Introduction to Sports Biomechanics: Analysing Human Movement ...

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CAUSES OF MOVEMENT – FORCES AND TORQUES DETERMINATION OF THE CENTRE OF MASS OF THE HUMAN BODY We have seen in the previous sections that the most important application of the laws of linear motion in sports biomechanics is in expressing, completely, the motion of a sports performer’s centre of mass, which is the unique point about which the mass of the performer is evenly distributed. The effects of external forces upon the sports performer can, therefore, be studied by the linear motion of the centre of mass and by rotations about the centre of mass. It is often found that the movement patterns of the centre of mass vary between highly skilled and less skilful performers, providing a simple tool for evaluating technique. Furthermore, the path of the centre of mass is important, for example, in studying whether a high jumper’s centre of mass can pass below the bar while the jumper’s body segments all pass over the bar, and how high above the hurdle a hurdler’s centre of mass needs to be just to clear the hurdle. The position of the centre of mass is a function of age, sex and body build and changes with breathing, ingestion of food and disposition of body fluids. It is doubtful whether it can be pinpointed to better than 3 mm. In the fundamental or anatomical reference position (see Chapter 1), the centre of mass lies about 56–57% of a male’s height from the soles of the feet, the figure for females being 55%. In this position, the centre of mass is located about 40 mm inferior to the navel, roughly midway between the anterior and posterior skin surfaces. The position of the centre of mass is highly dependent on the orientation of a person’s body segments. For example, in a piked body position the centre of mass of a gymnast may lie outside the body. Historically, several techniques have been used to measure the position of the centre of mass of the sports performer. These included boards and scales and manikins (physical models). They are now rarely, if ever, used in sports biomechanics, having been superseded by the segmentation method. In this method, the following information is required to calculate the position of the whole body centre of mass: The masses of the individual body segments, usually as proportions of the total body mass or as regression equations. The locations of the centres of mass of those segments in the position to be analysed. This requirement is usually met by a combination of the pre-established location of each segment’s centre of mass with respect to the end points of the segment, and the positions of those end points on a video image, for example the ankle joint in Figure 5.13(a). The end points can be joint centres or terminal points and are estimated in the analysed body position from the camera viewing direction (Figure 5.13). The anatomical landmarks used to estimate joint centre positions are shown in Table 5.1 and Box 6.2. It is worth noting in Figure 5.13(b) that even for this apparently sagittal plane movement in which the markers were carefully placed according to Table 5.1 and Box 6.2 with the lifter in the fundamental reference posture but with the palms facing backwards as when holding the bar, some markers no longer exactly overlay joint axes of rotation. 189
INTRODUCTION TO SPORTS BIOMECHANICS Figure 5.13 Determination of whole body centre of mass. (a) Simplified example for Study task 3; blue circles represent appropriate joint centres (for clarity, H = hip and W = wrist) or terminal points (see also Table 5.1). (b) Normal example with surface markers placed on the body in the reference position. 190
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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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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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