Introduction to Sports Biomechanics: Analysing Human Movement ...

QUANTITATIVE ANALYSIS OF MOVEMENT
Once the field of view has been set, the focal length of the lens must be kept
constant – this means that the auto-focus option available on most digital video
cameras must be switched off.
For particularly long movements, such as the long and triple jumps, consideration
should be given to using two or more cameras to cover the filming area (Figure 4.5);
with digital video cameras, which do not allow genlocking, another method for
event and time synchronisation will then be needed, as described in the next section.
The movement plane should be perpendicular to the optical axis of the camera. This
requires careful attention and can be done in various ways, including the use of laser
levelling devices, spirit levels, plumb lines and right-angled (3–4–5) triangles. If the
movement takes place in a vertical movement plane on a level horizontal surface,
this requirement means that the optical axis of the camera must be both horizontal
and at 90° to the vertical movement plane.
Horizontal and vertical length scales – two 1-m rules are often adequate – and a
vertical reference, for example a plumb line, must be included in the field of view.
The length scales must be positioned in the plane of motion. Their lengths should
be at least that required to give a scaling error, when digitised, of no more than 0.5%
of the field of view. Other means of performing scaling include the use of objects
resembling chequerboards – if large enough, these would allow calibration and
accuracy checks across the whole field of view. This is rarely practical if filming in
sports competitions, and is unnecessary providing strict experimental procedures are
adopted throughout.
The background should be as uncluttered as possible, plain and non-reflective. In a
laboratory study, contrasting performer and background colours can be helpful.
The use of colour contrast markers on the appropriate body landmarks is sometimes
recommended (see also Box 6.2). These correspond to an axis through the appropriate
joint centre, as seen from the camera, when the performer is standing in a
specific position and posture: these markers are sometimes referred to as ‘joint
centre’ markers. The use of such markers is not normally possible in competition,
and has some potential disadvantages when digitising (see below). If using skin
markers, then marking the skin directly is often less problematical than using
adhesive markers, which can fall off. Very small markers consisting of a miniature
light and battery usually show up very well on video, but like other adhesive markers
can fall off.
To reduce the risk of marker detachment, and to help cope with segmental movement
out of the photographic plane, bands can be taped around a segment at the
landmarks used to identify the ‘joint centre’. This works well for the wrist, elbow,
knee and ankle joints but is not possible for some other joints, such as the shoulder
and hip.
If axes of rotation are being estimated from anatomical landmarks, skin markers are
not essential, although their use probably saves time when digitising. For standard
placement of markers for a two-dimensional study, see Table 5.1 and Box 6.2.
A sufficiently high sampling rate should be used; 25 Hz is usually adequate for
swimming; 50 Hz is often adequate for activities such as the tennis serve, if the
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