Introduction to Sports Biomechanics: Analysing Human Movement ...

QUALITATIVE ANALYSIS OF SPORTS MOVEMENTS
Let us start at the left branch of the model in Figure 2.12 and work to the right. We
have already identified ‘fast run-up speed’ as a critical feature (CF1) for this event.
We have already noted that maximising the mean force and the acceleration path are
desirable to maximise take-off speed (but see below), so we now need only to translate
these terms into things we can observe. The mean forces are maximised by the jumper
maximising force generation (Figure 2.13). This can be done in three ways: directly, by
a fast and full extension of the take-off leg (CF2) increasing the force on the take-off
board and, indirectly, by a fast, high and coordinated swing of the free leg (CF3) and
the two arms (CF4). If the indirect contributions are not clear, refer to Figures 1.21 and
1.22 (pages 24–5), in which the normal and model swings of the arms in the standing
vertical jump both increased jump height.
Moving on to maximising the vertical acceleration path, we first note that this is
expressed as the difference between the heights of the athlete’s centre of mass at take-off
and touchdown. The jumper can achieve a high centre of mass at take-off by a combination
of critical features 2 to 4 (CF2–4). A lowish centre of mass at touchdown
might suggest a pronounced flexing of the knee at touchdown. Although knee flexion
will occur to some extent and this will reduce impact forces and thereby injury risk, it
would be a mistake for the jumper to try to increase this flexion – it would lower the
centre of mass height at touchdown but have far more important and deleterious effects
on the take-off speed. A mechanism that good long jumpers tend to use to lower the
centre of mass at touchdown is a lateral pelvic tilt towards the take-off leg. This is clearly
evident from a front-on view and illustrates two important points for a successful
qualitative movement analysis: know your sport or event inside out and never view a
sporting activity just from the side, even when it seems two-dimensional.
Now let’s consider the horizontal acceleration path. This is more tricky because, in
the first part of board contact, until the centre of mass has passed forward of the support
foot, the horizontal velocity will be decelerating, not accelerating. The last thing the
jumper would want to do is to plant the take-off foot too far ahead of the centre of
mass, which would increase the deceleration of the centre of mass – yet another blind
alley. Instead, the jumper minimises this distance by seeking an ‘active’ landing – one in
which the foot of the take-off leg would be moving backwards relative to the take-off
board at touchdown – to reduce the horizontal deceleration (CF6). Then, once the
centre of mass has passed over the take-off foot, the jumper needs to lengthen, within
reason, the acceleration path of the centre of mass up to take-off, which can be done by
taking off with the centre of mass ahead of the foot (CF7). This also serves to minimise
any tendency for the take-off distance (Figure 2.4) to be negative.
Well, we have finished with take-off velocity, which covers both take-off speed and
angle. Take-off height (level 3 of the model, see Figure 2.5) is mainly dealt with by
critical feature 4 (CF4), which has now covered flight distance from level 2 of the long
jump model – although we still need to look at the landing component of the take-off
height, which we will do in the next paragraph – leaving us with the take-off and
landing distances.
From Figure 2.4, it should be obvious that the take-off distance is the distance of the
centre of mass in front of the take-off foot at take-off minus the distance of the take-off
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