SACRUM LOAD-DISPLACEMENT BEHAVIOUR AND SI LOCKING MECHANISM IN WEIGHT LIFTERS

Abstract

INTRODUCTION: The presence of movement at the SI joint has been increasingly investigated and discussed in the past, and even though it has been contested, the presence of movement at this articulation is now accepted. Furthermore, it has been shown that abnormal motion at the SI modifies performance in weightlifters. To better understand the relationship between SI joints and function, models were developed: The Erector spinae, Gluteus maximus, Biceps femoris, and Latissimus dorsi muscles form a kinetic chain permitting load transfer via the different layers of the thoracolumbar fascia. This mechanism is thought to increase the compressive forces, much as a pelvic belt might do, on the SI joint surfaces and provide greater pelvic stability during activity. We challenged this concept by 1) evaluating the capacity of the hip extensors to lock the SI; 2) inducing tension in the thoracolumbar fascia by placing the subjects in a hip flexion position. METHODS: 10 experienced weight lifters aged between 18 and 25 years old took part in the study. Subjects were first placed prone on a flat surface and again on a triangular support structure producing 30o hip flexion. A previously described loaddisplacement apparatus was used to apply a total of 250 N in 50 N increments on S1 and then on S5. The loads were applied 1) with muscles relaxed and at a minimum of 80 % maximal voluntary isometric contraction of the hip extensors. RESULTS: Generally, the range of motion increased significantly as the load applied increased. There was a significant difference in the results between movement in nutation and contra-nutation (p<0.05). MVC of hip extensors reduced the sacrum mobility significantly (p<0.001) in both positions. Furthermore, the locking efficiency was still significant at loads of 250 N. On average, the MVC reduced the movement by 41%, and there was no difference in the efficiency in relation to position. There was no significant difference between the sacrum movement with the patient in a neutral or flexed position for either nutation or contra-nutation and this tendency was not influenced by the different loads applied. CONCLUSION: In our experiment, at 30 o of hip flexion, the thoraco-lumbar fascia does not contribute to the SI locking mechanism. This study has demonstrated that MVC of the hip extensors in weight lifters significantly limits movements of the sacrum. Medical staff, trainers and coaches involved in weight lifting and other physical activities are encouraged to consider specific hip extensor exercises with the objective of increasing the SI locking and potentially preventing injuries. Finally, muscular synergy studies in symptomatic athletes may in some instances indicate an inefficient SI locking mechanism. These studies could provide interesting approaches to the treatment of individuals suffering from SI hypermobility.