Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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Figure 3.21 Partitioning of variance (as mean squares, MS): (a) individual operator with markers; (b) individual
operator without markers; (c) group with markers; (d) group without markers.
predominant source of variance. Movement variability can, therefore, be assessed both reliably
and objectively in these conditions. Indeed, each human operator was not much more inconsistent
than auto-tracking, which was 99.99% consistent (and, therefore, reliable). Without
markers, however, the picture changed dramatically: true movement variability (across trials) is
now obscured by inter-operator (across people, Figure 3.21(d)) and intra-operator (across days,
Figures 3.21(b) and (d)) variance. Without markers, therefore, movement variability cannot be
assessed reliably or objectively; this is a dramatic finding for applied movement analysts, like me,
who have focused much of their research on performance in competition, where markers cannot
be attached to the performer.