The presence of movement at the SI joint has been increasingly investigated and discussed. Even though it has been contested in the past, the presence of movement at this articulation is now accepted. Since this joint must be relatively mobile, it is now considered that abnormal movement might be related to low back pain.
Biomechanical models have been developed recently in order to better understand the relationship between SI joints and low back pain. It appears that the sacrum mobility control might necessitate the action of pelvic girdle muscles. The erector spinae, gluteus maximus, biceps femoris and latissimus dorsi muscles would produce moments necessary to lock the SI joint via the thoraco-lumbar fascia. This mechanism would increase the compressive forces on the joints surfaces and provide greater stability to and a more efficient load transfer from the spine to the lower extremity. Therefore the aim of this study is to determine the effect of increased tension of the thoraco-lumbar fascia and hip extension muscles on sacrum mobility.
Ten male, weight-lifter subjects, aged between 18 and 25 participated in this study. A load displacement apparatus was developed in our laboratory. The subject is placed prone on a horizontal patient subject board or on a 30° triangular shaped board in order to produce tension on the thoraco-lumbar fascia. A total force of 250 N in 50 N increments is applied for every experimental situation.
An isometric contraction of hip extensors of more than 80% of maximal muscle force is produced in each experimental condition.Results demonstrate that the overall force/displacement ratio obtained in this study is inferior to the data obtained by Miller et al. (1987) but similar to Brunner et al. (1991) and Vleeming et al. (1992b) in cadaveric specimens. Generally, the movement range of motion increases significantly (p < 0.001) in relation to the level of force applied. In our weight-lifters, hip contraction reduced sacrum mobility significantly (p < 0.001). Furthermore, the efficiency of the hip extensors in reducing the movement is still significant at 250 N. Our results indicate that the thoraco-lumbar fascia does not seem to participate in the locking of the SI joint, although other biomechanical variables must be evaluated before concluding on its implication.
This study has demonstrated the efficiency of the experimental in vivo SI locking mechanism of the SI articulation.
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