IS THE FEEDFORWARD CONTROL OF HEAD-TRUNK AXIS VERTICALICALITY, IN DANCERS; VISION-DEPENDENT ?

Abstract

During unilateral leg movements performed while standing, it is necessary to displace the center of gravity towards the other leg in order to maintain equilibrium. In addition, the orientation of particular segments, such as head and trunk, which are used as reference values for organizing the motor act, needs to be preserved. The coordination between equilibrium control and the ability to maintain the orientation of given segments (head, trunk) was previously studied in standing subjects instructed to raise one leg lateral to an angle of 45" in response to a light. Two sources of light placed at eye level indicated the side on which the movement was to be performed. Two control strategies were identified (Mouchnino et al.. 1992). An 'inclination" strategy was used by the naive subjects (n=S). This consisted of an external rotation of the body around the antero-posterior ankle joint axis; a counter-rotation of the head with respect to the trunk was observed, which ensured the stabilization in the horizontal plane of the interorbital line. A 'translation" strategy was used by the dancers (n=5! Here the external rotation of the leg around the ankle joint was associated to a feedforward counter-rotation of the trunk around the coxofemoral joint so that the horizontality of the interorbital line and the verticality of the trunk axis were maintained. This new coordination resulted from long term training and indicates that a new motor program has been elaborated. The present investigation was aimed at exploring the role of vision in the feedforward counter-rotation performed at the hl joint in dancers. Three visual no visual information (blurred vision). conditions were compared: eyes open, eyes closed an 'f' translucent goggles which provide It was shown that in dancers the counter-rotation at the hip joint and the external rotation of the ankle joint occur simultaneously under both conditions of perturbed vision as it was under normal condition (eyes open: 415ms +/-59). The counterrotation of the trunk preceded the inclination of the le towards the supporting side by 7.3ms (+/-46) with eyes closed and by -28ms (+/-119) with translucent goggles. The ankle rotation and the hip counter-rotation were hig hly correlated (r=0.95junder both perturbed conditions of vision (eyes closed and translucent go les) whereas in normal condition the correlation coefficient was lower (r=057). In addition, the maintenance of the shoulder line in the horizontal plane was also reserved as it was in condition eyes open (2' +/-1.2) with both eyes closed (1.5 +/-1.2 ) and translucent goggles (1.8" +/- 1.5). These results indicate that the maintenance of the vertical orientation of the head-trunk axis in dancers was not dependent on vision; other sensors such as otolith graviceptors in the head or possibly muscle proprioceptors of the trunk might be used to maintain the axis of the head-trunk vertical. Mouchnino L Aurenty R., Massion J., PedoMi A (1992) Coordination between equilibrium and head-trunk orientation during leg movement: a new strategy built up by training. J. Neurophysiol., in press.