Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
96
knee angle curve, when it is driving the knee from an extended to a flexed position; we
can call this a flexing acceleration. Trace through the entire patterns of Figure 3.10 until
you thoroughly understand them and can explain them to your fellow students; this is a
very important step towards becoming a good movement analyst.
Another noteworthy geometric feature of Figure 3.10 is the sequence of the three
movement patterns: an extending (positive) angular acceleration, caused by muscle
tension, occurs before the angular velocity changes from flexing (negative) to extending
(positive). The resulting sequence of peaks (or troughs) is: acceleration, velocity,
angle. Also notable is the inverse phase relationship between the angle and angular
acceleration patterns; one is increasing while the other is decreasing; this is typical of
cyclic joint movements, but not always so apparent in movement patterns in discrete
sports skills, such as jumping and throwing.
THE COORDINATION OF JOINT ROTATIONS
Before looking at how we interpret graphical representations of coordination, let us
begin by considering what we mean by this important term. In Chapter 1, we saw how
well-coordinated arm movements can improve the height achieved in a standing vertical
jump. So, we could study coordination of arm movements with vertical forces for
vertical jumps; this would be an ambitious starting point, however. In Chapter 2,
when considering movement principles, one of the universal principles we noted was
‘Mastering the many degrees of freedom involved in a movement’. This is one
explanation of what coordination involves. A rather longer definition, which elaborates
on the one in the previous sentence, introduces the idea of ‘coordinative structures’.
This viewpoint sees the acquisition of coordination as constraining the degrees of
freedom into coordinative structures, which are functional relationships between
important anatomical parts of a performer’s body, to perform a specific activity. An
example would be groups of muscles or joints temporarily functioning as coherent units
to achieve a specific goal, such as hitting a ball. As muscles act around joints, this
explanation leads us to look at joints and their inter-relationships to gain an initial
insight into how sports movements are coordinated. We look at two common ways of
doing this in the following subsections.
Angle–angle diagrams
In the previous section, we looked at joint angles as a function of time – a ‘time series’.
However, times series involving several angles, as in Figure 3.11, can be difficult to
interpret for coordination. An alternative is to plot angles against each other – these are
called angle–angle diagrams. We could plot three angles in this way to form a threedimensional
plot, but this is rarely done.
Several forms of coordination can be brought to light through angle–angle diagrams.