THE INFLUENCE OF SUDDEN PERTURBATIONS ON TRUNK MUSCLE-ACTIVITY AND INTRAABDOMINAL PRESSURE WHILE STANDING

Unexpected ventral and dorsal perturbations and expected, self-induced ventral perturbations were delivered to the trunk by suddenly loading a vest strapped to the torso. Six male subjects were measured for int ra-abdominal pressure (IAP) and intra-muscular electromyography of the transversus abdominis (TrA), obliquus internus abdominis (OI), obliqu us externus abdominis (OE) and rectus abdominis (RA) muscles. Erector spinae (ES) activity was recorded using surface electromyography. Disp lacements of the trunk and head were registered using a video-based sy stem. Unexpected ventral loading produced activity in TrA, OI, OE and RA, and an IAP increase well in advance of activity from ES. Expected ventral loading produced pre-activation of all muscles and an increase d IAP prior to the perturbation. The TrA was always the first muscle a ctive in both the unexpected and self-loading conditions. Of the two v entral loading conditions. forward displacement of the trunk was signi ficantly reduced during the self-loading. Unexpected dorsal loading pr oduced coincident activation of TrA, OI, OE, RA and ES. These results indicate a response of the trunk muscles to sudden expected and unexpe cted ventral loadings other than the anticipated immediate extensor to rque production through ES activation. It is suggested that the increa se in IAP is a mechanism designed to improve the stability of the trun k through a stiffening of the whole segment.