However, during speech at a normal loudness or chest and head singing, deeper layers of the vocal folds are set into vibration. The tension of the vocal fold is the primary determinant of Fo control. Just as a guitar string or a rubber band can be stretched to produce a higher pitch when plucked, increased stress on vocal fold fibers produces an increase in Fo. It is also important to understand that these muscles can work independently of one another in regulating Fo. Researchers theorize that humans naturally are inclined to use all or none of a particular muscle.
Training can help vocalists use just part of a particular laryngeal muscle's capability.
Also, researchers have found that a rise in Fo is generally obtained by an increase in TA activity so long as CT activity is not near its maximum. The following animation demonstrates how the CT and TA muscles move in an excised human larynx. The relationship between vocal fold length and Fo is non-linear. In other words, incremental increases in vocal fold length do not produce similar step-wise increases in Fo. You can test this phonemenon yourself.