Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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she will sink. It is easier to float in sea water, which has a density of around 1020 kg/m 3 ,
than in fresh water. For the human body, the relative proportions of tissues will determine
whether sinking or floating occurs. Typical densities for body tissues are: fat,
960 kg/m 3 ; muscle, 1040–1090 kg/m 3 ; bone, 1100 (cancellous) to 1800 kg/m 3 (compact).
The amount of air in the lungs is also very important. Most Caucasians can
float with full inhalation whereas most Negroes cannot float even with full inhalation
because of their different body composition, which is surely a factor contributing to the
shortage of world-class black swimmers. Most people cannot float with full exhalation.
Women float better than men because of an inherently higher proportion of body fat
and champion swimmers have, not surprisingly, higher proportions of body fat than
other elite athletes.
Fluid dynamic forces
Basic fluid mechanics
All sports take place within a fluid environment; the fluid is air in running, liquid in
underwater turns in swimming, or both, for example in sailing. Unlike solids, fluids
flow freely and their shape is only retained if enclosed in a container; particles of the
fluid alter their relative positions whenever a force acts. Liquids have a volume that stays
the same while the shape changes. Gases expand to fill the whole volume available by
changing density. This ability of fluids to distort continuously is vital to sports motions
as it permits movement. Although fluids flow freely, there is a resistance to this flow
known as the ‘viscosity’ of the fluid. Viscosity is a property causing shear stresses
between adjacent layers of moving fluid, leading to a resistance to motion through the
fluid.
In general, the instantaneous velocity of a small element of fluid will depend both on
time and its spatial position. A small element of fluid will generally follow a complex
path known as the path line of the element of fluid. An imaginary line that lies
tangential to the direction of flow of the fluid particles at any instant is called a
streamline; streamlines have no fluid flow across them.
An important principle in fluid dynamics in sport is Bernoulli’s principle, which, in
essence, states that reducing the flow area, as for fluid flow past a runner, results in an
increase in fluid speed and a decrease in fluid pressure. There are two very important
ratios of fluid forces in sport. The ‘Reynolds number’ is important in all fluid flow in
sport; it is the ratio of the inertial force in the fluid to the viscous force, and is calculated
as v l / υ where v is a ‘characteristic speed’ (usually the relative velocity between the fluid
and an object), l is some ‘characteristic length’ of the object and υ is the ‘kinematic
viscosity’ of the fluid. The ‘Froude number’ is the square root of the ratio of inertial to
gravity forces; it is important whenever an interface between two fluids occurs and
waves are generated, as happens in almost all water sports. The Froude number is
calculated as v/√(l g) where v and l are the characteristic speed and length, as above, and
g is gravitational acceleration.