Introduction to Sports Biomechanics: Analysing Human Movement ...

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Phase planes MORE ON MOVEMENT PATTERNS – THE GEOMETRY OF MOTION made of the loss of ‘time’ as a variable, although marking the data, or time, points as in Figure 3.16 adds some indication of how the angles change with time. We do, however, lose access to time-series shape patterns, that is slope = velocity; curvature = acceleration; such relationships do not apply to angle–angle diagrams. Perhaps the main criticism of angle–angle diagrams is that they do not show coordination changes very clearly without painstaking analysis of the patterns, as for the hip–knee coupling above. Only the ankle–knee coupling of the three joint couplings that we compared between walking and running was qualitatively different topologically between the two modes of gait. Phase planes, a totally different approach, are based on the notion that any system, such as a body segment, can be graphed as diagrams of two variables; for the phase planes used in human movement analysis, these variables are usually joint angle and angular velocity. As it turns out, although the relevance of a phase-plane for a single joint to coordination between joints may seem hard to fathom, phase planes turn out to be pivotal for our understanding of movement coordination, as will be evident later in this section. Example phase planes, for the hip (Figure 3.17(a)) and the knee (Figure 3.17(b)) joints in a running stride, are shown in Figure 3.17. Its description – although not its analysis – is ‘child’s play’ compared to both angle–time series and angle–angle diagrams. As we define flexion as a decrease in joint angle and extension as an increase, then flexion must be from left to right and extension from right to left in Figure 3.17. Similarly, as we saw in the previous section on time series, a flexion velocity is negative and an extension velocity is positive; so, flexion must be below the horizontal (zero) line in Figure 3.17 and extension above it. And that is about that; the diagrams of Figure 3.17 must progress clockwise. Why? Well let’s assume the opposite – they proceed anticlockwise. In this case, in the top half of Figure 3.17(a) the joint would be flexing – from right to left – while the angular velocity was positive, indicating extension. We have proved a contradiction; therefore, our phase planes must progress clockwise with time. The value of phase planes starts to become evident when we define the so-called ‘phase angle’ as shown for ‘real data’ in Figure 3.18. We have changed the graph so that it is ‘centred’ on its mean value, for reasons that need not concern us in this book. If we now subtract the phase angle – defined anticlockwise from the right horizontal – for one joint from that for a second joint at the same instant, we define a variable known as relative phase. Here, we subtract the knee phase angle from that for the hip, rp = pah – pak. We can do this for every time instant in the cycle to arrive at values of this relative phase as a function of time, which is known as ‘continuous relative phase’. A graph of continuous relative phase as a function of time is shown in Figure 3.19. Continuous relative phase is simply another coordination ‘pattern’, although more difficult to interpret than angle–angle diagrams. In Figure 3.19, for example, we see that 103
INTRODUCTION TO SPORTS BIOMECHANICS Figure 3.17 Phase planes for one running stride for: (a) hip joint; (b) knee joint. 104 the relative phase angle is around 30–60° for much of the swing phase (after toe-off). The value then changes gradually to about 180° at touchdown and fluctuates about that value at the start of the stance phase before gradually returning to around 30–60° before toe-off. So what, you might be tempted to say! Well, relative phase has been found to be the variable that best expresses coordination changes in a wide range of biological phenomena, including human movement. Examples in human movement include the transitions between walking and running, and bimanual coordination changes. For a further discussion of these and other biological examples of the use of relative phase, see Kelso (1995; Further Reading, page 112).
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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 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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