Introduction to Sports Biomechanics: Analysing Human Movement ...

CAUSES OF MOVEMENT – FORCES AND TORQUES
DETERMINATION OF THE CENTRE OF MASS OF THE HUMAN BODY
We have seen in the previous sections that the most important application of the
laws of linear motion in sports biomechanics is in expressing, completely, the motion of
a sports performer’s centre of mass, which is the unique point about which the mass
of the performer is evenly distributed. The effects of external forces upon the sports
performer can, therefore, be studied by the linear motion of the centre of mass and
by rotations about the centre of mass. It is often found that the movement patterns
of the centre of mass vary between highly skilled and less skilful performers, providing
a simple tool for evaluating technique. Furthermore, the path of the centre of mass
is important, for example, in studying whether a high jumper’s centre of mass
can pass below the bar while the jumper’s body segments all pass over the bar,
and how high above the hurdle a hurdler’s centre of mass needs to be just to clear the
hurdle.
The position of the centre of mass is a function of age, sex and body build and
changes with breathing, ingestion of food and disposition of body fluids. It is doubtful
whether it can be pinpointed to better than 3 mm. In the fundamental or anatomical
reference position (see Chapter 1), the centre of mass lies about 56–57% of a male’s
height from the soles of the feet, the figure for females being 55%. In this position, the
centre of mass is located about 40 mm inferior to the navel, roughly midway between
the anterior and posterior skin surfaces. The position of the centre of mass is highly
dependent on the orientation of a person’s body segments. For example, in a piked
body position the centre of mass of a gymnast may lie outside the body.
Historically, several techniques have been used to measure the position of the
centre of mass of the sports performer. These included boards and scales and
manikins (physical models). They are now rarely, if ever, used in sports biomechanics,
having been superseded by the segmentation method. In this method, the
following information is required to calculate the position of the whole body centre of
mass:
The masses of the individual body segments, usually as proportions of the total body
mass or as regression equations.
The locations of the centres of mass of those segments in the position to be analysed.
This requirement is usually met by a combination of the pre-established location of
each segment’s centre of mass with respect to the end points of the segment, and the
positions of those end points on a video image, for example the ankle joint in Figure
5.13(a). The end points can be joint centres or terminal points and are estimated in
the analysed body position from the camera viewing direction (Figure 5.13). The
anatomical landmarks used to estimate joint centre positions are shown in Table 5.1
and Box 6.2. It is worth noting in Figure 5.13(b) that even for this apparently
sagittal plane movement in which the markers were carefully placed according to
Table 5.1 and Box 6.2 with the lifter in the fundamental reference posture but with
the palms facing backwards as when holding the bar, some markers no longer exactly
overlay joint axes of rotation.
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