EDU 2462 Biophysical Foundations of Human Movement 1

1. EDU 2462 Biophysical Foundations of Human Movement 1 Lecture 2 BIOMECHANICS

2. 2 Broad Categories • Kinematics • - describes movement • - “How far?” • - “How fast?” • - “Where did it go?” • - “At what rate?” • Kinetics • investigates causes of movement and measures the underlying forces at work

3. Function of Biomechanics • Internal and external forces acting on the human body determine the performance of a motor skill • The nature of these forces, their impact and our adaptations to them is crucial to the effectiveness of the skill’s performance

4. FORCES • Objects move when acted upon by a force greater than the resistance to movement provided by the object. • The force may: • produce motion (a golf club striking a ball) • stop motion (brakes on a bike) • positively accelerate (from jog to sprint) • negatively accelerate (from sprint to jog) • change the direction of the object (tennis serve).

5. Forces can be measured by: • magnitude eg. size/number of muscle fibres • direction eg. running = horizontal movement • point of force application eg. feet placement prior to jump • line of action of force eg. topspin in tennis

6. Summation of Forces • If several forces act at any one time, the net force will cause the object to move or deform. Summation of forces may be...

7. 1. Sequential • When the correct sequence of bodyparts is used to generate great force • To achieve the maximum power, each preceding segment must contribute maximally before the next segment follows on eg. Volleyball spike; T-ball hit; cricket bowl

8. Larger body parts are the slower movers A Throw • F1 hips F2 shoulders F3 arms F4 wrists The speed of the projectile depends on the speed of the last part of the body at the time of release/contact Smaller body parts are the fastest movers

9. 2. Simultaneous • summation of the forces involves an explosive action of all body parts at the same time to generate the greatest force eg. the high jump; vault take-off

10. INERTIA • If a body is at rest, it will tend to stay at rest • Once an object is moving in a straight line, it will continue to move with constant velocity unless a force acts upon it to change its speed/direction Inertia = the object’s resistance to this change in its state of motion.

11. In linear motion... • In linear motion (motion in a straight line), inertia = the mass of the object • The larger the mass, the greater the inertia (and vice versa)  the harder it is to move. eg. to increase inertia, a greater mass is added

12. Angular Motion • Refers to circular motion around an imaginary line called anaxis of rotation. • There are 3 primary axes of rotation: 1.Longitudinal (top-bottom) 2. Transverse (left-right) 3. Sagittal (front-back)

13. A body’s resistance to angular motion is called itsmoment of inertia. • It is dependent on two factors: • the mass of the object • how this mass is distributed relative to the axis about which the rotation is occurring Longitudinal axis eg. A figure skater when doing a spin has his arms and free leg extended away from the longitudinal axis of his body. His spin is slow. When he brings these limbs closer to his body, he is bringing his mass closer to the axis of rotation, reducing the moment of inertia and spinning faster. Photo : http://www.northstarnet.org/eakhome/skating/kevin/spin.html - Technical Figure Skating Spin page

14. Film: http://www.northstarnet.org/eakhome/skating/kevin/spin.html - Technical Figure Skating Spin page

15. In the picture the red triangle is the fulcrum or the • Balancing point. • Generally the Centre of Gravity of the body is • about half an inch above the navel and straight back • near the backbone. • At this point there would be equal weight on the • left and right side of the fulcrum and thus the • body can be balanced as shown in the diagram. Centre of Gravity • The center of the mass of an object. • The location around which the mass of an object is balanced. • Not necessarily within the mass of the object.

16. The human body’s centre of gravity can be altered by merely lifting your arms over your head (heightens the C of G). • Males and females have different C of G locations: • Adult males - more mass in chest and shoulders  higher C of G (57% of standing height) • Adult females - 55% of standing height

17. Centre of Gravity and Stability • The force of gravity always acts downwards • Stability (or equilibrium) is attained when the Centre of Gravity is over the base of support. • Stability = the body’s resistance to movement Photos: http://j-views.com/content/culture/Sumo/ - Grand Sumo Photos Jan 21, 1999

18. Stability depends on: a) The height of the C of G - the closer it is to the base of support, the more stable. eg. more stable -standing or lying? b) The size of the base of support - the larger the base of support, the more stable Photos: http://j-views.com/content/culture/Sumo/ - Grand Sumo Photos Jan 21, 1999

19. c) Line of gravity - a body becomes less stable as the line of the C of G moves closer to the edge of the base of support Photos: http://j-views.com/content/culture/Sumo/ - Grand Sumo Photos Jan 21, 1999

20. d) Mass - generally the larger the mass, the greater the stability IF the force is applied e) Friction - applies when you move any two surfaces against each other. Friction works in a direction opposite to the direction of motion.Generally, the greater the degree of friction, the greater stability eg. studs in football boots  friction  more stability and more effective motion * too much friction however can lead to injuries (eg. long studs in dry ground)

21. TORQUE • All bodies will rotate easiest about their Centre of Gravity. • If a force is applied on either side of the Centre of Gravity, the object will rotate. • This rotational force is called torque. • Torque = the product of the amount of force applied  the distance from the centre of gravity at which the force applied.

22. Practical Application: The Tackling Process Coaches often tell their players to tackle a runner low. In this way, the runner's feet will be rotated in the air in the direction of the tackle. REMEMBER! • Centre of Gravity - the point in a body's distribution of mass at which all of the mass can be considered to be concentrated. • Torque -a force that tends to produce rotation or twisting

23. eg. Compare the force required to push a person off-balance when they have their arms folded in front of themselves and when their arms are outstretched. • Because torque is a product, the same torque can be applied to an object at different distances from the center of mass by changing the amount of force applied • Less force is required farther out from the Centre of Gravity than closer in.

24. So, by tackling a runner low, far from the centre of gravity, it takes less force to tackle him than if he were tackled high Diagram: http://www.howstuffworks.com/physics-of-football1.htm

25. Furthermore, if a runner is hit exactly at his centre of gravity, he will not rotate, but instead will be driven in the direction of the tackle eg. the “big hit” or impact tackle Photos: Planet-Rugby Photo gallery

26. LEVERS Velocity = the rate at which a body moves from one location to another • a bar or rigid structure • hinged at one point around which the structure can turn when a force is applied to it • the bones act as levers which are pulled and moved by the forces generated by muscles. • can be used to enhance either the force, or the velocity of movement

27. Every lever has three components: Load Force Resistance arm Force arm Fulcrum/Axis • There are three different types of levers, that vary in the position of the load, fulcrum and appliedforce. • The names are: • Class I • Class II • Class III

28. What would happen if we moved the axis closer to a) the load? b) the force? a) this lengthens the force arm  increases power b) this lengthens the resistance arm increases speed

29. Types of Levers Load Force • Class I lever • axis between force and resistance eg. see-saw Fulcrum/Axis

30. Class II lever • increases power  slow speed eg. wheelbarrow Load Force Fulcrum/Axis Allows person to lift heel off ground

31. Class III lever • increases speed and range of motion  little power eg. catapult Load Force Fulcrum/Axis Elbow Allows person to lift a weight with their hand

32. Levers such as racquets, bats, golf clubs and hockey sticks increase the speed and range of motion of the arm to which they are attached. • Why? Because the end of a lever moves faster than any other point on that lever.

33. A A1 d 1 Time (same) B B1 d 2 Time (same)

34. Bernoulli’s Principle • In a moving fluid, an area of high velocity ~ relative low pressure • An area of low velocity has a relative high pressure. • The further distance the particles have to move the faster they have to move. An aircraft’s wing is designed to give lift as it moves through the air

35. Applying this to Sport... • Note the angle of the ski-jumper’s body vs that of the aircraft’s wing • Spin • applying spin to a ball creates a pressure differential • Top Spin • High velocity on top of the ball, low velocity on the bottom  ball will lift • Back Spin is the reverse.

36. Curving and Swinging • The ball can be made to swing by shining one side of the ball or presenting the seam to the wind. • This different type of surface on either side of the ball causes a pressure differential in flight. The ball on top is smooth and has a laminar flow. It has more drag. This ball has a rough surface (a turbulent flow) and has less drag  travels more easily through the air Photos: Science of Baseball (www.exploratorium.org)

37. Streamlining & Drag • Because water does not flow smoothly around the body, low pressure turbulence forms behind the swimmer - effectively tugging them backwards (drag) • streamlining decreases the turbulent flow and  the pressure differential  less drag. Photos; AIS Swimming homepage www.ais.org.au

38. Resources and Further Reading • ssep.bwfund.org/stu/activities/sports.html - heaps of sport science activities • http://www.exploratorium.org - huge science site with special sport section looking at science of baseball, ice hockey, cycling, skateboarding etc. • http://www.howstuffworks.com/physics-of-football1.htm - great site which talks about biomechanics and kicking, momentum and impulse when tackling etc. • http://www.hittingacademy.com/ohalibrary/library1 - the physics and science behind baseball • Text:Abernethy et al p.105-180