Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
Figure 5.4 (a) Training shoe on an inclined plane and (b) its free body diagram.
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materials and nature (such as roughness) of the contacting surfaces and is, to a large
extent, independent of the area of contact. The coefficient of friction should exceed 0.4
for safe walking on normal floors, 1.1 for running and 1.2 for all track and field events.
In certain sports in which spikes or studs penetrate or substantially deform a surface,
the tangential (usually horizontal) force is transmitted by interlocking surfaces –
traction – rather than by friction. Form locking then generates the tangential force,
which is usually greater than that obtainable from static friction. For such force
generation, a ‘traction coefficient’ can be defined similarly to the friction coefficient
above.
Once the two surfaces are moving relative to one another, as when skis slide over
snow, the friction force between them decreases and a ‘coefficient of kinetic friction’ is
defined such that: F t = µ k F n, noting µ k < µ s. This coefficient is relatively constant up to a
speed of about 10 m/s. Kinetic friction always opposes relative sliding motion between
two surfaces.
Friction not only affects translational motion, it also influences rotation, such as
when swinging around a high bar or pivoting on the spot. At present there is no agreed
definition of, nor agreed method of measuring, rotational friction coefficients in sport.
Frictional resistance also occurs when one object tends to rotate or roll along another, as
for a hockey ball rolling across an AstroTurf pitch. In such cases it is possible to define
a ‘coefficient of rolling friction’. The resistance to rolling is considerably less than the
resistance to sliding and can be established by allowing a ball to roll down a slope from
a fixed height (1 m is often used) and then measuring the horizontal distance that it
rolls on the surface of interest. In general, for sports balls rolling on sports surfaces, the
coefficient of rolling friction is around 0.1.
Reducing friction
To reduce friction or traction between two surfaces it is necessary to reduce the normal
force or the coefficient of friction. In sport, the latter can be done by changing the
materials of contact, and the former by movement technique. Such a technique is