Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
technique in the flight phase of the long jump, in which the arm and leg motions
transfer angular momentum from the trunk to the limbs to prevent the jumper from
rotating forwards too early in the flight phase.
Trading of angular momentum
The term ‘trading’ of angular momentum is often used to refer to the transfer of angular
momentum from one axis of rotation to another. For example, the model diver – or
gymnast – in Figure 5.19 takes off with angular momentum (L = L som) about the
somersault, or horizontal, axis. The diver then adducts her left arm, or performs some
other asymmetrical movement, by a muscular torque that evokes an equal but opposite
counter-rotation of the rest of the body to produce an angle of tilt. No external torque
has been applied so the angular momentum (L) is still constant about a horizontal
axis but now has a component (L twist) about the twisting axis. The diver has ‘traded’
some somersaulting angular momentum for twisting, or longitudinal, angular
momentum and will now both somersault and twist. It is often argued that this method
of generating twisting angular momentum is preferable to ‘contact twist’ (twist
generated when in contact with an external surface), as it can be more easily removed by
re-establishing the original body position before landing. This can avoid problems
in gymnastics, trampolining and diving caused by landing with residual twisting
angular momentum. The crucial factor in generating airborne twist is to establish a tilt
angle and, approximately, the twist rate is proportional to the angle of tilt. In practice,
many sports performers use both the contact and the airborne mechanisms to acquire
twist.
Figure 5.19 Trading of angular momentum between axes of rotation: (a) at take-off; (b) after asymmetrical arm movement.
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