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Asymmetry of the Cranio‐cervico‐omo‐clavicular complex (CCOCC) in Right‐handed Humans is Functionally Related to Handedness
Author(s) -
Osborn Michelle L.,
Homberger Dominique G.
Publication year - 2009
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.23.1_supplement.822.4
Subject(s) - clavicle , anatomy , medicine , shoulders , occiput , skull , scapula , trapezius muscle , orthodontics , physical medicine and rehabilitation , surgery , electromyography
In humans, the clavicle and scapula are suspended from the occiput and the uniquely large mastoid process by the upper trapezius and sternocleidomastoid (SCM) muscles, thereby creating the cranio‐cervico‐omo‐clavicular complex (CCOCC). While it is known that the humerus is more robust and the clavicle shorter on the dominant side of individuals, other skeletal characters of the CCOCC have not been analyzed in this respect. When weights are carried or lifted, the contracting upper trapezius muscle offsets the added weight on the arms and stabilizes the shoulder by using the clavicle as a lever arm. The SCM muscle counteracts the action of the upper trapezius on the skull, preventing the skull from being retroflexed. To test these premises using a natural experiment, we analyzed the skeletal asymmetry of 54 right‐handed individuals. 71% had a wider right mastoid process. 90% of a subsample of 9 individuals had a higher right superior nuchal line and a wider right mastoid process, thereby providing larger attachment sites for the larger trapezius and SCM muscles of right‐handed individuals. These results suggest that the upper trapezius and SCM muscles act as synergists to stabilize the head and shoulders when loads are carried. Our inference is supported by the innervation and developmental pattern of the trapezius and SCM muscles.