can be approximated with three rotations [18]
that are denoted with 1, 2, and 3 (Fig. 2a). These are
rotations of the humerus bone (i.e., upper arm) at the glenohumeral
rotation centre (i.e., shoulder joint). The elevation of
the humerus as it moves away from and toward the thorax
is 1 (i.e., flexion, extension, adduction, and abduction), the
cardinal direction of the humerus (i.e., north, south, east,
west) is 2 (i.e., transverse flexion, extension, adduction, and
abduction), and the internal/external rotation of the humerus2
is 3. Positions of the forearm relative to the humerus (i.e.,
at the elbow joint) can be approximated with two rotations
[18]: the flexion and extension at the elbow (Fig. 2b) that are
denoted with 1, and the axial rotation of the forearm (Fig. 2c)
that is denoted with 2 (i.e., pronation, supination).