Adaptation of postural control to weightlessness

Abstract

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").

Contribution of ankle, knee, and hip joints to the perception threshold for support surface rotation

“…It is possible that the signal-to-noise ratio originating from this length-tension relationship activated the compensatory mechanism in order to keep the signal-to-noise ratio rather constant or close to its reference value. As suggested by several authors (Clement, Gurfinkel, Lestienne, Lipshits, & Popov, 1984;Lestienne & Gurfinkel, 1988;Massion, 1992;Paillard, 1990), with experience, subjects elaborate an internal reference based in part on information originating from gravity forces. It is possible that subjects integrate such a reference, not (or not only) by means of specific receptors such as the otoliths, but also by means of the dynamic signals originating from the muscles activated during body oscillations.…”

Postural muscle activity patterns during standing at rest and on an oscillating floor

“…Invariably, only the posterior muscles in the lower legs are activated to maintain quiet stance [1,18,19]. The relative positional relationship of body segments during this quiet stance is similar under conditions of weightlessness [7], suggesting that relatively stable mechanisms for posture maintenance exist. However, many previous studies have shown that remarkable individual variation exists regarding patterns of postural muscle activity in the trunk and thigh during quiet stance.…”

Modulation of cutaneous reflexes by load receptor input during human walking

“…First, from animal studies, it is known that the load-related input, derived from proprioceptive and cutaneous afferents, can induce reflexes, some of which act directly on the central pattern generator (Conway et al 1987;Duysens and Pearson 1980;Gossard et al 1994;McCrea 1998;Pearson 1995;Pearson et al 1992;Whelan 1996). Proprioceptive influences, for example body loading and unloading, have strong facilitatory effects on the electromyographic activity (EMG) of extensor muscles such as the gastrocnemius (GM) and soleus (SO), but only marginal effect on the flexor muscles both in cats (Gossard et al 1994;Pearson et al 1998;Whelan et al 1995) and humans (Clement et al 1984;Dietz and Colombo 1998;Dietz et al 1992).…”

“…Such interactions have been described for postural tasks. From earlier research on cats (Whelan 1996) and humans (Clement et al 1984;Dietz and Colombo 1998;Dietz et al 1992;Rossi and Decchi 1994), it has been shown that postural reflexes depend on body load. Interactions between body load and cutaneous reflexes in humans were investigated during standing (Rossi and Decchi 1994).…”