4. Discussion
Our study compared body sway of clerical workers at the
beginning and at the end of a workday, using five common posturographic
parameters. Only two of those parameters differed
significantly between morning and afternoon; both probably
reflect the same postural process, since they could be reduced to a
single factor (F1). The three remaining parameters probably reflect
another postural process, since they could be reduced to another
single factor, orthogonal to the first (F2).
It should be noted that the parameters loading on F2 quantify
COP excursions about the mean COP position; since larger excursions
will shift COP closer to the point of losing balance, F2 is a
direct measure of postural stability. By contrast, the parameters
loading on F1 quantify COP movement irrespective of excursions
about the mean COP position; F1 is therefore not a direct measure
of postural stability. In light of these considerations, we conclude
that our participants used similar safety margins for posture in the
morning and in the afternoon, but moved their COP inside those
margins more in the morning than in the afternoon.
One possible interpretation of this finding is that postural control
was less effective in the morning, since the participants
expended more energy to attain the same stability criterion.
However, we favor an alternative interpretation. Dynamic systems
theory posits that self-organizing motor systems benefit from
movements through state space, which allow them to explore
alternative performance solutions [25e28]; if postural mechanisms
controlling the head, limbs, and trunk are considered as components
of such a self-organizing coordinative system, then greater
COP movement in the morning could indicate more exploration of
the state space, and thus a better starting point for postural responses
to unexpected perturbations. Accordingly, less COP
movement in the afternoon would be indicative of a less agile
system, reduced to postural maintenance without postural exploration.
This interpretation is supported by our finding that older
participants tended to produce less COP movement than younger
ones, i.e., less COP movement might be a sign of age-related decay.
Further support comes from the fact that COP movement was
negatively associated with self-rated fatigue1 (r ¼ 0.170; p < 0.05),
i.e., less COP movement was associated with higher rather than
lower levels of fatigue.
In conclusion, our data suggest no change of postural stability
over the workday of clerical workers, but a decrease of COP movement,
possibly due to a fatigue-induced reduction of exploratory
postural activity. One limitation of our study is that the effects of
fatigue modifiers such as caffeine, nicotine, drugs, accomplishments
and frustrations at work, as well as weather conditions were not