Register and pitch are two distinct perceptual entities of the human voice. Without clear evidence for the use of the terminology, sources have begun to refer to lighter, or "falsetto", register as being "cricothyroid dominant" and heavier, or "chest", register as being "thyroarytenoid dominant" (Hirano, 1987; Miller, 1996; McCoy, 2004; Henrich, 2006; Spivey, 2008; Edwin, 2008; Phillips, Williams, & Edwin, 2012). The same intrinsic laryngeal musculature (i.e. the cricothyroid, CT, and thyroarytenoid, TA, muscles) play a role in the control of both register and pitch. Higher-pitched phonation, typically used to produce falsetto register, is mediated primarily by the cricothyroid (CT) muscle. The thyroarytenoid (TA) muscle plays a larger role in controlling lower-pitched voicing, the pitch range in which chest register tends to be used (Titze, 1989b; Shipp and McGlone, 1971). Despite their frequent co-occurrence, high and low pitched phonation are not controlled in the same way as light and heavy register productions.
The purpose of this study was to examine the ratio of CT and TA muscular activity in the control of chest and falsetto registers. Data were collected from untrained voice users: four females and one male. Hooked-wire electrodes were inserted into both the CT and TA muscles of each participant in order to collect electromyographic (EMG) data during glissando from low to high pitch on the vowel /i/ twice per subject, and tasks eliciting maximal activation of CT and TA muscles. A trained singing instructor with 17 years of experience determined and recorded the occurrence of register transition during each glissando. CT and TA EMG activity data from the glissando were normalized relative to maximum elicited CT and TA EMG activity, and were then retrospectively analyzed. CT muscular dominance was defined as a ratio of percentage of maximum CT EMG activity to percentage of maximum TA EMG activity greater than 1 (i.e. CT:TA greater than 1). TA dominance is a ratio of CT:TA activity less than 1 (i.e. CT:TA less than 1).
During glissando, all subjects experienced register transition from chest to falsetto register. In all subjects, the majority of chest register, and all of falsetto register, was produced with CT muscular dominance. Only the 3-4 lowest semitones, on average, in chest register were TA dominant. The transition from chest to falsetto register consistently did occur when the CT muscle was dominant, however, register transition did not occur as CT muscle activity began to dominate TA muscle activity. Results of the study showed that CT muscular dominance did not define falsetto register, nor was chest register defined by the TA muscular dominance.