The Scapular Contribution to the Amplitude of Shoulder External Rotation on Throwing Athletes - Technologies for Medical Sciences - page 223

1.1.2 The Shoulder Girdle Structure and Function

The shoulder girdle is a morphofunctional unit composed by the scapula and
the clavicle bones, resting on the thorax. Scapula and clavicle are connected via
acromioclavicular joint. Both bones are linked to the thorax via sternoclavicular
joint and the functional scapulothoracic joint. In this context, the thorax acts as
a stable base for the movements of the upper limb. Together the thorax and the
shoulder girdle form a closed kinematic chain mechanism with some degree of inter-
dependence. As consequence, the shoulder girdle moves with respect to the thorax
at the same time that is used as a stable base for muscles acting on the humerus.

1.2 Overhead-Throwing Athletes

Thrower athletes also called overhead-throwing athletes include throwers (e.g.
baseball pitchers), swimmers, water polo, handball, and volleyball players. From
a functional standpoint, these sports require repetitive overhead motions, which are
discontinuous and ballistic in nature, and where the throwing arm is forced forward
from maximal external to near maximal internal rotation, while the arm is kept in
an elevated position.
Kinematics of the throwing arm motion (with ball) is frequently described as
a particular sequence of phases, the “ throwing cycle ”[ 1 , 21 ], that includes the
initial and late cocking phases, where the arm assumes an elevated-external rotated
position, followed by an acceleration and a follow-through (deceleration) phases.
At the end of the acceleration phase the object (ball) is released or stroked. On
throwers, during the deceleration phase, the posterior rotator cuff musculature acts
eccentrically in order to decelerate or “ brake ” the internal and horizontal adduction
arm motion, generated during the acceleration phase. The act of throwing requires
a coordinated motion that progresses from the toes to the fingertips. This sequence
of events has been described conceptually as a kinetic chain. For the kinetic chain
to work effectively, sequential muscle activity is required so that the energy that is
generated in the lower body can be transmitted to the upper body through the arm,
hand, and fingers, and finally to the ball. The speed of the ball is then determined
by the efficiency of this process. Body rotation, timing and positioning of the
scapula are key elements in the kinetic chain. Any physical condition that alters
the components of the kinetic chain, especially one that affects the so called “ core ”
(trunk, back and proximal parts of the lower limbs), will alter more distal segments
and may result in the development of a dysfunctional shoulder [ 1 ].
The inherent contradiction for overhead athletes is the fact that the shoulder must
be loose enough to perform overhead activity and yet stable enough to prevent
the joint from “ giving way ” or sub-luxating [ 22 ]. In elite-level throwers, there is
a delicate balance between shoulder mobility and stability. The shoulder needs to
be mobile enough to reach extreme positions of rotation so that velocity can be
imparted to the ball, but at the same time the shoulder needs to remain stable so that