Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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are ethical considerations about video recording, particularly with minors and the
intellectually disadvantaged. Our systematic observation strategy should have addressed
both what to focus on and how to record, and observe, the movements of interest.
Clearly, we should focus on the critical features of the movement identified in stage 1,
but we need to prioritise these. Secondly, we need to decide on the environment in
which to videograph, the best camera locations within that environment and how many
trials of the movement to record for analysis.
Prioritising critical features can vary with the skill of the performer, the activity being
analysed, and whether a movement-phase approach (Appendix 2.2) is used, as in the
long jump example later in this chapter (pages 62–71). Our prioritising strategy
might, for example, put the critical features in descending order of importance for the
performance outcome; or work from the general to the specific, for example from
the whole skill to the role of the trunk and the limbs; or focus on balance, in skills in
gymnastics.
The other main issues in videography for qualitative analysis are:
Choice of camera shutter speed.
Where to conduct the study.
Choice of camera locations (sometimes, particularly in North America, called vantage
points) and whether the cameras are to be stationary (usually mounted on
tripods) or moved to follow the analysed movements.
How many trials to record, when relevant.
Use of additional lighting, which must be adequate for the shutter speed and frame
rate. The latter is normally fixed for ‘domestic-quality’ video cameras, at 50 fields per
second in Europe and 60 in North America, or 25 frames per second in Europe and
30 in North America (the unit hertz, Hz, is normally used for events per second).
Who and what to observe.
The background should be plain and uncluttered to help objective observation, but
this is not always feasible, particularly when videographing in competitions.
Participant preparation – briefing, clothing, habituation, debriefing.
Size of the performer on the image – the bigger the better, but this might require
zooming the camera lens (assuming that your camera has a zoom lens) while also
panning and tilting the camera during filming.
Checks for reliability (within, or intra-, observer) and objectivity (among, or inter-,
observer) in any study.
So, let us now look at these points in a little more detail. The shutter speed is the
time that the camera shutter stays open for each ‘picture’ that the camera records. If too
slow, the picture will blur; if too fast for the lighting conditions, the picture may be too
dark. A guide to the slowest satisfactory shutter speeds is given in Table 2.1. Not all
digital video cameras for the domestic market will have the fastest of these shutter
speeds, and do regard with suspicion the ‘sports’ option that cheaper cameras tend to
use rather than a range of shutter speeds. We should also note that field rates, also
known as sampling rates, of 50 or 60 Hz are far from ideal for the fastest activities in