Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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normal force include the use of inverted aerofoils on racing cars and the technique used
by skilled rock and mountain climbers of leaning away from the rock face. To increase
the coefficient of friction, a change of materials, conditions or locking mechanism is
necessary. The last of these can lead to a larger traction coefficient through the use of
spikes and, to a lesser extent, studs in for instance javelin throwing and running, in
which large velocity changes occur.
Starting, stopping and turning
As noted above, form locking is very effective when large accelerations are needed.
However, the use of spikes or studs makes rotation more difficult. This requires a
compromise in general games, in which stopping and starting and rapid direction
changes are combined with turning manoeuvres necessitating sliding of the shoe on the
ground. When starting, the coefficient of friction or traction limits performance. A
larger value on synthetic tracks compared with cinders, about 0.85 compared with 0.65,
allows the runner a greater forward inclination of the trunk and a more horizontally
directed leg drive and horizontal impulse. The use of starting blocks has similar benefits,
with form locking replacing force locking. With lower values of the coefficient of
friction, the runner must accommodate by using shorter strides. Although spikes can
increase traction, energy is necessary to pull them out of the surface and it is questionable
whether they confer any benefit when running on dry, dust-free synthetic tracks.
When stopping, a sliding phase can be beneficial, unless a firm anchoring of the foot is
required, as in the delivery stride of javelin throwing. Sliding is possible on cinder tracks
and on grass and other natural surfaces; this potential is used by the grass and clay court
tennis player. On synthetic surfaces, larger friction coefficients limit sliding; an athlete
has to accommodate by unweighting by flexion of the knee. When turning, a large
coefficient of friction requires substantial unweighting to permit a reduction in the
torque needed to generate the turn. Without this unweighting, the energy demands of
turning increase with rotational friction. In turning and stopping techniques in skiing,
force locking is replaced by form locking using the edges of the skis; this substantially
increases the force on the skis.
Effects of contact materials
The largest friction coefficients occur between two absolutely smooth, dry surfaces,
owing to microscopic force locking at an atomic or molecular level. Many dry, clean,
smooth metal surfaces in a vacuum adhere when they meet; in most cases, attempts to
slide one past the other produce complete seizure. This perfect smoothness is made use
of with certain rubbers in rock climbing shoes and racing tyres for dry surfaces. In the
former case, the soft, smooth rubber adheres to the surface of the rock; in the latter,
localised melting of the rubber occurs at the road surface. However in wet or very dusty
conditions the loss of friction is substantial, as adhesion between the rubber and surface
is prevented. The coefficient of friction reduces for a smooth tyre from around 5.0 on a
dry surface to around 0.1 on a wet one. A compromise is afforded by treaded tyres with