Influence of a variation in the position of the arms on the sagittal connection of the gravity line with the spinal structures.

PURPOSE

To evaluate the influence of the position of the arms on the location of the body's gravity line.

PREVIOUS DATA

The sagittal balance of the pelvi-spinal unit is organized so that the gravity line is localized in a way that limits the mechanical loads and the muscle efforts. This position of the gravity line was analyzed in vivo, in standing position, the arms dangling, by the barycentremeter, a gamma rays scanner. Then, several teams had the same purpose but using a force platform combined with radiographies. Their results differed significantly among themselves and with the data of the barycentremetry. However, in these studies, the positions of the arms varied noticeably, either slightly bent forwards on a support, or the fingers on the clavicles or on the cheeks.

METHODS

We estimated, for each varied posture of the arms, the sagittal coordinates of the masses of the upper limbs and their influence on the anatomical position of the gravity line of the whole body.

RESULTS

Using a simple equation and the data of the barycentremeter, we observed that the variations in the location of the gravity line were proportionally connected to the changes of the sagittal position of the mass of the upper limbs induced by the various positions of the arms.

CONCLUSIONS

We conclude in a validation of the data of the barycentremeter, as well as of the data obtained by the force platforms as long as the artifact of the position of the arms is taken into account.