Conclusions about normal and pathologic shoulder motion are frequently made from studies using skin surface markers, yet accuracy of such sensors representing humeral motion is not well known. Nineteen subjects were investigated with flock of birds electromagnetic sensors attached to transcortical pins placed into the scapula and humerus, and a thermoplastic cuff secured on the arm. Subjects completed two repetitions of raising and lowering the arm in the sagittal, scapular and coronal planes, as well as shoulder internal and external rotation with the elbow at the side and abducted to 90°. Humeral motion was recorded simultaneously from surface and bone fixed sensors. The average magnitude of error was calculated for the surface and bone fixed measurements throughout the range of motion. ANOVA tested for differences across angles of elevation, raising and lowering, and differences in body mass index. For all five motions tested, the plane of elevation rotation average absolute error ranged from 0-2°, while the humeral elevation rotation average error ranged from 0-4°. The axial rotation average absolute error was much greater, ranging from 5° during elevation motions to approaching 30° at maximum excursion of internal/external rotation motions. Average absolute error was greater in subjects with body mass index greater than 25. Surface sensors are an accurate way of measuring humeral elevation rotations and plane of elevation rotations. Conversely, there is a large amount of average error for axial rotations when using a humeral cuff to measure glenohumeral internal/external rotation as the primary motion.
Bibliographical noteFunding Information:
This manuscript was supported in part by NIH Grant K01HD042491 from the National Institute of Child Health and Human Development . The content is solely the responsibility of the authors and does not necessarily represent the views of the National Institute of Child Health and Human Development or the National Institutes of Health. Study sponsors had no involvement in the data analysis, interpretation of the data, writing of the manuscript or the decision to submit for publication. The authors would like to thank Cort J. Cieminksi, Ph.D., PT, ATC, CSCS; Daniel R Hassett, PT; Fred Wentorf, Ph.D.; Ed Gonda; Michael McGinnity, RN; and Kelley Kyle, CST/CFA for their assistance with completing various aspects of data collection and analysis for this project.
- Skin motion error