1st Joint ESMAC-GCMAS Meeting - Análise de Marcha

(Fig. 1). Three markers on the acromion and medial and lateral epicondyles tracked motion of
the humerus. Coordinates for these segments were formed consistent with ISB
recommendations [2]. A static (anatomic) pose is captured with the elbow fully extended, the
arm at neutral abduction and flexion, and the palm facing the subject’s thigh. The local
(humeral) coordinate system (HCS) is transformed into the clinical flexion/extensionab/adduction
coordinate system (CCS) for both the static and dynamic (activity) trials. The
CCS is comprised of the previously defined humeral ZH, and transformed x’- and y’-axes that
are defined as follows: (1) the x’-axis is taken to be the intersection of the humeral XHYH plane
and the thoracic XTZT plane, which is, by definition, perpendicular to the humeral z-axis; (2)
the y’-axis is the cross product of the humeral z- and x’- axes. Relative rotations are calculated
between the Thoracic and CCS, and helical-axis decomposition is used to decompose the
rotation matrices as described previously [3]. The components of the helical axis in the CCS
x’-, y’-, and z-axes will represent, respectively, the amount of clinical abduction, flexion, and
internal rotation for a given shoulder motion.
Results
Results of an able-bodied subject
performing five shoulder flexion/extension
cycles are shown (Fig. 2). Decomposition
of the angles in the HCS revealed 75
degrees of peak rotation about the HCS
XH-axis and 115 degrees of peak rotation
about the HCS YH-axis. Alternatively, the
measures of maximal abduction were
found to be around 42 degrees, while there
was a maximum 144 degrees of shoulder
flexion. The second approach more closely
represents the motion observed clinically.
While there was still some amount of
ab/adduction, this is due to the fact that the
subject was not constrained in their chosen
path during flexion/extension trials.
Discussion
While use of decomposition of the relative rotations of shoulder motion generated using the
HCS yields correct and useful information for the purposes of generating biomechanical
models, this data does not represent the motion in terms that are always meaningful in the
clinical setting. Use of the proposed CCS for decomposing shoulder motion transforms relative
motions from those occurring about humerus-fixed axes to axes that relate to the thorax are
more clinically relevant. Additionally, since this method uses helical-axis decomposition, there
is no need to alter rotation orders to compensate for the possibility of gimbal-lock due to either
prescribed motions or unanticipated compensations. While this method is presented using
relative motion of the humerus with respect to the trunk, it can easily be applied to examination
of glenohumeral motion as well.
References
[1] An, Kai-Nan, (1991), Journal of Orthopaedic Research, 9, 143-9.
[2] Wu, GE, (2005), Journal of Biomechanics, 38, 981-992.
[3] Morrow, DA, (2003), Gait & Posture, 18, S35-S36.
Figure 2. Kinematics from a subject
performing shoulder flexion trials for helicalaxis
decomposition of the motions in (A) the
HCS, and (B) the CCS.
- 105 -
A B