V.S. Gurfinkel

Postural control in weightlessness: a dual process underlying adaptation to an unusual environment.

Abstract The control of posture is a well-known example of an extraordinarily complex anti-gravitational system. How does the brain organize the combination of the multibody joints necessary to maintain stably an erect two-legged body? Studies of postural activity in weightlessness offer effective means to formulate and develop theories about what underlies this sensorimotor control. Based on postural studies performed during prolonged space flights, this article will review the most pertinent results suggesting the existence of a dual system (conservative and operative) underlying the process of postural control in an unusual environment.

Anticipatory neck muscle activity associated with rapid arm movements

This study reveals the existence of a backward acceleration of the head prior to the onset of voluntary raising movements of the upper limb. This backward acceleration is produced by the displacement of the head-trunk as a whole. The anticipatory head movement is organized according to a sequence of activation and desactivation of the neck muscles, time locked with the anticipatory leg muscle activity. These findings highlight the existence of a complex postural behavior selected in advance of movement. It is proposed that the feedforward type of neural control of neck muscles should not be interpreted as a compensation to postural perturbation. This anticipatory process might play an important role in the widespread postural fixation of the cervical and dorsal spine.

Human equilibrium on unstable support: the importance of feet-support interaction.

Healthy humans maintained equilibrium on rocking supports (seesaw) of different curvatures and heights. We recorded platform tilt, horizontal displacements of the upper body, ankle joint angle and activity of ankle joint muscles. Subjects maintained balance by making seesaw rotations placing the support under the bodys centre-of-gravity. Forward displacement was balanced by compensatory plantariflexion: thus the relation between muscle activity and ankle joint angle differed from that on a rigid floor. Mechanical analysis of stability showed that standing on low seesaws requires ankle torque increase during forward body shift (as on a rigid floor) and torque decrease on high seesaws (when the seesaw height exceeded its radius). In the latter case, balancing was impossible with eyes closed. The results suggest that directionally specific torque changes in response to centre-of-gravity shifts provide important information for maintenance of orthograde posture.

Is there an effect of weightlessness on mental rotation of three-dimensional objects?

We studied the performance of eight cosmonauts in a mental rotation paradigm with simultaneously presented perspective views of three-dimensional objects. The cosmonauts were tested successively on earth, in microgravity aboard the Russian MIR station and again on earth. Their performance was compared to performance of a control group of five subjects tested on earth on the same dates. We particularly tried to disambiguate the effect of microgravity, procedural bias and practice. Our results show that the microgravity did not alter the mental rotation process. The performance of cosmonauts increased with practice, similarly to the performance of control groups subjects suggesting that the weightlessness did not impair implicit learning as well. Finally, we propose an explanation of previous contradictory results.

Muscular after-contraction and ongoing postural reactions in standing and sitting humans

Muscular after-contraction (MA-C) and the ongoing postural reaction of the body was studied in standing and sitting subjects in two visual situations: eyes open or closed. EMG of trapezius and latissimus dorsalis and 3D kinematic recordings of the left scapula were analysed. The release of the long-lasting sustained isometric contraction at the level of the scapula produced a muscular after-contraction consisting of involuntary muscular contraction associated with a trunk movement similar to the unroll of a spiral. The unroll of the spiral is in the opposite direction when we compare the standing and sitting situations. We suppose that the muscular after-contraction reveals the activity of central tonigenic structures in evoking involuntary trunk movements in humans and stresses the importance of the initial postural situation (standing or sitting) and the visual condition in the characteristics of these involuntary movements.

Influence of graviceptives cues at different level of visual information processing: The effect of prolonged weightlessness

We evaluated the influence of prolonged weightlessness on the performance of visual tasks in the course of the Russian-French missions ANTARES, Post-ANTARES and ALTAIR aboard the MIR station. Eight cosmonauts were subjects in two experiments executed pre-flight, in-flight and post-flight sessions. In the first experiment, cosmonauts performed a task of symmetry detection in 2-D polygons. The results indicate that this detection is locked in a head retinal reference frame rather than in an environmentally defined one as meridional orientations of symmetry axis (vertical and horizontal) elicited faster response times than oblique ones. However, in weightlessness the saliency of a retinally vertical axis of symmetry is no longer significantly different from an horizontal axis. In the second experiment, cosmonauts performed a mental rotation task in which they judged whether two 3-D objects presented in different orientations were identical. Performance on this task is basically identical in weightlessness and normal gravity.

Does gravity play an essential role in the asymmetrical visual perception of vertical and horizontal line length

The eye perceives the length of vertical and horizontal lines with an inherent asymmetry. A vertical line having the same length as a horizontal one is usually perceived to be longer. In this experimental investigation we tested the hypothesis that gravity has a direct role in producing the observed perceptual asymmetry. To this end we performed experiments in weightlessness during long-orbital space flights onboard the MIR station. Subjects performed a psychophysical task in which the length of a visually-presented vertical line was adjusted to match the length of a horizontal reference. On Earth, almost all subjects produce errors in adjusting the length of the vertical line, consistently under-estimating the length of the horizontal reference. The asymmetry of perception of the line lengths persisted in weightlessness. From these results we conclude that the phenomena of asymmetry of perception of the lengths of vertical and horizontal lines is not dependent on gravity, but is instead defined by properties of the system of internal representation. Grant numbers: 99-04-48450.

Muscle resistance to slow ramp weakly depends on activation level

The mechanical response of human m. flexor pollicis longus to slow (3.2 degrees/s) linear stretch by 5.5 degrees was measured during sustained (45-60 s, 9-13.5 p.p.s.) unfused tetanus evoked by electrical stimulation. The stiffness increased during unfused tetanus. At the late phase of unfused tetanus it was 1.8 +/- 0.2 (mean +/- S.D.) times greater than at the early phase. The sensitivity of the isometric tension level to a short change in a stimulation frequency also increased. At the late phase of unfused tetanus force oscillations increased 1.2 +/- 0.2-fold during slow stretch or shortening and immediately reached a smaller amplitude after the cessation of length change. This was probably related to the friction and thixotropy in muscles. Muscle resistance to slow ramp depended only weakly on activation level. In the late phase of unfused tetanus the stiffness per unit force was 1.5 +/- 0.4 times greater at 9-13.5 p.p.s. than at 20-25 p.p.s. Thus, the relative value of muscle stiffness was greater for smaller activation levels typical for maintenance of posture. The enhancement of muscle stiffness during sustained unfused tetanus and a weak stiffness dependence on the activation level indicated a non-additivity of processes occurring in active muscle.