Introduction to Sports Biomechanics: Analysing Human Movement ...

known as unweighting, in which the performer imparts a downward acceleration to his
or her centre of mass (Figure 5.5(a)), thus reducing the normal ground contact force to
below body weight (Figure 5.5(b)). This technique is used, for example, in some
turning skills in skiing and is often used to facilitate rotational movements.
Large coefficients of friction are detrimental when speed is wanted and friction
opposes this. In skiing straight runs, kinetic friction is minimised by treating the base
of the skis with wax. This can reduce the coefficient of friction to below 0.1. At the
high speeds associated with skiing, frictional melting occurs, which further reduces the
coefficient of friction to as low as 0.02 at speeds above 5 m/s. The friction coefficient is
also affected by the condition of the snow–ice surface. In ice hockey and in figure and
speed skating, the sharpened blades minimise the friction coefficient in the direction
parallel to the blade length. The high pressures involved cause localised melting of the
ice which, along with the smooth blade surface, reduces friction. Within the human
body, where friction causes wear, synovial membranes of one form or another excrete
synovial fluid to lubricate the structures involved, resulting in frictional coefficients
as low as 0.001. This occurs in synovial joints, in synovial sheaths (such as that of
the biceps brachii long head), and synovial sacs and bursae that protect tendons, for
example at the tendon of quadriceps femoris near the patella.
Increasing friction
A large coefficient of friction or traction is often needed to permit quick changes of
velocity or large accelerations. To increase friction, it is necessary to increase either
the normal force or the friction coefficient. The normal force can be increased by
weighting, the opposite process to unweighting. Other examples of increasing the
Figure 5.5 Unweighting: (a) forces acting on jumper; (b) force platform record.
CAUSES OF MOVEMENT – FORCES AND TORQUES
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