Journal of Voice Vol. 11, No. 1, pp. 23-32 © 1997 Lippincott-Raven Publishers, Philadelphia

The Role of Strap Muscles in In Vivo Canine Laryngeal Model

Ki Hwan Hong, *Ming Ye, *Young Mo Kim, *Kevin F. Kevorkian, and *Gerald S. Berke

Department of Otolaryngology, Chonbuk National University, Medical School, Chonbuk, Korea; and *Division of Head and Surgery, UCLA School of Medicine, Los Angeles, California, U.S.A.

Summary: In spite of the presumed importance of the strap muscles on laryn- geal valving and speech production, there is little research concerning the physiological role and the functional differences among the strap muscles. Generally, the strap muscles have been shown to cause a decrease in the fundamental frequency (Fo) of phonation during contraction. In this study, an in vivo canine laryngeal model was used to show the effects of strap muscles on the laryngeal function by measuring the F o, subglottic pressure, vocal in- tensity, vocal fold length, cricothyroid distance, and vertical laryngeal move- ment. Results demonstrated that the contraction of sternohyoid and sternothy- roid muscles corresponded to a rise in subglottic pressure, shortened cricothy- roid distance, lengthened vocal fold, and raised F o and vocal intensity. The corresponded to lowered subglottic pressure, widened cricothyroid distance, shortened vocal fold, and lowered F 0 and vocal inten- sity. We postulate that the mechanism of altering F o and other variables after stimulation of the strap muscles is due to the effects of laryngotracheal pulling, upward or downward, and laryngotracheal forward bending, by the external forces during strap muscle contraction. Key Words: Strap muscles--Fo- Intensity--Subglottic pressure--Cricothyroid distancewVocal fold length.

It is generally agreed that the extrinsic laryngeal The literature has described several human studies muscles are important for laryngeal function in relating the functional role of the strap muscles to deglutition to lift and tilt the as part of bio- the larynx, attributing changes in the length of the logical valving and phonation (1,2), but their pho- vocal folds to external forces such as contraction of natory function is less well defined. A large number the strap muscles (3-7). However, there still re- of muscles participate indirectly or directly in the mains some controversy on the function of these functioning of the larynx, including infrahyoid muscle acts. (strap), suprahyoid, pharyngeal constrictor, and ex- Sonninen (8) stimulated the trinsic tongue muscles. These can be divided mainly of a patient under local anesthesia with the neck into two groups: the strap muscles and the supra- extended and found lowering of the pitch. Faaborg- hyoid muscles (1). Anderson and Sonninen (9) studied laryngeal posi- The location of the origin and insertion of the tion and the electrical activity in the extrinsic laryn- strap muscles are unique for each muscle, implying geal muscles during phonation at different pitches. functional differences among the strap muscles. They reported that the electrical activity of ster- nothyroid muscle showed pronounced activity at Accepted May 24, 1996. low pitch and decreased activity at high pitch. In Address correspondence and reprint requests to Dr. Ki Hwan Hong, Department of Otolaryngology, Chonbuk National Uni- the thyrohyoid and mylohyoid muscles, electrical versity, Medical School, Chonju, Chonbuk, 560-180, KOREA. activity increased at high pitch. Hirano et al. (5)

23 24 K. H. HONG ET AL.

reported that greater electrical activity of the ster- glottic pressure (Psub), and vocal intensity were nohyoid muscle at lower pitches appears to be re- calculated from waveform signals. Vertical laryn- lated chiefly to the lower position of the larynx, but geal movement, cricothyroid distance, and vocal also the muscular activity increased at higher fold length were measured using monitored video pitches. Simada and Hirose (10) also reported that a images (19,20). The study allowed us to evaluate the consistent increase in the activity of the sternothy- role of each individual strap muscle on stimulation roid muscle was observed in association with pitch and then postulate the mechanism involved in alter- lowering with decreased activity during pitch rise. ing pitch. However, the activity of the showed no consistent correlation with pitch lower- ing. Ohala and Hirose (l l) reported that the sterno- MATERIALS AND METHODS hyoid muscle is active in both lowering the pitch In vivo canine model and in achieving extremely high pitch, but in speech The in vivo canine model of this experiment (Fig. the involvement in pitch lowering is most notice- I) was similar to that used in previous reports able. Erickson et al. (12,13) reported that the strap (17,18). Four healthy mongrel dogs (-25-30 kg) muscles have a negative relationship to frequency were premedicated with acepromazine maleate in- and a strong negative correlation with the activity of tramuscularly. Intravenous pentobarbital sodium the geniohyoid and cricothyroid muscles. Simada (Nembutal) was administered to a level of corneal and Hirose (I0) also reported that the sternohyoid anesthesia. Additional pentobarbital sodium was muscle helps at least to maintain a low pitch at a low used to maintain this level of anesthesia throughout level during an utterence. the procedure. Each dog was placed supine on the There are a few reports that the strap muscles operating table. Orotracheal intubation was per- have a positive relation to frequency with stimula- formed. tion of strap muscles resulting in pitch elevation. A midline incision was made to expose the strap Sonninen (14) reported that the function of the ex- muscles from the mandible to the sternal notch. The trinsic laryngeal muscles, the so-called "external sternohyoid, sternothyroid, and thyrohyoid mus- frame function," are considered to lengthen or cles were dissected carefully. The recurrent laryn- shorten the vocal folds and regulate pitch by chang- geal (RLN) was isolated about 5 cm inferior ing the relation of the to the cricoid carti- to the larynx, and bilateral nerve lage. He argued that simultaneous contractions of branches to the sternohyoid, sternothyroid, and sternothyroid muscles resulted in a forward force thyrohyoid muscles were dissected leaving l-cm on the tending to increase the ten- segments. The most active nerve branch to each sion of the vocal folds. He postulated that section- strap muscle was identified via stimulation. A low ing the various external laryngeal muscles would tracheotomy was performed at the level of the su- result in lowering the voice, loss of range, and fail- prasternai notch and an endotracheal tube was ure of the to close completely. Murakami and placed for ventilation. A second tracheotomy was Kirchner (15,16) suggested that the external laryn- performed in a more superior location and a cuffed geal muscles appear to make two contributions to endotrachei tube was passed superiorly with the tip the tensor mechanism of the larynx, shortening of positioned about l0 cm below the glottis. For direct the cricothyroid distance and contraction of the thy- visualization of the larynx through the oral cavity, a roarytenoid muscle. Niimi et al. (6) supposed that button was used to suspend the . the sternothyroid muscle could serve as a pitch raiser. They speculated that the thyroid cartilage Nerve stimulation rotates downward around the cricothyroid joint, or Harvard subminiature electrodes (South Natick, the frontal part of the cricoid ring comes closer to MA) were applied to the isolated RLNs. A constant the thyroid cartilage and the , current nerve stimulation (WR Medical electronics which results in higher tension of the vocal folds. RLN Stimulator, Model S2LH, St. Paul, MN) was The present study used an in vivo canine laryn- used to provide constant amounts of current to the geal model (17,18) to evaluate the effects of strap RLNs equal bilaterally. The frequency of stimula- muscle stimulation on the vocal folds. Stimulation tion was 80 Hz with a pulse duration of 1.5 milli- to the sternohyoid, sternothyroid, and thyrohyoid second for both nerve stimulators. A Grass model muscles was performed. The frequency (Fo), sub- 54H stimulator (Quincy, MA) was used to provide

Journal of Voice, Vol. 11, No. 1, 1997 THE STRAP MUSCLES 25

video recorder TV monitor video camera macro lens ---. (~Ysound \ endoscope ~ ~/~¢ level meter ~

oscilloscope printer FIG. 1. Schematic presentation of in vivo laryngeal model. RLN = recurrent laryngeal nerve; SH ...... ~¢ n erve = sternohyoid muscle; ST -- sternothyroid muscle; TH = thy- / image rohyoid muscle. processing

• ° ..

pressure transducer A A computer digitization RLN stimulation varying amounts of current to the isolated of OR) before recording. Then the signals were re- the strap muscles. Voltage varied from 0 to 3 V for corded on a personal computer with a Labmaster strap muscle stimulation, and was classified as low analog-to-digital microprocessor. The acoustic or high levels according to the contraction of strap waveforms were recorded with subglottic pressure muscles. Low-level stimulation was the level at simultaneously and these signals were low-pass fil- which the strap muscles contracted mildly on direct tered at 3,000 Hz and digitized at a rate of 20 kHz. visual observation. Maximal stimulation was deter- A multipurpose computer program (Cspeech 3.1) mined by lack of additional contraction of the strap was used to analyze the subglottic and acoustic sig- muscles. During maximal stimulation of the isolated nals. The fundamental frequency of each trial was nerves of the strap muscles, no contraction of the calculated by measuring the vocal period from the was noted. subglottic pressure curve. Airflow and pressure system Room air was warmed and humidified by bub- Measurements of vocal fold length bling through 5 cm of water at 37°C and air flow was Videoendoscopy was performed using a Storz tel- controlled by a needle valve (Whitey, Highland escope connected with a fiberoptic cable for mea- Heights, OH) and measured with a flowmeter suring vocal fold length. Images were recorded us- (model FI500; Gilmont instruments, Great Neck, ing a CCD camera (Toshiba IKC30A, Buffalo NY). The rate of air flow was about 400 ml/sec. A Grove, IL) and ¥4-inch videotape recorder (SONY Millar catheter-tipped pressure transducer (model VO-9850, Park Ridge, N J). Recorded video images SPC-303 Millar Instruments, Houston, TX) was in- were viewed on a SONY video monitor (PVM 1341) serted through the superior tracheotomy to rest 2 and images were printed out by a SONY video color cm below the glottis. The transducer was calibrated printer. The calibration method was used to mea- at the temperature of the animal's by sub- sure change of vocal fold length before and after merging it in a water bath at 37°C to a depth just stimulation as in Fig. 2. A standard centimeter ruler covering the sensor (0.5 cm) and then calibrating it was lowered to the level of the glottis and measured against a mercury manometer from 0 to 100 mmHg. with the software, so that pixel units could be con- Waveform signals during phonation verted to square millimeters (19,20). The camera The subglottic and acoustic signals were verified was kept at a constant distance from the larynx on a Tektronix oscilloscope (model 5116 Beaverton, throughout each experiment,

Journal of Voice, Vol. 11, No. 1, 1997 26 K. H. HONG ET AL.

FIG. 2. Photograph with ruler measuring the vocal fold length between two markers on the vocal fold demonstrating the calibration method.

Measurements of the vertical level of larynx and nals were digitized. Data were evaluated at 300- cricothyroid distance millisecond intervals. Subglottal pressure, F 0, and Downward movements of thyroid and cricoid vocal intensity were averaged from ten consecutive cartilages were calculated by measuring the vertical cycles selected at random from a stable section of lowering of the larynx. Small needles with markers phonation. Videoendoscopic images were used to were placed on the midline of the thyroid lamina at measure the change of vocal fold length. The move- the vocal cord level and at the midline of the ante- ments of the cricoid and thyroid cartilages were cal- rior cricoid lamina. Images were recorded using a culated from video images of the external thyroid CCD camera (Toshiba IKC30A) and a 3/4-inch vid- and needles. eotape recorder (SONY VO-9850). Recorded video images were viewed on a SONY video monitor RESULTS (PVM 1341) and images were printed out using a SONY video color printer. Using video images, the A two-way analysis of variance (ANOVA) with extent of vertical laryngeal movements during stim- replication was undertaken for all dependent vari- ulation were calculated by the movement of the ables. Stimulation condition (with or without the needle on the thyroid cartilage while the ruler was strap muscle) was a within-subjects factor; dogs, constantly adjusted by holding it at the side of the stimulation site, and stimulation level were be- larngotrachea. The changes of cricothyroid distance tween-subjects factors. Table I shows the mean were measured by the differences in the needle changes of all dependent variables for each dog. movements on the thyroid and the cricoid cartilages For the sternohyoid muscle, Fo, vocal intensity before and after stimulation. and subglottic pressure showed significant increase after stimulation in F o and subglottic pressure, but Experimental design not in vocal intensity (see Table 2 and Fig. 3). Sig- Four dogs were studied by varying the stimula- nificant interaction occurred between dogs and lev- tion level to the isolated nerves to the strap mus- els of stimulation for dependent variables (F3,16 = cles. Low and high levels were employed using con- 670.3, p < 0.01) and vocal intensity (F3.16 = 10.6, stant air flow and bilateral RLN stimulation. Three p < 0.05), and subglottic pressure (F3,16 = 26.3, p trials were performed for each condition separated < 0.01). Some of the dependent measures were by 3 to 5 minutes to reduce fatigue effects. During moderately correlated using Pearson's correlation each trial, the subglottic pressure and acoustic sig- adjusted for multiple comparisons (Fo and vocal in-

Journal of Voice, Vol. II, No. I, 1997 THE STRAP MUSCLES 27

TABLE 1. Mean changes of dependent variables with stimulations, a

Dog ! Dog 2 Dog 3 Dog 4 Muscle variables Low High Low High Low High Low High

SH F o 8.3 27.0 19.5 35.3 25.7 38.3 19.1 76.8 Intensity 0.2 0.3 1.4 0.7 0.5 0.3 0.1 1.3 Psub 8.7 15.8 4.3 6.0 17.1 20.7 9.9 35.6 CTD 0.0 - 2.0 - 1.0 - 2.0 0.0 - 2.0 - 1.0 - 2.0 VFL -3.0 -7.6 -5.3 5.8 0.0 7.4 2.4 8.0 VLM -2.4 -9.5 -2.5 - 11.5 -2.0 - 13.5 - 1.0 -6.0 ST F o 7.8 23.2 10.0 20.6 5.1 16.4 12.7 57.3 Intensity 0.5 1.4 1.6 1.5 0.1 0,2 0.3 0.7 Psub 11.2 12.5 5.6 8.9 1. I 12,6 5.4 9.9 CTD 0.0 -2.0 0.0 -2.0 0.0 -2.0 - 1.0 -2.0 VFL 0.0 5.4 3.0 6.0 -4.3 5.6 -5.7 5.5 VLM - 2.0 - 7.5 - 1.5 - 8.0 - 2.0 - 9.5 - 3.0 - 6.0 TH F o -3.6 -8.1 -4.1 ~7.4 -6,3 - 10,9 -3.0 - 10.8 Intensity - 0. I - 0.3 - 0.2 - 0.3 - 0.1 0.0 0.0 - 0.2 Psub - 1.4 4.5 -0.9 -2.2 -0.6 - !.6 -2.3 -5.4 CTD 0.0 2.0 1.0 2.0 0.0 2.5 1.0 2.5 VFS 0.0 - 3.8 0.0 - 5.0 0.0 - 6.4 - 3.4 - 5.4 VLM 1.0 3.5 2.0 6.0 0.0 5.5 !.0 8.0

Airflow was held constant at 400 cc/s relatively. SH. sternohyoideus; ST, sternothyroideus; TH, thyrohyoideus; F o, fundamental frequency (Hz); Intensity, acoustic power (volt); Psub, subglottic pressure (mmHg); CTD, cricothyroid distance (ram); VSM, vertical laryngeal movement (mm); VFL, vocal fold length (mm). tensity: r = .46, p < 0.05; intensity and subglottic stimulation, but significantly increased in the high pressure: r = 0.16, p > 0.05; F0 and subglottic pres- level stimulation as shown in Fig. 4. Significant in- sure: r = 0.83, p < 0.01). teraction occurred between dogs and levels of stim- Vertical laryngeal movement, cricothyroid dis- ulation for the dependent variables vocal fold length tance, and vocal fold length for the sternohyoid (F3,16 = 12.7, p < 0.01) and vertical movement muscle showed significant difference before and af- (F3,16 = 3.5, p < 0.05), but not for cricothyroid ter stimulation and between levels of stimulation as distance (F3,16 = 0.7, p > 0.05). These dependent shown in Table 3. The vertical levels of the larynx measures were significantly correlated using Pear- were lowered and the cricothyroid distances were son's correlation between vocal fold length and ver- shortened after stimulation of this muscle. The vo- tical movement (r = 0.779, p < 0.01), but not be- cal folds were lengthened slightly after low level tween vocal fold length and cricothyroid distance (r

TABLE 2. Overall changes and statistical data for F o, vocal intensity, and subglottic pressure after stimulation

Overall changes ANOVA (A) ANOVA (B) Muscle variables Low High df F P df F P

SH Fo (Hz) 18.2 44.3 i,40 71.7 <0.01 1,16 161.8 <0.01 Intensity (volt) 0.6 0.7 1,40 42.4 <0.01 i, 16 0.38 >0.05 Psub (mmHg) 10.0 19.5 1,40 87.2 <0.01 1,16 78.5 <0.01 ST F o (Hz) 8.9 29.4 1,40 41.7 <0.01 1,16 77.4 <0.01 Intensity (volt) 0.6 0.9 1,40 75.5 <0.01 1,16 5.2 <0.05 Psub (mmHg) 5.8 i !.0 1,40 60.4 <0.01 I, 16 12.8 <0.01 TH F o (Hz) -4.3 - 9.3 1,40 65.1 <0.01 1,16 14.8 <0.01 Intensity (volt) -0.1 - 0.2 1,40 30.5 <0.01 I, 16 11.7 <0.01 Psub (mmHg) - 1.3 - 3.4 1,40 50.6 <0.01 1,16 21.9 <0.01

Airflow was held constant at 400 cc/s relatively. SH, sternohyoideus; ST, sternothyroideus; TH, thyrohyoideus; Psub, subglottic pressure (mmHg). ANOVA (A), between before and after stimulation; ANOVA (B), between the levels of stimulation.

Journal of Voice, Vol. 11, No. 1, 1997 28 K. H. HONG ET AL.

50 A 44 ~. 40 30 18 ~: 20 o- 10 " 0

-10 low -4.2 ...~,, -9.3

0.93 I B FIG. 3. Plots for the changes of 0.8 0.59 0.61 Fo(A), vocal intensity (B), and 0 subglottic pressure (C) with levels 0.6 of stimulation of the strap mus- 0.4 t~ cles; SH = sternohyoid muscle; e" 0.2 ST = sternothyroid muscle; TH m = thyrohyoid muscle. 0 low -0.2 -0.09 ..0.2

19 20 C

E 10 E 10

5

D. 0 -5 low -1.3 ,,,w,, -3.4

= -0.327, p > 0.05) and between cricothyroid dis- pressure were significantly correlated using Pear- tance and vertical movement (r = 0.372, p > 0.05). son's correlation adjusted for multiple comparisons For the sternothyroid muscle, Fo, vocal intensity, (r = 0.58, p < 0.01), but not between Fo and inten- and subglottic pressure also showed significant dif- sity (r = 0.06, p > 0.05) and vocal intensity and ferences with stimulation (Table 2). A significant subglottic pressure (r = 0.1, p > 0.05). Figure 4 interaction occurred between dogs and levels of depicts overall changes for laryngeal movement stimulation (F3,16 = 12.2, p < 0.01), but not for the (lowered), cricothyroid distance (shortened), and subgiottic pressure (F3,16 = 1.86, p > 0.05) and vocal fold length (lengthened) after stimulation of intensity (F3,16 = 2.3, p < 0.05). F o and subglottic the sternothyroid muscle. The vocal fold length was

TABLE 3. Overall changes and statistical data for the lalwngeal movement"

Overall changes ANOVA (A) ANOVA (B) Muscle variables Low High df F P df F P SH VFL (%) - 1.5 7.2 1,40 38.2 <0.01 I, 16 360.7 <0.01 CTD (ram) - 0.5 - 2.3 1,40 35.8 <0.01 1,16 36.5 <0.01 VLM (mm) -2.0 - 10.1 1,40 7.4 <0.01 1,16 400.8 <0.0t ST VFL (%) - 1.7 5.6 1,40 52.6 <0.01 1,16 189.8 <0.01 CTD (mm) - 0.3 - 2.0 1,40 20.3 <0.01 I, 16 28.0 <0.01 VLM (mm) - 2.1 - 7.7 1,40 4.5 <0.01 I, 16 417.7 <0.01 TH VFL (%) - 0.8 - 5.2 1,40 33.5 <0.01 I, 16 196.9 <0.01 CTD (mm) 0.5 2.3 1,40 52. I <0.01 I, 16 85.9 <0.01 VLM (mm) 1.0 5.8 1,40 31.4 <0.01 1,16 163.7 <0.01

" Cricothyroid distance and vocal fold length after stimulation. SH, sternohyoideus; ST, sternothyroideus; TH, thyrohyoideus; VFL, vocal fold length; CTD, cricothyroid distance; SLM, vertical laryngeal movement. ANOVA (A), between before and after stimulation; ANOVA (B), between the levels of stimulation.

Journal of Voice, Vol. 11, No. 1, 1997 THE STRAP MUSCLES 29

10 A 5.8

E 5

0 E -2 o -5

-10 -7.7 -10

3 B 2 E 1 0.5 FIG. 4. Plots for the changes of vertical level of larynx (A), CT distance IB) and vocal fold length '- 0 --q (C) with levels of stimulation of strap muscles; SH low = sternohyoid muscle; ST = sternothyroid mus- -0.5 -0.3 I- cle; TH = thyrohyoid muscle. 0 -2

-3 -2.3

lO C 8.6 8.1 8

A 6 4 2 e- 0 I _J -2 low -0.8 -4 -1.5 -1.7 -6 -5.4 significantly increased in the high-level stimulation subglottic pressure (r = 0.5, p < 0.05), vocal fold state (Table 3). These variables were significantly length and cricothyroid distance (r = -0.853, p < correlated adjusted for cricothyroid distance and 0.01), vocal fold length, and vertical movement (r = vertical movement (r = 0.69, p < 0.01), vocal fold -0.759, p < 0.01), and cricothyroid distance and length and cricothyroid distance (r = 0.535, p < vertical movement (r = 0.835, p < 0.01), but not 0.01), and vocal fold length and vertical movement between Fo and intensity (r = 0.08, p > 0.05) and F0 (r = -0.666, p < 0.01). and subglottic pressure (r = 0.37, p > 0.05). In contrast, the thyrohyoid muscle showed differ- ent responses compared with the sternohyoid and DISCUSSION sternothyroid muscles. The values for all variables were significantly different at the 0.05 level for the The strap muscles have been considered to comparison of no stimulation versus low (or high) lengthen or shorten the vocal folds by changing the stimulation levels. The F 0, vocal intensity, and sub- relationship of the thyroid cartilage to the cricoid glottic pressure were lowered after stimulation (Fig. cartilages (14). However, the action of the strap 4 and Table 2). Vocal fold length was decreased, but muscles during phonation is controversial and has cricothyroid distance was increased and the larynx differed from subject to subject depending on vocal was raised after stimulation of the thyrohyoid mus- habits or vocal training (9,10). So it is difficult to cle (Fig. 4 and Table 3). No significant interactions draw general conclusions from human studies. In occurred between dogs and levels of stimulation for this canine study, contraction of the sternohyoid the dependent variable (F3,16 = 0.543, p > 0.05), and sternothyroid muscles corresponded to raising vocal intensity (F3,16 = 2.3, p < 0.05), subglottic the subglottic pressure, F o, and vocal intensity. The pressure (F3,16 = 1.4, p > 0.05), vocal fold length sternothyroid muscle corresponded to greater ef- (F3,16 = 8.2, p < 0.01), cricothyroid distance fects on subglottal pressure and F o than the ster- (F3,16 = 3.6, p < 0.05), and vertical movement nothyroid muscle. Both muscles produced narrow- (F3,16 = 7.6, p < 0.01). The dependent measures ing of the cricothyroid distance and lengthening of were moderately correlated between intensity and the vocal folds according to the level of stimulation.

Journal of Voice, Vol. II, No. I, 1997 30 K. H. HONG ET AL.

The thyrohyoid muscle showed opposite activity tissue when compared with the thyroid cartilage, from the sternohyoid and sternothyroid muscles; creating restricted vertical movement of the cricoid that is, significant decrease in F 0, subglottal pres- cartilage compared with the thyroid cartilage. Dur- sure, and intensity. ing lowering of the larynx, this results in shortening In terms of larynx and hyoid position, the con- the cricothyroid distance. During contraction of the traction of the sternohyoid and sternothyroid mus- sternohyoid and sternothyroid muscle, the cricoid cles pulls the downward and forward, and thyroid cartilages move downward together, thus lowering the larynx or preventing its elevation. but the thyroid cartilage moves more than the In this study, these muscles produced laryngeal and cricoid, which results in shortening of the cricothy- tracheal backward and downward tilting. The thy- roid distance. rohyoid muscle made the larynx and hyoid bone Shipp et al. (22) postulated that the F 0 raising raise upward without tilting. The movements of the might be due to increased vertical tension of the larynx and trachea by sternohyoid and sternothy- vocal folds caused by high lung volumes accompa- roid muscle contraction have been described as nied by greater laryngotracheal pull. Murakami and laryngotracheal pulling (14). We observed an addi- Kirchner (15) reported that the external laryngeal tional type of movement of the larynx and trachea, muscles tend to stabilize or pull the thyroid carti- laryngotracheai bending, during stimulation of the lage downward and to pull the cricoid cartilage up- sternohyoid and sternothyroid muscles. Thus, we ward. This narrows the anterior midline distance, propose that the movement of the laryngotrachea serves as a counterforce to the contraction of the by the external laryngeal muscles can be classified thyroarytenoid muscle, and thus contributes to ten- by the laryngotracheal pull, upward and downward, sion within the vocal folds. Therefore, the strap and laryngotracheal bending, forward and back- muscles may participate in the reflex protection ward (Fig. 5). mechanism of the larynx. In this canine study, we The effect of laryngotracheal downward pull dur- could not evalute the effect of vertical tension di- ing sternothyroid contraction has two possible ex- rectly during laryngotracheal downward pull. We planations. Zenker and Zenker (21) reported that a could observe, however, that the trachea was di- laryngotracheal downward pull can contribute to lated during laryngotracheal downward pull, allow- lowering F 0 by increasing the cricothyroid distance, ing higher air volume in the laryngotrachea, with and thus changing the longitudinal tension of the higher subglottic and tracheal pressures. We can vocal folds. But in this canine study, the cricothy- therefore postulate that the increase in frequency roid distance was decreased and F o increased dur- and vocal intensity might be due to increased sub- ing laryngotracheal downward pull. This phenome- glottic pressure during laryngotracheal pulling or a non can be explained anatomically. The cricoid car- counterforce to the contraction of the thyroaryte- tilage is relatively fixed to the adjacent connective noid muscle, and thus contribute to tension within

forward ~ forward~ FIG. 5. The movements of the bent ling / t~;';.":~l I bending(~ ~:~:~/~/.... . ~ ... hyoid bone, thyroid and cricoid cartilages,after contractionof (A) sternohyoid muscle (SH), (B) sternothyroid muscle (ST), and (C) thyrohyoidmuscle for C. CTD = cricothyroiddistance. ~]~fb '~ ~ '~/pressure ~ : ,ressure I,~ ~p il pressure 1/""~] ~ i ~'~inereased ~A~_ , ! nereased [~" ~[d=reased d°Wn:l I:ddwn: I: :i

Journal of Voice, Vol. 11, No. 1, 1997 THE STRAP MUSCLES 31 the vocal folds. Another possible mechanism affect- muscles caused the laryngotracheal downward pull ing the pitch during sternohyoid and sternothyroid and high air volume in the subgiottic air space, and contraction might be the effect of laryngotracheal correspond to increasing the subglottic pressure. forward bending. During lowering of the larynx af- This downward pull resulted in shortening the ter contraction of these muscles, the cricoid carti- cricothyroid distance; anterior downward bending lage moves toward the cervical vertebrae and the caused shortening of the anterior cricothyroid dis- thyroid cartilage moves anterocaudally. This results tance and resulted in lengthening the vocal folds in shortening of the cricothyroid distance causing and raising the frequency. The contraction of the an increase in the length and tension of the vocal sternothyroid muscle also resulted in raising F0, but folds. less than the sternohyoid muscle because the laryn- We can explain the different results of pitch be- gotracheal pull and bending was less prominant, tween the sternohyoid and sternothyroid muscles which was most likely due to the direction of mus- on an anatomical basis. The sternohyoid muscle is cle pull. The contraction of the thyrohyoid muscle the largest among the strap muscles and takes its caused the larynx to move upward. The thyrohyoid origin from the posteromedial surface of the muscle made the thyroid cartilage move more up- and and is inserted into the inferior border ward than the cricoid cartilage and caused length- of the body of the hyoid bone. The sternothyroid ening of the anterior cricothyroid distance, resulting muscle runs from the upper posterior part of the in lowering of the frequency by decreasing vocal clavicle and sternum to the oblique line of the thy- fold tension. It also caused lower air volume in the roid lamina. The oblique line is roughly equidistant subglottic air space, and corresponded to decreased from the isthmus of the thyroid laminae and cri- subglottic pressure and pitch. coarytenodi joint. When these muscles contract, the thyroid cartilage rotates downward around the Acknowledgment: This research was supported by the cricothyroid joint and the anterior part of the NIDCD Grant No. ROlDC0085-01 and by Veterans Ad- cricoid cartilage comes closer to the thyroid and the ministration Merit Review Funds. This study was per- formed in accordance with the PHS Policy on Human arytenoid cartilages. In this study, the stenothyroid Care and Use of Laboratory Animals, the NIH Guide for muscle showed less effective pitch raising than the the Care and Use of Laboratory Animals, and the Animal sternohyoid muscle. We suggest that this difference Welfare Act (7 U.S.C. et sequ.); this animal use protocol might be due to the directions of muscle contrac- was approved by the Institutional Animal Care and Use tion. The more anteriorly directed of the two mus- Committee (IACUC) of the University of California, Los Angeles. cles is the sternohyoid muscle. Thus, the contrac- tion of the sternothyroid muscle resulted in less de- gree of laryngotracheal pull and bending than the REFERENCES sternohyoid muscle. I. Moore KL. Clinically oriented anatomy, 2nd ed. Baltimore, On the other hand, the thyrohyoid muscle acted MD: Williams & Wilkins, 1985:1057-9. to lower the pitch. The thyrohyoid muscle runs 2. Atkinson JE, Erickson D. 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