RELIABILITY OF A KINEMATIC MODEL OF THE UPPER EXTREMITY
Kristof Kipp, B.S., Michelle Sabick, Ph.D., Mark DeBeliso, Ph.D.
Center for Orthopedic and Biomechanics Research, Boise State University,
Boise, Idaho, USA
E-mail: MSabick@boisestate.edu
Web: http://coen.boisestate.edu/cobr
INTRODUCTION
In an effort to expedite comparison and
exchange of upper extremity motion data,
the Standardization and Terminology
Committee of the International Society of
Biomechanics (ISB) has put forth
recommendations for the use of standardized
Joint Coordinate Systems for the shoulder,
elbow, and wrist (van der Helm et al, in
press). The purpose of this study was to
establish the reliability of joint angle
measurements using a simplified version of
the standardized JCS adapted for automated
motion capture using surface markers.
Measurements of isolated joint motions
determined using both manual goniometry
and the simplified JCS (sJCS) were
compared to determine if the sJCS would
produce joint angle measurements similar to
those commonly obtained clinically.
extremity tracked with an infrared motion
capture system (Vicon Motion Systems,
Lake Forest, CA).
The standardized JCS was adapted for
automated motion capture by placing
retroreflective markers on easily identifiable
surface landmarks (Fig. 1). The originally
proposed model was simplified by using the
acromion process to estimate the location of
the center of the humeral head. All other
landmarks coincide with the standardized
JCS and represent points that are used to
estimate joint centers. Euler angles based on
the sequences described in the standardized
JCS were used to compute shoulder, elbow,
and wrist joint angles for each movement.
METHODS
Five subjects performed isolated upper
extremity joint movements in the cardinal
planes of the body. Each subject performed
twelve different movements corresponding
to individual upper extremity joint rotational
degrees of freedom. Each motion was
performed twice. Total joint range of
motion in each trial was measured using two
techniques: (1) manually with a goniometer
in accordance with the American Academy
of Orthopedic Surgeon's guidelines (Greene
and Heckman, 1994), and (2), calculated
using the sJCS from the positions of
retroreflective markers on the upper
Figure 1. Surface marker locations.
The two sets of joint angle data were
analyzed for repeatability and agreement
using the methods of Bland and Altman
(1986). The aim of the study was to
determine if the sJCS angle measurements
using surface markers would produce
similar results to the clinical standard,
goniometry.
RESULTS AND DISCUSSION
Goniometry: The intra-observer interclass
reliability coefficient for all joints was
r≥0.99. The mean and standard deviation
(SD) of the intra-observer differences for all
joints were -0.23± 3.3°. 93% of the intraobserver differences were within ± 2 SD of
the mean difference.
sJCS: The interclass reliability coefficient
for all joints was r≥0.99. The mean and SD
of the differences for all joints was
-1.8± 3.4°. 94% of the differences were
within ± 2 SD of the mean difference.
Goniometry vs. sJCS: The interclass
reliability coefficient between the two
measurement techniques was r=0.95 (Fig.
2). The mean and SD of the technique
differences for all joints was 13.6± 12.1°.
98% of the technique differences were
within ± 2 SD of the mean difference.
These results show a high agreement
between the two joint measurement
techniques and suggest that the sJCS is
suitable for the measurement of human
upper extremity motion.
Goniometer Angle (deg)
180
The goniometer generally produced greater
range of motion values than did the sJCS
method. The one exception was internal
rotation of the humerus, where the sJCS
measurements were greater. The maximum
difference between techniques was 38° for
shoulder abduction and the minimum
difference was 1° for ulnar deviation.
Even though the acromion process was used
to estimate the glenohumeral joint center,
the shoulder angle measurements calculated
using the sJCS were still significantly
related to the goniometer measurements.
For many applications, this simplification
may be sufficiently accurate. Refining the
methods for locating the shoulder joint
center is a focus of our ongoing research.
SUMMARY
The simplified ISB Joint Coordinate System
shows promise for use in upper extremity
motion capture using surface markers. The
landmarks used are prominent, easy to
identify, and yield highly repeatable
measures. A better method for identifying
the shoulder joint center is still needed as the
calculations using the acromion process to
approximate the humeral head produced the
greatest measurement variability.
160
REFERENCES
140
120
100
r=0.95
80
60
40
20
0
0
20
40
60
80
van der Helm, F.T.C. et al. (in press). J.
Biomechanics
Greene, W.B., Heckman, J.D. (1994). The
clinical measurement of joint motion.
Amer. Acad. of Orthopaedic Surgeons.
Bland, J. M., Altman, D. G. (1986).
Lancet, 1, 307-310.
100 120 140 160 180
sJCS Angle (deg)
Figure 2. Correlation between goniometer
and sJCS measurements of joint angle for
all 13 upper extremity motions.
ACKNOWLEDGEMENTS
The authors would like to thank Kristin
Bovey for her invaluable support.