Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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Impact forces
flow will occur on the top surface and turbulent on the bottom surface. The wake will
be deflected upwards, the opposite from the normal Magnus effect discussed above, and
the ball will plummet to the ground under the action of the negative lift force. Reynolds
numbers in many ball sports are close to the critical value, and the negative Magnus
effect may, therefore, be important. It has occasionally been proposed as an explanation
for certain unusual ball behaviour in, for example, the ‘floating’ serve in volleyball.
Impact forces occur whenever two or more objects collide. They are usually very large
and of short duration compared to other forces acting. The most important impact
force for the sport and exercise participant is that between that person and some
external object, for example a runner’s foot striking a hard surface. These forces can
be positive biologically as they can promote bone growth, providing that they are not
too large; large impact forces are one factor that can increase the injury risk to an
athlete. An example of an impact force is shown by the force peak just after the start of
the landing phase (C) in Figure 5.2.
Impacts involving sports objects, such as a ball and the ground, can affect the
technique of a sports performer. For example, the spin imparted by the server to a
tennis ball will affect how it rebounds, which will influence the stroke played by the
receiver. Impacts of this type are termed oblique impacts and involve, for example, a ball
hitting the ground at an angle of other than 90°, as in a tennis serve, and a bat or racket
hitting a moving ball. If the objects at impact are moving along the line joining their
centres of mass, the impact is ‘direct’, as when a ball is dropped vertically on to the
ground. Direct impacts are unusual in sport, in which oblique impacts predominate.
COMBINATIONS OF FORCES ON THE SPORTS PERFORMER
In sport, more than one external force usually acts on the performer and the effect
produced by the combination of these forces will depend on their magnitudes and
relative directions. Figure 5.10(a) shows a biomechanical system, here a runner, isolated
from the surrounding world. The effects of those surroundings, which for the runner
are weight and ground reaction force, are represented on the diagram as force vectors.
As mentioned on page 167, such a diagram is known as a ‘free body diagram’, which
should be used whenever carrying out a biomechanical analysis of ‘force systems’. The
effects of different types of force system can be considered as follows.
Statics is a very useful and mathematically simple and powerful branch of mechanics.
It is used to study force systems in which the forces are in equilibrium, such that they
have no resultant effect on the object on which they act, as in Figure 5.6(b). In this
figure, the buoyancy force, B, and the weight of the swimmer, G, share the same line of
action and are equal in magnitude but have opposite directions, so that B = G. This