Dance Anatomy & Kinesiology

The Upper Extremity 401

of the humerus upward (stabilizing component)

rather than produce the angular rotation needed for

shoulder abduction. However, this natural position-

ing and upward pull of the deltoid are countered

by members of the rotator cuff—the subscapularis,

infraspinatus, and teres minor (figure 7.24). These

muscles are collectively referred to as the SIT force

couple (S for subscapularis, I for infraspinatus, and

T for teres minor). The SIT force couple, perhaps

aided by some additional muscles (Hamill and

Knutzen, 1995), functions to depress the head of

the humerus and counter the upward movement of

the head of the humerus toward the acromion that

could pinch the interposed soft tissues (shoulder

impingement). The lower positioning of the humeral

head into the wider portion of the glenoid cavity

produced by the SIT force couple also facilitates the

desired rotation of the shaft of the humerus (e.g.,

shoulder abduction).

Synergies

Many of the muscles of the shoulder complex have

multiple actions, and sophisticated use of synergies

is necessary to achieve the desired positions and

movements of the arms, scapulae, and torso. For

example, when the arms are raised forward from low

fifth to high fifth, several synergies are operative. At

the glenohumeral joint, the two primary shoulder

FIGURE 7.24 The SIT force couple. When the arm is down by the side and the middle deltoid contracts (FD) to pro-
duce shoulder abduction, a large component of this force (fsd, stabilizing component) tends to pull the head of the
humerus upward while only a small component of this force (frd, rotary component) is capable of producing the desired
joint movement. However, the SIT muscles provide a force (Fd, dislocating component) that acts to pull the humerus
downward and a larger rotary component (fr) such that the resultant force of the SIT muscles (Fsit) acts as a force
couple to counter the upward pull of the deltoid and facilitate the desired shoulder abduction.

flexors used to raise the arm (anterior deltoid and

pectoralis major) also tend to produce internal

rotation of the shoulder. This undesired internal

rotation can be neutralized by the synergistic action

of external rotators such as the teres minor and infra-

spinatus. Meanwhile, another synergy is occurring to

produce the desired upward rotation of the scapula

(figure 7.25). In this case, the lower trapezius and

lower fibers of the serratus anterior neutralize out

the undesired action of scapular elevation produced

by the upper portion of the trapezius. Similarly, the

serratus anterior neutralizes out the undesired action

of scapular adduction that would be produced by the

trapezius if it were unopposed. These are examples

of helping synergists (chapter 2), as they assist with

the desired action of upward rotation of the scapula

while neutralizing out potential undesired elevation

or adduction of the scapula.

Muscular Analysis of Fundamental Shoulder Movements

As suggested by the previous discussion, analysis

of shoulder movements is complicated by the use

of muscles to stabilize the joint, prevent impinge-

ment, effect the linked movements of the scapula,

fd

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