Original articleDoes reaching the back reflect the actual internal rotation of the shoulder?
Section snippets
Subjects
Seven male volunteers without any history of shoulder disease were enrolled in this study. Their mean age was 28 years (range, 25-33 years). Only the right dominant shoulders were studied.
Measurement device
By use of an electromagnetic tracking device (3Space Fastrak; Polhemus, Colchester, VT) and analyzing software (3Space-Win; MP Japan, Tokyo, Japan) (Figure 1), the position and orientation of the sensors relative to the source were recorded. There were 4 sensors, attached to the ulnar head, the lateral
Results
Repeatability coefficients are summarized in Table I. With regard to rotation measurements, the angle of shoulder internal rotation increased greatly between the hanging-arm position and the buttock position. At this single interval, internal rotation increased by a mean of 39.8°, which occupied 54.3% of the entire range of internal rotation (Figure 4). From the buttock to the sacrum, 11.7% of internal rotation occurred (mean, 8.6°). In total, 66% of internal rotation occurred from the
Discussion
The error of the electromagnetic tracking device was found previously to be less than 0.5 mm for position and 0.5° for angulation.7 With the device attached to the surface of the skin, there may be greater error. To evaluate the error of this measurement system, we used the coefficient of repeatability, which is defined as 2 SDs of the differences between repeated measurements.1 The coefficients of repeatability varied among the individuals, but they were small enough (<12% of measured values)
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