Most of the glenohumeral joint is composed of a large hemispheroidal
humeral head articulating with an ovoid, concave glenoid
(Fig. 6a). Typically, this kind of joint geometry permits three
degrees of freedom: rotation (supination/pronation), flexion/
extension (elevation/depression) and protraction/retraction. In the
case of Tiktaalik, however, some restriction of humeral mobility is
engendered by cranial extensions of the articular surfaces of the
glenoid and humeral head (Figs 3d and 6a; see also Supplementary
Information). These accessory facets—a shallowly concave, elongate
humeral facet and a convex glenoid facet—are brought into contact
as the humerus is pronated, flexed and protracted. Pronation, flexion
and protraction are movements that could have been effected by
the large musculotendinous apparatus passing posterolaterally
through the coracoid foramen and inserting on the ventral surface
of the humerus. The simultaneous apposition of the reversed
concavoconvex geometries of the anterior and posterior parts of
the articulation represents a close-packed, or most stable, joint
position. Additional stability would be contributed through the
action of the trans-coracoid musculature. With the anterior facets
of the glenohumeral joint in full contact, however, protraction and
supination are inhibited, and flexor forces simply compress and
stabilize the humerus against the anterior portion of the glenohumeral
joint.
Most of the glenohumeral joint is composed of a large hemispheroidalhumeral head articulating with an ovoid, concave glenoid(Fig. 6a). Typically, this kind of joint geometry permits threedegrees of freedom: rotation (supination/pronation), flexion/extension (elevation/depression) and protraction/retraction. In thecase of Tiktaalik, however, some restriction of humeral mobility isengendered by cranial extensions of the articular surfaces of theglenoid and humeral head (Figs 3d and 6a; see also SupplementaryInformation). These accessory facets—a shallowly concave, elongatehumeral facet and a convex glenoid facet—are brought into contactas the humerus is pronated, flexed and protracted. Pronation, flexionand protraction are movements that could have been effected bythe large musculotendinous apparatus passing posterolaterallythrough the coracoid foramen and inserting on the ventral surfaceof the humerus. The simultaneous apposition of the reversedconcavoconvex geometries of the anterior and posterior parts ofthe articulation represents a close-packed, or most stable, jointposition. Additional stability would be contributed through theaction of the trans-coracoid musculature. With the anterior facetsof the glenohumeral joint in full contact, however, protraction andsupination are inhibited, and flexor forces simply compress andstabilize the humerus against the anterior portion of the glenohumeraljoint.
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