Introduction to Sports Biomechanics: Analysing Human Movement ...

Figure 5.11 Levers as examples of parallel force systems: (a) first-class lever; (b) third-class lever.
when analysing the various soft tissue forces acting on a body segment. These force
systems will not be covered further in this book.
The vector equations of statics and the use of inertia forces can aid the analysis of the
complex force systems that are commonplace in sport. However, many sports
biomechanists feel that the use of the equations of static equilibrium obscures the
dynamic nature of force in sport, and that it is more revealing to deal with the dynamic
equations of motion, an approach that I prefer. In sport, force systems almost always
change with time, as in Figure 5.2, which shows the vertical component of ground
reaction force recorded from a force platform during a standing vertical jump. The
effect of the force at any instant is reflected in an instantaneous acceleration of the
performer’s centre of mass. The change of the force with time determines how
the velocity and displacement of the centre of mass change, and it is important to
remember this.
MOMENTUM AND THE LAWS OF LINEAR MOTION
Inertia and mass
CAUSES OF MOVEMENT – FORCES AND TORQUES
The inertia of an object is its reluctance to change its state of motion. Inertia is directly
measured or expressed by the mass of the object, which is the quantity of matter of
which the object is composed. It is more difficult to accelerate an object of large mass,
such as a shot, than one of small mass, such as a dart. Mass is a scalar, having no
directional quality; the SI unit of mass is the kilogram (kg).
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