### Physiological Basis  
Prior to the development of the RFF metric, several researchers examined cycle-specific changes in \(f_{\rm o}\) to describe phonatory offset and onset behaviors in similar phonetically constrained contexts. It has been postulated that two primary laryngeal adjustments occur to cease vocal fold vibration during intervocalic offsets: vocal fold abduction and increased laryngeal muscle tension [14]. Vocal fold abduction is primarily attributed to the posterior cricoarytenoid muscle and has been shown to reduce transglottal pressure as well as reduce the duration of vocal fold contact [15] to inhibit vocal fold vibration. At the same time, increased laryngeal muscle tension from increased activity of the cricothyroid (CT) [16] and vocals [17] muscles are suspected to help cease vibration, though there is additional evidence to suggest that overall laryngeal height and/or tilt (mediated primarily by the extrinsic laryngeal musculature) may also play a role [18]. Intervocalic onset behavior is influenced by the preceding voiceless consonant [19; 20], with laryngeal tension established during the voiceless consonant carrying over into the initial voicing cycles. The existing tension in the CT and vocals are thought to increase \(f_{\rm o}\)[16; 21] while vocal fold abduction acts to raise transglottal pressure and assist in the re-initiation of voicing. Prior work has shown that initial voicing onset cycles exhibit relatively higher \(f_{\rm o}\) when compared to subsequent voicing cycles [22; 23]. [PERSON] et al. [24] integrated information from these previous works to propose a theoretical model to interpret RFF in speakers with and without voice disorders. The authors hypothesized that offset and onset RFF values rely on three laryngeal factors: kinematics, aerodynamics, and muscle tension. Intervocalic offset cycles generally stabilize around 0 ST in speakers with typical voices, indicating a balance between the increasing laryngeal muscle tension needed to stop vocal fold vibration (increasing vocal fold vibration and cycle speed) and reduced transglottal pressure due to vocal fold abduction for the voiceless consonant production (decreasing vocal fold vibration and cycle speed). Conversely, intervocalic onset cycles in speakers with typical voices are consistently more positive than offset values at the start of the second vowel, hovering between 2 and 3 ST for onset cycle 1. The high RFF values observed during initial onset cycles are thought to occur due to increased transglottal pressure and peak flow during adduction [20] in combination with elevated longitudinal tension carried over from the consonant production. It follows that someone with a voice disorder who has impacted airflow, laryngeal kinematics, and/or laryngeal muscle tension would show aberrant RFF. [PERSON] et al. [24] hypothesized that patients with voice disorders characterized by excessive tension of the intrinsic and/or extrinsic laryngeal musculature would exhibit lower RFF values. Specifically, the authors proposed that elevated laryngeal muscle tension at baseline would prohibit increases in tension levels during voicing offset, which would typically help to counteract the effects of abduction. A lack of increased tension from baseline was speculated to result in more negative RFF offset cycle values compared to adults with typical voices, where values hover around 0 ST for voicing offset. It was also hypothesized that voicing onset would be affected by this inability to further modulate laryngeal tension, resulting in lower RFF onset values when compared to adults with typical voices. [PERSON] et al. [6] expanded on this model by distinguishing the effects of longitudinal and transverse laryngeal muscle tension for specific patient populations. The authors proposed that patients with phonotraumatic lesions may have elevated longitudinal and transverse tension--or tight adduction during phonation [25]--that interacts with the laryngeal kinematics proposed in the [PERSON] et al. [24] model.