Humerothoracic Motions

Humeral movement with reference to the thorax is commonly described in terms of four motions: elevation, forward flexion, horizontal flexion-extension, and axial rotation (Figure 1.9). These motions result from motion of both the humerus and the scapula and can be more precisely described using three independent rotations of each bone. The rotations of the humerus relative to the scapula can be described as the rotation which defines the plane in which the humerus elevates (plane of elevation), the rotation which moves the humerus away from the body (elevation), and the rotation about the long axis of the humerus (axial rotation). The scapula’s rotations relative to the thorax are in turn rotation of the scapula across the chest and away from the mid-line (elevation), rotation about a superior axis which changes the direction of the glenoid (internal-external rotations), and rotation about a lateral axis tilts the glenoid (anterior-posterior tilting) (Figure 1.10).

Figure 1.9: Humeral rotations used to describe motion.

Sequence of rotations used to describe humeral orientation relative to the scapula. (A) Humerus at rest with epicondyles in coronal plane; (B) humerus with epicondyles in scapular plane following ‘plane of elevation’ rotation indicated in (A); (C) humerus parallel to ground following ‘abduction’ rotation indicated in (B); (D) humerus ‘externally’ rotated following rotation indicated in (C).

Figure 1.10: Scapular rotations used to describe motion.

Displayed are rotations about the axes most commonly used to describe scapular orientation relative to the thorax. Version rotation (left) showing scapula moving into anteversion; elevation rotation (middle) showing scapula upwardly rotating; tilting rotation (right) showing scapula moving into a posterior tilt.

‘Elevation’ is the common term used to describe the motion of the arm away from the body in the lateral direction. Although this motion may be thought of as movement in the coronal plane, it is more common for this motion to occur in the scapular plane – which

lies ~30° anterior – in which the deltoid and supraspinatus are optimally aligned for

elevation. (Itoi et al., 2009; Poppen & Walker, 1976). This motion may exceed 180° of rotation but on average is limited to 171° for women and 167° for men (Itoi et al., 2009). This range of motion, however, is influenced by individual anatomy and laxity, and decreases with age due to joint stiffening and degenerative muscle changes (Barnes, Van Steyn, & Fischer, 2001). In order to achieve optimal joint function, stability, and kinematics, this large range of motion is divided between glenohumeral elevation and scapulothoracic elevation. Combining these two motions optimizes function in three ways. First, it enables an arm range of motion unattainable through glenohumeral joint only, because a 180° rotation at this joint would trap the muscles within the articulation, rendering them non-functional (Van der Helm & Pronk, 1995). Second, the decreased ROM of each joint allows the muscles crossing the joint to function within the optimal region of their length-tension curve (Itoi et al., 2009). Third, the presence of scapular rotation moves the glenoid beneath the humeral head, providing support against the weight of the arm, and thus maintaining optimal joint kinetics, kinematics, and stability (Itoi et al., 2009).

The manner in which the arm’s motion is divided between the glenohumeral and scapulothoracic joints during elevation is termed the ‘scapulohumeral rhythm.’ In 1944, Inman and Abbott, described that when averaging the rhythm over a full range of motion, its ratio is 2:1 (glenohumeral:scapulothoracic). Plane of abduction, direction of motion (raising vs. lowering), shoulder dominance, gender, level of resistance, and level of muscle fatigue have been widely studied in relation to this rhythm since then (see Crosbie, Kilbreath, Hollmann, & York, 2008; Crosbie, Kilbreath, & Dylke, 2010; Dayanidhi, Orlin, Kozin, Duff, & Karduna, 2005; Ebaugh, McClure, & Karduna, 2005; Fayad et al., 2006; Forte, de Castro, de Toledo, Ribeiro, & Loss, 2009; Graichen et al., 2000; Inman & Abbott, 1944; Ludewig et al., 2009; Ludewig, Cook, & Nawoczenski,

1996; Matsuki et al., 2010; McClure et al., 2001; McClure, Michener, & Karduna, 2006; McQuade & Smidt, 1998; Nagai et al., 2013; Poppen & Walker, 1976; Prinold, Villette, & Bull, 2013; Scibek & Carcia, 2012; Yano et al., 2010). While these investigations have resulted in varying and often contradictory conclusions, some supporting one constant ratio, and others, a variable ratio across motion, the commonly accepted ratio remains Inman & Abbott's (1944) 2:1 (glenohumeral:scapulothoracic) for a full range of abduction.

In addition to elevating during humeral elevation, the scapula has also been shown to undergo internal-rotation and tilting. Reports of the initial direction and magnitude of movement of these two rotations vary; however, it has been shown that across a full range of abduction, the scapula tends to externally rotate and tilt posteriorly relative to its initial

resting posture, with the majority of this motion occurring beyond 90° of humerothoracic

elevation (Itoi et al., 2009; McClure et al., 2001). The function of these rotations is not well understood, but McClure et al. (2001) posit that they allow the humeral head and rotator cuff tendons to more easily pass beneath the acromion as the humerus elevates beyond 90°. The ability of the humeral head and rotator cuff to clear beneath the acromion is also aided by external rotation of the humerus, which occurs during elevation in any plane anterior to the scapula. Browne, Hoffmeyer, Tanaka, An, and Morrey (1990) demonstrated that maximal elevation occurs 23° anterior to the scapular plane and is facilitated by 35° of external rotation. This external rotation has the additional effect of relaxing the inferior capsuloligamentous tissues, thus permitting further motion. In contrast, maximal elevation in planes posterior to the scapula requires internal rotation and peaks at 115° in a plane ~20-30° posterior to the scapula (Browne et al., 1990).

The term ‘forward flexion’ is commonly used to describe motion of the arm away from the body in the anterior direction. This motion can be considered a special case of elevation in a plane anterior to the scapula; however, this is only the case if the humerus is internally rotated prior to elevation. If the humerus is instead left in neutral rotation, the glenohumeral kinematics and contact mechanics would be significantly different. It is the latter case which is commonly termed ‘forward flexion.’ Despite this motion’s unique

glenohumeral kinematics, the scapula’s motion is remarkably similar to that observed during elevation in the scapular plane (McClure et al., 2001).

Horizontal flexion-extension is a common motion – especially in throwing sports – which involves changes in the humeral plane of elevation while the elevation level is maintained parallel to the ground, and for which internal-external rotation can be constant or variable. The boundary between flexion and extension is commonly considered to be the scapular plane. Itoi et al. (2009) have found that when the arm is placed in this plane and externally rotated, the glenohumeral joint achieves maximum stability. However, in patients with anterior shoulder instability, orientation in this plane, as well as planes posterior to the scapular plane, have been described as the most unstable (Speer, Hannafin, Altchek, & Warren, 1994). Scapular kinematics for horizontal flexion- extension have not been evaluated during independent rotation about this one axis, but rather only during throwing motions. However, results from throwing motions have

shown that scapular upward rotation is decreased to approximately 20° with the arm in

full humeral horizontal extension from approximately 40° with the arm in the scapular

plane (Ludewig et al., 2009). Additionally, the scapula progressively internally rotates

from approximately 20° in maximal horizontal extension to its resting posture of 30°

when the humerus is in the scapular plane.

For the remainder of this thesis, glenohumeral elevation in the scapular plane will be termed ‘abduction,’ while the rotation defining this plane will be termed ‘plane of elevation,’ and rotation about the humeral longitudinal axis will be termed ‘internal- external rotation’.