface Drag - Drag is always a resistive force in aerodynamics, but in hydrodynamics it can be a motive force Profile / Form Drag (Drag Force #1): - Primary factor influencing magnitude of drag - Magnitude is proportional to the area of leading edge of the projectile - A higher-pressure zone is found on the leading side, a lower-pressure zone is found on the trailing side - Can be reduced by streamlining - An indirect relationship between magnitude of flow velocity and pressure created (high flow = low pressure) - EX: When running, the air hits your stomach (high pressure), but is not moving that fast when it hits your stomach (low flow velocity)… when it tails off your stomach it speeds up (high flow velocity), but little pressure is found (low pressure) - When crouching on a bike, you reduce the profile drag (less pressure) Skin Friction / Surface Drag (Drag Force #2): - Secondary factor influencing the magnitude of drag - Air sticks to the outside surface of a projectile - The rougher the surface the greater the friction - Smooth surfaces (ex: swim suits) reduce the skin friction and surface drag Projecting for max V Vertically: - Gravity is at first resistive, then motive - Fluid drag is always resistive in aerodynamics - The greater the V of projectile, the greater the Fluid Drag, but also greater height - On descent, acceleration is influenced by fluid drag - Fluid drag has the same role on ascent and decent (a resistive force) V Vertical Depends On: - Height of C of G at takeoff (higher C of G at takeoff = higher apex of C of G in flight) - Vvert of C of G at takeoff (greater velocity of C of G at takeoff = g
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