Short-Term Effects of Lessac-Madsen Resonant Voice Therapy on Maximum Phonation Time and Its Association With Acoustic Features in Young Healthy Japanese Women.

Healthy individuals carry a risk of developing voice disorders from vocal abuse, prompting the development of preventive measures. We hypothesized that resonant voice exercises, typically prescribed for individuals with vocal problems, might positively affect phonatory performance in those without existing voice disorders. Here, 39 healthy female university students participated in an experimental session featuring an introductory Lessac-Madsen Resonant Voice Therapy (LMRVT). The LMRVT consisted of vocal musculature relaxation exercises and the repetition of words initiated by the nasal consonant /m/. For assessment, we recorded maximum phonation time (MPT), sustained single-vowel phonation, and a passage-reading task before and after the LMRVT intervention. The results demonstrated a significant prolongation of MPT following the intervention, whereas no acoustic feature showed a significant group-level change after correction for multiple comparisons. In multiple regression analyses, we tested whether acoustic changes were associated with individual differences in ΔMPT. These analyses consistently identified a reduction in the Hammarberg Index, reflecting increased high-frequency regional resonance, as a significant correlate of ΔMPT in both sustained-vowel phonation and passage-reading tasks. The multiple regression analysis for the passage-reading task also showed a trend for a decrease in vowel-space area. An exploratory Bayesian structural equation modeling (BSEM) was then conducted to characterize the shared variation among ΔMPT and the acoustic measures, assuming "vocal economy" as a latent factor related to the intervention-associated change. The ΔHammarberg Index showed the strongest negative loading on the vocal economy factor, which was consistently associated with ΔMPT across tasks. These findings suggest that introductory resonant voice exercises may improve MPT in healthy individuals and that the Hammarberg Index may be a candidate acoustic correlate of individual differences in MPT improvement.