Comparison between tripod and skin-fixed recording of scapular motion

Non-invasive dynamical measurements of 3D scapular motion can be performed easily by attachment of a 6 DOF electromagnetic receiver onto the skin above the acromion. To quantify the introduction of possible errors due to skin displacement, we assessed 3D scapular positions on n = 8 subjects by both tripod and skin-fixed method. Error analysis included the variables method (tripod, skin-fixed simultaneously with tripod, separate skin-fixed at 0 and 0.25 Hz of elevation speed), plane of elevation (0° and 90°) and observation (receiver replacement: n = 3). Inter-individual 'group' differences depended on elevation plane and showed an average underestimation of scapular rotation of 6.5° (worst case 13°) using the skin-fixed method. Only the group RMSE, not the individual RMSE, could be successfully lowered using linear regression (to about 2°). Inter-trial reliability (RMSE <3.24°, ICC>0.94) and RMSE between 0 and 0.25 Hz recordings (about 2.5°) were satisfactory. Intra-observer RMSE after replacement of the skin-fixed receiver was 5°. The skin-fixed method is suitable for dynamic recordings of scapular rotations; however, measurements are precise only when the acromion receiver is not replaced. Combined with a relatively low accuracy, we conclude that the skin-fixed method should be used only in combination with tripod 'calibration'.