The variability of shoulder motions recorded by means of palpation

OBJECTIVE: The objective of this methodological study is the quantification of the sources of variability in the recorded three-dimensional motions of the shoulder mechanism for comparative purposes. BACKGROUND: The palpation and subsequent digitization of skeletal landmarks of the shoulder mechanism is a non-invasive and relatively easy method to quantify shoulder orientations. Comparison of individual motions is subject to the accuracy of the palpation method, the magnitude of kinematic redundancy of the shoulder mechanism and inter-subject differences in morphology and physiology. Quantification of these sources of variance, i.e. the palpation error, motoric noise and inter-subject differences, demonstrated the accuracy of the method and the potential validity of the descriptive motion parameters, e.g. Cardan angles, in intra- and inter-individual studies for clinical, ergonomical and biomechanical studies. METHODS: The orientations of the shoulder bones were recorded five times for each of five subjects by palpation and digitization of 12 skeletal landmarks for 10 equidistant arm elevation postures in the scapular plane. The orientations were described by means of Cardan angles. The palpation error was determined at a standardized initial rest position and expressed by Cardan angles for each recorded posture. Adding motoric noise and inter-subject differences gave the inter-individual variance. RESULTS: The palpation error was approximately 2 degrees. The major recorded variance originated from motoric noise (+/-33%) and inter-subject difference (+/-55%). CONCLUSIONS: The palpation method is an accurate means of recording the three-dimensional orientations of the shoulder mechanism and for intra-individual studies. However, inter-subject variability is large.