Introduction to Sports Biomechanics: Analysing Human Movement ...

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THE GEOMETRY OF ANGULAR MOTION MORE ON MOVEMENT PATTERNS – THE GEOMETRY OF MOTION In the previous section we considered linear motion – the movement patterns of a point. The sports performer or sports object can be represented as a point situated at the centre of mass. The linear motion of this point is defined independently of any rotation taking place around it. The centre of mass generally serves as the best, and sometimes the only, point about which rotations should be considered to occur. All human motion however involves rotation, or angular motion, for example the movement of a body segment about its proximal joint. Angular motion is far more complex than linear motion. The rules that apply to the rotation of a rigid body can be directly applied to an object such as a cricket bat or, as an approximation, body segments. They can also be applied to a non-rigid body that, instantaneously, is behaving as though it was rigid, such as a diver holding a fully extended body position or a gymnast holding a tuck. Such systems are classified as quasi-rigid bodies. Applications of the laws of angular motion to non-rigid bodies, such as the complicated kinematic chains of segments that are the reality in most human movements, have to be made with considerable care. The theory of the rotation of even rigid bodies in the general case is complicated and many problems in this category have not yet been solved. The movement variables in angular motion are defined similarly to those for linear motion. Angular displacement is the change in the orientation of a line segment. In two-dimensional motion, also known as planar motion because it takes place in a two-dimensional plane, this will be the angle between the initial and final orientations regardless of the path taken. Angular velocity and acceleration are, respectively, the rates of change with time of angular displacement and angular velocity. Joint angles are the most important examples of angular motion – first, as here, when we look at the change of the angle over time and then, in this next section, for other combinations of variables. Joint angle patterns are, in general, far more important than linear motion patterns because they open the way to so many fascinating representations of human movement patterns. The angle–time pattern of Figure 3.9, a time-series pattern, shows how the knee angle changes with time over one stride of treadmill running. As with the linear motion example of Figure 3.6, we can learn a lot about this pattern by studying its geometry – the gradients and curvatures of the graph – as we move through time from left to right. As this analysis focuses on the qualitative aspects rather than the numbers, you need to remember the angle convention that we adopted in Chapter 1: here, a fully extended knee would be a larger angle than with the knee flexed – the angle increases upwards in the figure. Before you read on, satisfy yourself that you can: Identify where the knee is flexing and where it is extending in Figure 3.9(a). Determine the gradient of the graph in each region of the overall pattern (Clue – would you be going uphill or downhill walking along the pattern?) 93
INTRODUCTION TO SPORTS BIOMECHANICS Figure 3.9 Variation of knee angle with time in treadmill running: (a) angle–time series only; (b) regions of flexion and extension (negative and positive gradient); (c) regions of positive and negative curvature. 94
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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 6.8 Main skeletal muscles: (
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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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