Introduction to Sports Biomechanics: Analysing Human Movement ...

causing an acceleration at another specific point, the centre of rotation. The above
equation shows that Q and R can be reversed. The centre of percussion is important in
sports in which objects, such as balls, are struck with other objects such as bats and
rackets. If the impact occurs at the centre of percussion, no force is transmitted to the
hands. For an object such as a cricket bat, the centre of percussion will lie some way
below the centre of mass, whereas for a golf club, with the mass concentrated in the
club head, the centres of percussion and mass more nearly coincide. Variation in grip
position will alter the position of the centre of percussion. If the grip position is a long
way from the centre of mass and the centre of percussion is close to the centre of mass,
then the position of the centre of percussion will be less sensitive to changes in grip
position. This is achieved by moving the centre of mass towards the centre of percussion,
as for golf clubs with light shafts and heavy club heads, and cricket bats for which
the mass of the bat is built up around the centre of percussion. Much tennis racket
design has evolved towards positioning the centre of percussion nearer to the likely
impact spot. The benefits of such a design feature include less fatigue and a reduction
in injury.
The application of the centre of percussion concept to a generally non-rigid body,
such as the human performer, is problematic. However, some insight can be gained
into certain techniques. Consider a reversal of Figure 5.18(e) such that Q is high in the
body and R is at the ground (similar to Figure 5.17(d)). Let F be the reaction force
experienced by a thrower who is applying force to an external object. If F is directed
through the centre of percussion, there will be no resultant acceleration at R (the
foot–ground interface). A second example relates Figure 5.18(e) to the braking effect
when the foot lands in front of the body’s centre of mass. The horizontal component of
the impact force will oppose relative motion and cause an acceleration distribution,
as in Figure 5.18(f), with all body parts below R decelerated and only those above R
accelerated. This is important in, for example, javelin throwing, where it is desirable not
to slow the speed of the object to be thrown during the final foot contacts of the
thrower.
Transfer of angular momentum
CAUSES OF MOVEMENT – FORCES AND TORQUES
The principle of transfer of angular momentum from segment to segment is sometimes
considered to be a basic principle of coordinated movement. Consider, for example, the
skater in Figure 4.11; if she moved her arms fully away from her, to a 90° abducted
position, she would decrease her speed of rotation; if she moved them into her body, she
would rotate faster. This is sometimes interpreted in terms only of the two ‘quasi-static’
end positions – fully abducted arms, and arms drawn in to the body. The former
position has a large moment of inertia, and hence low speed of rotation; the latter
position has a low moment of inertia and high speed of rotation. However, from the
abducted-arms to the tucked-arms position, the arms lose angular momentum as they
move towards the body, ‘transferring’ some of their angular momentum to the rest of
the body which, therefore, turns faster. A more complex example is the hitch kick
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