Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
critical feature – the jumper must have a fast run-up; we need to develop an ‘eye’ for
this as qualitative analysts whereas a quantitative analyst would need to devise the best
way of measuring this speed. The second component – speed added on the board – is of
little use, being unobservable (although measurable), so we need to replace it by a
biomechanical relationship.
The one relationship that should spring to mind most easily – because it is the
simplest – is the impulse–momentum relationship outlined in Appendix 2.1 (see
pages 77–8) and explained in more detail in Chapter 5. This is represented graphically in
Figure 2.6(b), in which the speed added on the board has been replaced by the take-off
impulse and the athlete’s mass (at the same level of the model). Take-off impulse
depends at the next level down on the mean force and the time on the take-off board.
This figure may not seem much of an advance on Figure 2.6(a) – the jumper’s mass
is no problem but observing take-off impulse, time on board or mean force is downright
impossible although, interestingly, all of these are easily measurable if the take-off
board is mounted on a force plate (see Chapter 5). In a way, this impossibility is
fortunate for the qualitative analyst, as this ‘branch’ of the model is a blind alley, down
which the quantitative analyst could easily wander. The reason is that this branch of the
model implies that we need to maximise both the mean force and the time on the board
to improve performance. Given the fast run-up of good long jumpers, trying to extend
the board contact time (about 15 ms) would be difficult at best; if feasible, it would
have a deleterious effect on the mean force. Fortunately we have escaped from this blind
alley simply by being unable to observe these features; that is not always the case.
A better approach altogether is to use the work–energy relationship outlined in
Appendix 2.1 (see page 78). This is shown in the new version of level 4 (and level 5) of
Figure 2.6 Level 4 of long jump model – factors affecting take-off speed: (a) initial model; (b) ‘blind alley’.
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