Many studies have already looked at the time-of-day dependence of postural parameters [1, 2] and the effect of physical activity on human postural control, see  for a review. Even if typically dual-task performances have been extensively studied (evidencing that quiet standing requires cognitive resources, [4, 5]), the novelty of the present experiment resides in studying the effects of a psychomotor vigilance test prior to the balance tests in young people. Apart from the fact of questioning the clinical utility of posturography , the reliability of various postural measurements (derived from the center of pressure – COP)  and the subsequent methodological recommendations , the determination of confounding variables (aging, gender, sleep deprivation…) (e.g. [9, 10]) when assessing postural balance in people is a major matter to be investigated. If postural instability increases as a function of (demanding physical and/or cognitive) activities prior to postural measurements, the relevance for using (clinical) information coming from the evaluation of postural stability should have to be raised. In this respect, the confounding variables must be clearly identified and controlled. Thus the purposes of the present study are twofold: first, it aims to confirm time-of-day effects on postural stability [1, 2] for young subjects; secondly, it explores the impact of an induced-experimental activity (i.e. an exhaustive physical exercise or a sustained-attention task or no specific activity) prior to the assessment of postural stability. To our knowledge, no study has yet investigated these potential confounding variables at the same time.
Among the numerous well-documented confounding variables when performing force plateform balance tests, two important categories of systems are of overriding interest: the neuromuscular system (and fatigue due to muscular exercise) and the cognitive system (and mental fatigue due to a demanding sustained-attention task). Concerning the effects of neuromuscular constraints on postural control, a considerable number of studies have demonstrated that both general and local exercises (such as walking, running, cycling, the repetition of simple segmental movements, or maintenance of isometric (or dynamic) muscular actions) contribute to alter the effectiveness of sensory inputs and motor output of postural control (see , for a recent review). The duration of postural disturbance after general (or local) muscular exercise was found to be relatively short (about 10–20 min.) [11–13]. Additionally, according to the context of muscle fatigue, different compensatory postural strategies (e.g. sensory compensation, ) have been identified to counteract or limit the balance control impairment. For example, Simoneau et al. confirmed the immediate decrement of the postural performance (COP displacements) measured just after moderate fatigue (experimentally induced by three periods of fast walking on a treadmill), but their participants were able to quickly compensate for this effect of moderate fatigue by a higher cognitive resources investment necessary to maintain active postural control [5, 15].
With respect to cognitive constraints, consistent findings underlined the importance of considering the interplay of cognitive load (or attentional resources investment) and stability of motor performance [16–18]. Specifically, if postural control has often been thought of as an automatic- or reflexively-controlled task, clear evidence of attentional processing requirements to postural control have been discovered (e.g. [19, 20]). Using typically the dual-task methodology, numerous studies have demonstrated that attentional demands associated with balance control have a dependence on the complexity of the dual postural tasks [5, 21]. In more challenging conditions (such as standing on a foam surface with eyes closed – no regulation of visual inputs is available), postural tasks are more cognitively demanding [4, 14]. More precisely, the extent to which the performance on either task declines indicates the extent to which the two tasks share attentional resources, as evidence of cognitive and specifically attentional allocation deficits. In the logic of this attentional depletion, we thus emphasise the impact of cognitive fatigue due to a sustained-attention task  experimentally induced before postural performance measurements. Recent studies have showed that the performance of demanding tasks during extensive periods of time leads to cognitive resource allocation deficits and a decrease in subsequent performance [18, 23, 24].
Apart from the effects of attentional resource depletion over time on motor performance stability , we also aim to isolate any time-of-day effect on postural control (e.g. ), a clinical factor to be controlled when assessing postural balance . For example, Gribble et al. tested young people (mean age: 21.8) on static and dynamic postural tasks at 10:00, 15:00, and 20:00. Overall, their results indicate that performance in postural control tasks was better in the morning than in the afternoon or evening. With older adults, Jorgensen et al. have recently confirmed the importance for controlling the time of day, with a main significant variation of postural balance between 9 a.m., 12:30 p.m., and 4 p.m. (poorer balance control in the afternoon).
Aims of the study and hypotheses
In this experiment, we aimed to characterize the postural balance of young participants as a function of the induced-experimental activity prior to the static postural measurements. We hypothesized that an exhaustive exercise and a high attention-demanding task should result in alterations of postural control. Concerning the time-of-day variable, we suggested that the well-documented effects of afternoon on postural control could be intensified due to the previous specific activities.