EMG RECORDINGS OF ABDOMINAL AND BACK MUSCLES IN VARIOUS STANDING POSTURES - VALIDATION OF A BIOMECHANICAL MODEL ON SACROILIAC JOINT STABILITY

In a biomechanical model we described that for stability of the flat s acroiliac joints (SIJ) muscle forces are required which press the sacr um between the two hip bones (self-bracing). Shear loading of these jo ints is caused by gravity and longitudinally oriented muscles. Protect ion against shearing can come from transversely oriented muscles like the internal oblique (OI) abdominal muscles. For validation we used st anding postures with significantly more or less OI activity compared t o activity in a standardized erect standing reference posture. OI acti vity decreased significantly when (a) resting on one leg (the contrala teral), as can be observed at bus stops, (b) tilting the pelvic backwa rd and (c) applying a pelvic belt. We explain this decrease of OI acti vity by, respectively, decrease of gravity load, decrease of load from the psoas major muscles, and a substitute of self-bracing. The outcom e of this study is in line with the biomechanical model on SIJ stabili ty. Clinical relevance of this study regards aspecific low back pain a nd is found in the effect of the use of a pelvic belt, of a trunk posi tion as adopted when wearing a small rucksack and of the benefit of ex ercising trunk muscles in extension and torsion. (C) 1998 Elsevier Sci ence Ltd. All rights reserved.