No abstract

The aim of this study has been to identify, in humans, the braking process underlying voluntary forearm movements performed at various velocities and amplitudes and against differential inertial loads. The procedure used to estimate the parameters of the braking process involved kinematic analysis of the movement and measurement of alpha-motoneural input to flexor and extensor muscles by recording electromyographic (EMG) activity. We have shown that when the agonist force does not exceed the passive viscoelastic tension developed by the extensor and flexor muscles, the movement can be braked by the viscoelastic forces alone. In contrast, above this force threshold, it was found that the motor output in agonist and antagonist muscles consists of well defined bursts of EMG activity, first in agonist then in antagonist muscles. The timing of these two bursts (duration of the agonist activity and onset of the antagonist activity) are clearly correlated with the value of the peak velocity. For the same peak velocity, the addition of inertial loads increase the excitation level of both agonist and antagonist muscles but does not change the timing of the EMG bursts. The discussion of these results focuses on the concept of "unit of movement" organized to reduce the large number of possible patterns of activation of the muscles acting on the same joint.

Adaptation of motor control to weightlessness was studied during a 7-day spaceflight. The maintenance of control of upright posture was examined during a voluntary raising movement of the arm and during the voluntary raising on tiptoe. In order to evaluate the contribution of visual cues, three types of visual situations were examined: normal vision, central vision, and without vision. On the basis of cinematographic and mechanographic data, the postural perturbations consecutive to the movement of a body part in conditions of weightlessness were found to be similar to those observed on earth. However, in weightlessness, in contrast to the ground-based situation, erectness of posture was maintained primarily due to the predominant contraction of the ankle flexor muscles. The sequences of postural leg muscle activity associated with the arm or foot movement were well structured and varied slightly in the course of the flight. In addition, the initial posture, that is the erect posture before the movement was executed, changed throughout the flight from an exaggerated oblique position to a terrestrial standing position. Visual information was preponderant at the beginning of the space mission for the recalibration of other sensory cues affected by weightlessness. The findings are indicative of two types of adaptation of the central program of posture regulation to weightlessness: fast, short-term adaptation, characterized by a quasi-instantaneous redistribution of motor commands between ankle flexors and extensors (an "operative process") and slow, long-term adaptation, exemplified by the loss of anticipatory activation of certain muscles by the end of the flight (a "conservative process").

Recent studies have highlighted interactions between state anxiety, sensory processing involved in motor performance, and personality traits such as trait anxiety. In the present study, we investigated the effects of moderate state anxiety on static balance performance with eyes open and eyes closed in two groups of healthy subjects with contrasting trait anxiety. We found that an anxiogenic condition induces larger and faster body swaying in both groups in the eyes-open test. This suggests that state anxiety could modify the processing of the different sensory inputs involved in balance control whatever the subjects' trait anxiety level. When vision is absent, precision of static balance control in individuals with intermediate level of trait anxiety - characterized by a higher visual dependence - is more disrupted than in low trait anxiety subjects. Moreover, moderate state anxiety seems to disturb the ability to use vestibular and/or somatosensory inputs in individuals with low anxiety, but not in individuals with intermediate trait anxiety. These results on inter-individual differences provide a first insight into the inter-dependence between trait anxiety, state anxiety and static balance control.