Craniofacial Muscles

9 Structure and Function of the Laryngeal and Pharyngeal Muscles
155
with increasing pitch. Vertical position of the larynx appears to be altered with pitch
change, but the level of change is highly variable across individuals (4–22.5 mm)
(Shipp 1975 ) .
The infrahyoid muscles are often referred to as the strap muscles. The strap
muscles typically include the sternohyoid (SH), omohyoid (OH), thyrohyoid (TH),
and sternothyroid (ST); with the SH and OH comprising the most super fi cial layer
of these muscles and the TH and ST comprising the deepest layer. All four strap
muscles are located deep to the platysma and sternocleidomastoid muscle.
Strap muscles have an important role in voice production, particularly with
regard to pitch modulation during phonation. By impacting the position of the
thyroid cartilage, they increase and decrease the distance between the thyroid and
cricoid, resulting in increases in vocal fold tension or relaxation (Kenyon 1992 ) .
In general, the literature has focused on the role of the TH, SH, and ST, but has typically
omitted the omohyoid. Figure 9.6 contains a photograph of a canine larynx
showing the ST and TH muscles.
Some studies examining the ST in human via EMG or canine or primate via
stimulation have found it to be active during pitch elevation (Faaborg-Andersen and
Sonninen 1960 ; Shipp 1975 ; Niimi et al. 1991 ; Ueda et al. 1972 ; Sapir et al. 1981 )
and/or pitch lowering (Shipp 1975 ; Ueda et al. 1972 ). Other studies (again human
EMG studies or animal stimulation studies), however, have indicated that the ST is
not active or inconsistently active with pitch decrease (Shin, Hirano, Maeyama,
Nozoe, and Ohkubo 1981 ; Collier 1975 ; Atkinson 1978 ; Niimi et al. 1991 ; Sapir
et al. 1981 ) and/or pitch increase (Collier 1975 ; Sapir et al. 1981 ) . TH activity has
been noted during increased pitch across a majority of human EMG studies (Faaborg-
Andersen and Sonninen 1960 ; Shipp 1975 ; Baer et al. 1976 ) ; although a few human
EMG studies indicated that the TH was not active during increases in pitch (Erickson
et al. 1977 ) , or active during pitch lowering in addition to pitch raising (Baer et al.
1976 ) . However, the majority of human EMG studies indicate that the TH is not
active during pitch lowering (Faaborg-Andersen and Sonninen 1960 ; Shipp 1975 ;
Collier 1975 ) . In canine stimulation and human EMG studies, the SH has generally
been associated with pitch increases (Baer et al. 1976 ; Ueda et al. 1972 ) and decreases
(Baer et al. 1976 ; Atkinson 1978 ; Ueda et al. 1972 ), although one human EMG study
did not fi nd any activity in this muscle during pitch increase (Atkinson 1978 ) . Thus,
the way in which these muscles truly affect pitch is still unclear due to the contradictory
fi ndings reported in the literature (Hong et al. 1997 ; Vilkman et al. 1996 ) .
As can be gleaned from the above paragraph, studies examining SH, TH, and ST
function have typically involved electrical stimulation in animals or humans (Sonninen
1956 ; Ueda et al. 1972 ; Sapir et al. 1981 ) or EMG (during singing and speech) in
humans (Erickson et al. 1977 ; Faaborg-Andersen and Sonninen 1960 ; Shipp 1975 ;
Niimi et al. 1991 ; Collier 1975 ; Atkinson 1978 ; Baer et al. 1976 ) varying from 1 to 25
participants (Vilkman et al. 1996 ) . Cadaver experiments have also been undertaken to
provide insight into strap muscle function (Sonninen 1956 ; Vilkman et al. 1996 ) .
These studies have typically drawn conclusions about function by manipulating muscle
position (Vilkman et al. 1996 ) . The hypotheses, models, and literature related to
SH, TH, and ST function have been extensively reviewed (Vilkman et al. 1996 ) .