Introduction to Sports Biomechanics: Analysing Human Movement ...

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Three-dimensional rotation Rotational movements in airborne activities in diving, gymnastics and trampolining, for example, usually involve three-dimensional, multi-segmental movements. The mathematical analysis of the dynamics of such movements is beyond the scope of this book. The three-dimensional dynamics of even a rigid or quasi-rigid body are far from simple. For example, in two-dimensional rotation of such bodies, the angular momentum and angular velocity vectors coincide in direction. If a body, with principal moments of inertia that are not identical – as is the case for the sports performer – rotates about an axis that does not coincide with one of the principal axes, then the angular velocity vector and the angular momentum vectors do not coincide. A movement known as ‘nutation’ can result. Nutation also occurs, for example, when performing an airborne pirouette with asymmetrical arm positions. The body’s longitudinal axis is displaced away from its original position of coincidence with the angular momentum vector, sometimes called the axis of momentum, and will describe a cone around that vector. Furthermore, the equation of conservation of angular momentum applies to an inertial frame of reference, such as one moving with the centre of mass of the performer but always parallel to a fixed, stationary frame of reference. The conservation of angular momentum does not generally apply to a frame of reference fixed in the performer’s body and rotating with it. MEASUREMENT OF FORCE BOX 5.4 WHY MEASURE FORCE OR PRESSURE? CAUSES OF MOVEMENT – FORCES AND TORQUES To provide further movement patterns for the use of the qualitative analyst. To highlight potential risk factors, particularly in high-impact activities. To evaluate, for example, the foot-strike patterns of runners or the balance of archers. To provide the external inputs for internal joint moment and force calculations (inverse dynamics; see Bartlett, 1999; Further Reading, page 222). The forces and pressures can be further processed to provide other movement information (see below). Most force measurements in sport use a force plate, which measures the contact force components (Figure 5.20) between the ground, called the ground contact force, or another surface, and the sports performer. The measured force acting on the performer has the same magnitude as, but opposite direction from, the reaction force exerted on the performer by the force plate, by the law of action–reaction. Force plates are widely used in research into the loading on the various joints of 201
INTRODUCTION TO SPORTS BIOMECHANICS the body. In such research, movement data from videography, or another motion analysis system, are used with force, torque and centre of pressure data to calculate the resultant forces and torques at body joints. The two recording systems need to be synchronised for such investigations. Force plates can be obtained in a variety of sizes. The most commonly used have a relatively small contact area, for example 600 × 400 mm for the Kistler type 9281B11 (Kistler Instrument Corporation, Winterthur, Switzerland; http:// www.kistler.com) or 508 × 643 mm for the AMTI model 0R6–5–1 (Advanced Mechanical Technology Incorporated, Watertown, MA, USA; http:// www.amtiweb.com) and weigh between 310 and 410 N, although much lighter plates have recently become available. They are normally bolted to a base plate set in concrete. Forces of interaction between the sports performer and items of sports equipment can also be measured using other force transducers, usually purpose-built or adapted for a particular application. Such transducers have been used, for example, to measure the forces exerted by a rower on an oar, or by a cyclist on his or her bike’s pedals. Many of the principles discussed in this chapter for force plates also apply to force transducers in general. Further consideration of the operation of these, usually specialist, devices will not be undertaken in this book. One limitation of force plates is that they do not show how the applied force is distributed over the contact surface, for example the shoe or the foot. This information can be obtained from pressure plates, pads and insoles, which will be considered in the next section. Force plates used to evaluate sporting performance are sophisticated electronic devices and are generally very accurate. Essentially, they can be considered as weighing Figure 5.20 Ground contact force (F x, F y, F z) and moment (or torque) (M x, M y, M z) components that act on the sports performer. 202
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Introduction to Sports Biomechanics
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First edition published 1997 This e
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Contents List of figures x List of
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Study tasks 216 Glossary of importa
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FIGURES 1.26 Underarm throw - femal
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FIGURES 5.9 Typical path of a swimm
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Tables 2.1 Examples of slowest sati
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Preface Why have I changed the cove
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Introduction MISSING TEXT The first
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INTRODUCTION principal movements in
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INTRODUCTION TO SPORTS BIOMECHANICS
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Figure 2.2 ‘Principles’ approac
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Figure 2.13 Identifying critical fe
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INDEX 282 balls air flow past 173 b
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INDEX 284 energy 219 kinetic 219 mi
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INDEX 286 isokinetic contraction 24
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INDEX 288 muscle length-tension rel
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INDEX 290 three-dimensional 146-7,
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INDEX 292 validity 220 vantage poin
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