The aim of this study was to check the combined and/or dissociated influences of time-of-day and sleep deprivation on postural control. Twenty subjects participated in test sessions which took place at 6:00 am, 10:00 am, 2:00 pm and 6:00 pm either after a normal night's sleep or after a night of total sleep deprivation. Postural control was evaluated by COP surface area, LFS ratio and Romberg's index. The results showed that postural control fluctuates diurnally according to three different periods, pronounced by sleep deprivation: (1) at 6:00 am, there was no modification by sleep deprivation; (2) at 10:00 am and 2:00 pm, an interaction effect was observed for COP surface area and LFS ratio after sleep deprivation. Values of COP surface area were significantly higher (P < 0.01) following the night of sleep deprivation than after the normal night's sleep (139.36 ± 63.82 mm² vs. 221.72 ± 137.13 mm² and 143.78 ± 75.31 mm² vs. 228.65 ± 125.09 mm², respectively); (3) at 6:00 pm, the LFS ratio was higher than during the two other periods (P < 0.001) whereas COP surface area decreased to the level observed at 6:00 am. At this time-of-day, only the LFS ratio was significantly increased (P < 0.05) by the night of sleep deprivation (0.89 ± 0.14 vs. 1.03 ± 0.30). This temporal evolution in postural control does not seem to be related to any deterioration in visual input as Romberg's index (150.09 ± 97.91) was not modified, regardless of the test session.

The principal goal of our study is to gain an insight into the coordinative structure of a complex body movement. As a first step, this paper describes the activity of multiple skeletal muscles associated with the drawing-like movements that resemble the Jeté, performed by skilled ballet dancers. The EMG activity of 18 muscles of the trunk, pelvis, and both legs was recorded when dancers standing on the left leg moved the toe of the right leg forward and backward along a straight line. A major finding is that the EMG activity of all right muscles, despite their functional and anatomical diversity, was minimised not only at the initial, vertical position but also in the reversal phase of movement when the moving leg was maximally deviated from the vertical position. In other words, the activity was minimal when torques of the weights of limb segments were minimal as well as when these torques were maximal. In contrast, in the static task when the maximally deviated leg position was maintained, there was substantial tonic activation of leg muscles, an activity that was necessary to balance these torques. The result is consistent with the hypothesis that movements of the body result from centrally induced changes in the muscle recruitment thresholds influencing the referent configuration of the body. The existence of minima in the overall EMG activity of skeletal muscles is not the only prediction of the referent configuration hypothesis. An immediate consequence of the hypothesis is that, in movements of the limb, the EMG patterns should be a direction-dependent phenomenon known as "directional tuning" of muscles. In combination with the principle of minimal interaction of neuromuscular system, the referent configuration hypothesis offers a dynamic approach to the problems of how control levels may guide multi-muscle and multi-joint systems without redundancy problems.

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