MUSCULAR AFTER-CONTRACTION AND ONGOING POSTURAL REACTIONS IN STANDINGAND 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 s ituations: eyes open or closed. EMG of trapezius and latissimus dorsal is and 3D kinematic recordings of the left scapula were analysed. The release of the long-lasting sustained isometric contraction at the lev el of the scapula produced a muscular after-contraction consisting of involuntary muscular contraction associated with a trunk movement simi lar to the unroll of a spiral. The unroll of the spiral is in the oppo site direction when we compare the standing and sitting situations. We suppose that the muscular after-contraction reveals the activity of c entral 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 o f these involuntary movements. (C) 1998 Elsevier Science Ireland Ltd. All right reserved.