Three Phases of Shoulder ABDUCTION Movement
The first phase of abduction: 0-60″
The
muscles involved are essentially the deltoid and
the supraspinatus, which form functional couple at the level of the
shoulder joint. It is in this joint that the movement of abduction starts. This first
phase ends near 90′,when the shoulder’locks’ as a result of the
greater tuberosity hitting the superior margin of the
glenoicl cavity. Lateral rotation and a slight flexion of the humerus
displace
the greater tuberosity posteriorly and delay this mechanical block. Thus abduction
combined with 30′ flexion and taking place in the plane
of the scapula is the trlle physiological movement of abduction
The second phase of abduction: 60-120′
As
the shoulder is lockecl, abduction can only proceed
with participation of the shoulder girdle. The
movements afe these:. A’swing’of the scapula with anticlockwise rotation
(for the right scapula), causing the glenoid cavity to face more superiody.
The
range
of this movement is 60′. Axial rotation mechanically linked at the
stefno-costo-clavicular
and the acromioclavicular joints, each joint contributing up to
30′.
The
muscles involved in this second phase are these :
.
trapezius
.
serrants anterior
These muscles form a functional couple of abduction at the level of the scapulo-thoracic ‘joint’.This movement is checked at about 150″ (90′ + 60′ due to rotation of the scapula) by the resistance of the stretched adductors: latissimus dot”si and pectoralis majcn.
The third phase of abduction: 120-180″
To
allow the limb to reach the vertical position, movement
of the spinal column becomes necessary. If only one arm is abclucted,lateral
bending of the spinal column produced by the contfalateral spinal
muscles (6) is adequate. If both arms are
abductecl,
they can come to lie parallel vertically only by being maximally flexed.
For the vertical position to be reached, exaggeration of the lumbar lordosis
is necessary and this is achieved by the action of the spinal muscles.
This
division of abduction into three phases is, of collrse,
artiflcial; in fact, these various combinations of muscular movements fun into one
another. Thus it is easy to observe that the scapula begins to’swing’before
the arm has reached 90′ abduction; likewise, the spinal column begins to bencl
before
150″ abduction is reachecl.
At
the end of abcluction all the muscles are in a
state
of contraction.
