Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
plane of the plate – the frictional torque, sometimes called the free moment (Figure
5.25(d)) – can be easily calculated. With appropriate knowledge of the position of the
performer’s centre of mass, in principle at one instant only, the component torques
(moments) of the ground contact force (Figure 5.25(d)) can also be calculated about the
two horizontal and mutually perpendicular axes passing through the performer’s centre
of mass and parallel to the plate.
From the force–time and torque–time data, integration can be performed to find
overall or instant-by-instant changes in centre of mass velocity and whole-body angular
momentum. Absolute magnitudes of these variables at all instants (Figures 5.25(e) and
(f)) can be calculated only if their values are known at least at one instant. These values
could be obtained from videography or another motion analysis system. They are easily
obtained if the performer is at rest on the plate at some instant, when both linear
and angular momentums are zero. If absolute velocities are known, then the changes
in position – displacement – can be found by integration. In this case, absolute values
of the position of the centre of mass with respect to the plate coordinate system
(Figure 5.25(g)) can be obtained if that position is known for at least one instant. Again,
that value could be obtained from videography or another motion analysis system.
Alternatively, the horizontal coordinates can be obtained from the centre of pressure
position with the person stationary on the plate, and the vertical coordinate as a fraction
of the person’s height.
Figure 5.26 (a) Side view of force vectors for a standing broad jump; (b) centre of pressure path from above. (S shows the start
of the movement and T take-off.) Note: (a) and (b) are not to the same scale.
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