Self-reported Voice-related Quality of Life in Adolescents With Paradoxical Vocal Fold Dysfunction

Paradoxical vocal fold dysfunction (PVFD) is a complex disorder of the larynx that primarily affects breathing. To date, no study has focused on the patient's own perception of how their quality of life related to voice is affected by the diagnosis of PFVD. The objective of this study is to characterize the self-reported voice-related quality of life (VR-QOL) patients diagnosed with PVFD using the Pediatric Voice Outcomes Survey (PVOS) instrument; an additional objective was to assess for the presence of any associations between laryngeal findings, concomitant medical disorders, and PVOS measures. The method used was a retrospective chart review. Twenty patients are identified in the study period; there were 17 females (mean age 14 years) and three males (mean age 11 years). The mean PVOS scores are uniformly normal for the subset of boys (n=3, mean PVOS 100). The mean PVOS for the 17 females was 89 (SD, 12.8), lower than that measured in healthy, age-matched females (n=13) who had a mean PVOS of 95 (SD, 7.4). The difference between these two groups was not statistically significant (P<0.17, unpaired t test). Although the absolute PVOS scores were affected by observed laryngeal abnormalities, asthma history, or reflux history, the absence of any of these were significantly more likely to be associated with a perfect score on the PVOS. The overall PVOS does not show a significant decline in the self-reported VR-QOL. Several cofactors may be associated with the detectable but not significant reduction in the PVOS scores for the females in this study.     

Japanese version of voice handicap index for subjective evaluation of voice disorder

Recently, the Voice Handicap Index (VHI), developed in the United States, has been highlighted as a means to assess a patient's perceptions of the severity of his or her voice disorder. The VHI is based on a self-administered questionnaire that quantifies the degree of a patient's disability related to his/her voice disorder. The questionnaire was translated into Japanese and applied to Japanese patients with various kinds of disordered voice or dysphonia. The results were analyzed and the usefulness discussed. In this study, 546 patients (281 males and 265 females) were included. Mean VHI scores were 36.2/120 in males and 44.1/120 in females. In the male patients, VHI scores were the highest among teens. However, VHI scores did not vary with age in the female patients. Patients with vocal fold paralysis, functional dysphonia, psychological dysphonia, and spasmodic dysphonia showed relatively high VHI scores, whereas those with laryngeal granuloma and laryngopharyngeal reflux disease showed low scores. In most diseases, functional and physiological scores were higher than emotional scores. In any treated patients, those with vocal nodule, vocal polyp, polypoid vocal fold, and recurrent laryngeal nerve paralysis, VHI scores decreased after therapeutic intervention. These findings suggest that the Japanese VHI is a useful tool for monitoring a patient's psychological status, choosing appropriate treatment, and assessing the therapeutic outcome.     

Incidence of supraglottic activity in males and females: a preliminary report

Supraglottic activity was rated from flexible endoscopic video recordings of subjects with normal laryngeal structure and function as they sustained vowels and repeated syllables and sentences. Judges rated these recordings for false vocal fold (FVF) adduction and anterior-to-posterior (A-P) compression at the initiation of the speech task, throughout the whole speech task (static supraglottic activity), and as brief individual adductions within a speech task (dynamic supraglottic activity). Significant differences in A-P (p < 0.0003) and FVF (p < 0.0000001) compression were found between tasks. Dynamic FVF activity was associated with glottal stops. Static A-P and FVF activities were present in males significantly more (p < 0.0001) than females. FVF activity associated with speech initiation was found in females significantly more (p = 0.0256) than males. Supraglottic activity plays a role in normal speech production, and should not necessarily be considered suggestive of a voice use pattern with excessive muscle tension.     

Differences Between Flexible and Rigid Endoscopy in Assessing the Posterior Glottic Chink

Objective

To determine whether different modalities of laryngeal examination produce differences in the assessment of the posterior glottic chink (PGC), and whether the prevalence of PGC differs by gender.

Introduction

The PGC has been described as a triangular laryngeal space between the posterior laryngeal wall and the vocal processes during glottic closure found commonly in human females, but less often in males. The purpose of this study was not only to identify whether there are gender differences in prevalence of posterior glottic but also to determine whether there is a difference in detection of this configuration dependent on the modality of laryngeal imaging, specifically flexible nasopharyngolaryngoscopy (FNPL) versus rigid laryngoscopy (RL).

Methods

A review of 104 consecutive initial laryngeal examinations was performed. All patients underwent both flexible laryngoscopy (FL) and RL performed under stroboscopic light. Patients with immobile vocal folds, masses causing glottic gaps, atrophy, or severe muscle tension dysphonia causing an inability to fully visualize the entire length of the vocal fold were excluded. In the remaining patients, the posterior glottic configuration showing a posterior chink in relation to the vocal process was graded on a 0–4 scale (called the Posterior Glottic Closure Score [PGCS]); 0 was used to indicate a closed glottis and 4 the most open configuration without creating a complete glottic gap. PGCSs for males were compared with those of females, and the PGCSs obtained by flexible nasopharyngolaryngoscopy was compared with RL.

Results

Fifty-two patients were included in the study. Twenty-four of the patients were male, and 28 were female. The average age of the patients was 48.4 years (SD ± 17.35), and the range was 15–81 years. On RL, eight males had a PGCS 1–4, that is, evidence of PGC, and 23 females had a PGCS 1–4. On flexible nasopharyngolaryngoscopy, only four males had a PGCS 1–4 and 24 females had PGCS 1–4. Twenty-two females had a PGC detected by both modalities, and the PGCS was significantly higher using RL (2.73 ± 0.70 vs 2.14 ± 0.834) than FL. For the four males in which PGC was detected by both modalities, there was no statistical significance when comparing the PGCS between RL and FL (1.75 ± 0.96 vs 1.75 ± 0.5, P < 0.05). When comparing only males and females who had a PGC (PGCS 1–4), females had a higher PGCS (2.65 ± 0.78) than males (1.75 ± 0.71, P < 0.05) on RL, indicating a more open posterior glottis in females. On flexible examination, there was no difference detected in the average PGCS, 2.08 ± 0.83 for females and males 1.75 ± 0.50. PGCs were more common in younger (age 43 years) than older (age 54–56 years) subjects for both laryngoscopic modalities.

Conclusion

From this pilot study, we determined that there is a difference in male and female PGC prevalence and size. PGC is more common in females than males. Prevalence (or the detection rate) is about the same with RL and FL in females, but higher with RL than FL in males. The average score of the glottic opening, when present, was statistically significantly different between RL and FL in females but not in males. Furthermore, females had a larger PGCS on both modalities when compared with males, although this difference was only found to be statistically significant on RL; and complete glottic closure was more common in older than in younger subjects.     

Vocal Nodules and Laryngeal Morphology

Our purpose was to study the occurrence of vocal fold nodules under conditions of habitual vocal abuse associated with increased laryngeal muscle tension, to identify the existence of a relationship between vocal nodules and laryngeal morphology. We studied one group of 30 subjects with vocal nodules, 18 to 50 years old, who were compared with two control groups, one of females and one of males, consisting of 30 subjects each. The parameters evaluated were: type of vocal folds coaptation, glottic proportion (GP) and abduction angle (AA), obtained by videotelelaryngoscopy. In the nodules group, the larynges presented a mean value of GP similar to that of the female group, both of which were lower than the mean GP value of the male group. On the other hand, the mean AA was lower than the one in the female group, and closer to the one in the male group. We concluded that vocal nodules were present only in larynges with a predominantly young female morphology, with functional limitations of abduction.     

Measuring the neck frequency response function of laryngectomy patients: implications for the design of electrolarynx devices

Measurements of the neck frequency response function (NFRF), defined as the ratio of the spectrum of the estimated volume velocity that excites the vocal tract to the spectrum of the acceleration delivered to the neck wall, were made at three different positions on the necks of nine laryngectomized subjects (five males and four females) and four normal laryngeal speakers (two males and two females). A minishaker driven by broadband noise provided excitation to the necks of subjects as they configured their vocal tracts to mimic the production of the vowels /a/, /ae/, and /I/. The sound pressure at the lips was measured with a microphone and an impedance head mounted on the shaker measured the acceleration. The neck wall passed low-frequency sound energy better than high-frequency sound energy, and thus the NFRF was accurately modeled as a low-pass filter. The NFRFs of the different subject groups (female laryngeal, male laryngeal speakers, laryngectomized males, and laryngectomized females) differed from each other in terms of corner frequency and gain, with both types of male subjects presenting NFRFs with larger overall gains. In addition, there was a notable amount of intersubject variability within groups. Because the NFRF is an estimate of how sound energy passes through the neck wall, these results should aid in the design of improved neck-type electrolarynx devices.     

Stroboscopic and acoustic measures of inspiratory phonation

Inspiratory phonation (IP) is the production of voice as air is taken into the lungs. Although IP is promoted as a laryngeal assessment and voice treatment technique, it has been described quantitatively in very few speakers. This study quantified changes in laryngeal adduction, fundamental frequency, and intensity during IP relative to expiratory phonation (EP). We hypothesized that IP would increase laryngeal abduction and fundamental frequency. The experiment was a within-subjects, repeated measures design with each subject serving as her own control. Participants were 10 females (ages 19-50 years) who underwent simultaneous transoral videostrobolaryngoscopy and acoustic voice recording. We found that membranous vocal fold contact decreased significantly during IP relative to EP, while the trends for change of ventricular fold squeeze during IP varied across individuals. Vocal fundamental frequency increased significantly during IP relative to EP, but intensity did not vary consistently across conditions. Without teaching or coaching, changes that occurred during IP did not carry over to EP produced immediately following IP within the same respiratory cycle.     

The effect of head position on glottic closure in patients with unilateral vocal fold paralysis

The present study was designed to assess the effect of head position on glottic closure as reflected in airflow rates (open quotient and maximum flow declination rate), in patients with unilateral vocal fold paralysis. Ten patients, 2 males and 8 females ranging in age from 40 to 75, with a mean age of 57.3, served as subjects. Airflow measures were taken during sustained phonation of two vowels (/i/ and /a/) in 3 head positions (center, right, left). Vowels /i/ and /a/ were produced at subject's comfortable pitch and loudness, with random ordering of both vowel order and head orientation. Subjects were trained to focus eye gaze on right and left markers (70-degree angle) and a central marker at eye level directly in front of the subject. Theoretically, if turning the head during phonation alters the laryngeal anatomic relationship by bringing the vocal folds in closer proximity to one another, then airflow rate should lessen. Our results indicate that head position does not improve glottic closure in these patients, which is in contrast to previously published research.(1) Our results question the utility and underlying theoretical construct for the use of head turning as a therapeutic technique for improvement of voice in patients with unilateral vocal fold paralysis.     

The False Vocal Folds: Shape and Size in Frontal View During Phonation Based on Laminagraphic Tracings

The geometry of the false vocal fold region during phonation is important to the understanding of the aerodynamics and acoustics of voice. The shape and dimensions of this region during phonation were estimated using laminagraphic tracings of the larynx. Laminagrams from two previous studies, one with non-singer subjects (Experiment I, Hollien and Colton, 1969) and the other with singers (Experiment II, Wilson, 1972), were traced, photocopied, and measured. Statistical analysis showed significantly greater false vocal fold height in males than females for both experiments. The false vocal fold gap was also significantly greater in males than females for Experiment II, but reached only borderline significance for Experiment I. For each gender, most of the linear measures were greater in Experiment I when compared to Experiment II; these differences may be passive in nature (due to actual differences in subject size) or active (due to muscle contraction that displaced the false vocal folds during singing).     

Relationship among glottal area, static supraglottic compression, and laryngeal function studies in unilateral vocal fold paresis and paralysis

In this study, we evaluated the relationship between laryngeal function measures and glottal gap ratio and normalized measures of supraglottic behaviors in patients with unilateral vocal fold paresis (UVFP). Thirty-one patients were found to have unilateral vocal fold paresis by videoendoscopy and laryngeal electromyography, and 13 controls participated in this study. Patients with UVFP demonstrated significantly larger glottal gap ratios (p = 0.016) than control subjects. The nonparalyzed or contralateral vocal fold was associated with significantly more static false vocal fold compression (p = 0.03) compared with the paralyzed vocal fold or with the controls. Patients with unilateral vocal fold paresis were divided into subgroups: those with normal or abnormal maximum phonation time, flow, or pressure measures. Smaller glottal gap ratios were identified in patients with normal maximum phonation times and flow measures. Greater false vocal fold activity was identified in unilateral vocal fold paresis patients with normal laryngeal function measures than in unilateral vocal fold paresis patients with abnormal measures. These findings suggest that some patients with documented unilateral paresis and glottal incompetence can compensate for vocal fold weakness such that their acoustic and aerodynamic measures are normal.     

Standardized laryngeal videostroboscopic rating: Differences between untrained and trained male and female subjects, and effects of varying sound intensity, fundamental frequency, and age

To determine the influence of the factors gender, vocal training, sound intensity, pitch, and aging on vocal function, videolaryngostroboscopic images of 214 subjects, subdivided according to gender and status of vocal training, were evaluated by three judges with standardized rating scales, comprising aspects of laryngeal appearance (larynx/pharynx ratio; epiglottal shape; asymmetry arytenoid region; compensatory adjustments; thickness, width, length, and elasticity of vocal folds) and glottal functioning (amplitudes of excursion; duration, percentage, and type of vocal fold closure; phase differences; location of glottal chink). The video registrations were made while the subjects performed a set of phonatory tasks, comprising the utterance of the vowel /i/ at three levels of both fundamental frequency and sound intensity. Analysis of the rating scales showed generally sufficient agreement among judges. With the exception of more frequently observed complete closure and lateral phase differences of vocal fold excursions in trained subjects, no further differences were established between untrained and trained subjects. With an α level of p = 0.005, men differed from women with respect to laryngeal appearance (larynx/pharynx ratio, compensatory adjustments, and the presence of omega and deviant-shaped epiglottises), and their vocal folds were rated thicker in the vertical dimension, smaller in the lateral dimension, longer, and more tense, with smaller amplitudes of excursion during vibration. Glottal closure in male subjects was rated more complete, but briefer in duration. Significant effects of the factors pitch, sound intensity, and age on vocal fold appearance and glottal functioning were ascertained. Awareness of the influence of these factors, as well as the factor gender, on the rated scales is essential for an adequate evaluation of laryngostroboscopic images.     

Nonlinear source-filter coupling in phonation: vocal exercises

Nonlinear source-filter coupling has been demonstrated in computer simulations, in excised larynx experiments, and in physical models, but not in a consistent and unequivocal way in natural human phonations. Eighteen subjects (nine adult males and nine adult females) performed three vocal exercises that represented a combination of various fundamental frequency and formant glides. The goal of this study was to pinpoint the proportion of source instabilities that are due to nonlinear source-tract coupling. It was hypothesized that vocal fold vibration is maximally destabilized when F(0) crosses F(1), where the acoustic load changes dramatically. A companion paper provides the theoretical underpinnings. Expected manifestations of a source-filter interaction were sudden frequency jumps, subharmonic generation, or chaotic vocal fold vibrations that coincide with F(0)-F(1) crossovers. Results indicated that the bifurcations occur more often in phonations with F(0)-F(1) crossovers, suggesting that nonlinear source-filter coupling is partly responsible for source instabilities. Furthermore it was observed that male subjects show more bifurcations in phonations with F(0)-F(1) crossovers, presumably because in normal speech they are less likely to encounter these crossovers as much as females and hence have less practice in suppressing unwanted instabilities.     

Effects of Inhalatory Abdominal Wall Movement on Vertical Laryngeal Position During Phonation

The configuration of the body resulting from inhalatory behavior is sometimes considered a factor of relevance to voice production in singing and speaking pedagogy and in clinical voice therapy. The present investigation compares two different inhalatory behaviors: (1) with a "paradoxical" inward movement of the abdominal wall, and (2) with an expansion of the abdominal wall, both with regard to the effect on vertical laryngeal position during the subsequent phonation. Seventeen male and 17 female healthy, vocally untrained subjects participated. No instructions were given regarding movements of the rib cage. Inhaled air volume as measured by respiratory inductive plethysmography, was controlled to reach 70% inspiratory capacity. Vertical laryngeal position was recorded by two-channel electroglottography during the subsequent vowel production. A significant effect was found; the abdomen-out condition was associated with a higher laryngeal position than the abdomen-in condition. This result apparently contradicted a hypothesis that an expansion of the abdominal wall would allow the diaphragm to descend deeper in the torso, thereby increasing the tracheal pull, which would result in a lower laryngeal position. In a post-hoc experiment including 6 of the subjects, body posture was studied by digital video recordings, revealing that the two inhalatory modes were clearly associated with postural changes affecting laryngeal position. The "paradoxical" inward movement of the abdominal wall was associated with a recession of the chin toward the neck, such that the larynx appeared in a lower position in the neck, for reasons of a postural change. The results suggest that the laryngeal position can be affected by the inhalatory behavior if no attention is paid to posture, implying that instructions from clinicians and pedagogues regarding breathing behavior must be carefully formulated and adjusted in order to ensure that the intended goals are reached.     

Analysis of Vocal-fold Vibrations from High-Speed Laryngeal Images Using a Hilbert Transform-Based Methodology

This paper presents a Hilbert transform-based approach to analyze vocal fold vibrations in human subjects exhibiting normal and abnormal voice productions. This new approach is applied to the analysis of glottal area waveform (GAW) and is capable of providing useful information on the vocal fold vibration. The GAW is extracted from high-speed laryngeal images by delineating the glottal edge for each image frame. An analytic signal is generated through the Hilbert transform of the GAW, which yields a recognizable pattern of the vocal fold vibration in the analytic phase plane. The vibratory pattern is comprehensive and can be correlated with specific voice conditions. Quantitative measures of the glottal perturbation are introduced using the analytic amplitude and instantaneous frequency obtained from the analysis. Examples of clinical voice recordings are used to evaluate and test the effectiveness of this approach in providing qualitative representation and quantitative characteristics of vocal fold vibratory behavior. The results demonstrate the potential of using this new analytical tool incorporated with the high-speed laryngeal imaging modality for clinical voice assessment.     

Documenting laryngeal change following prolonged loud reading: A videostroboscopic study

This study investigated the effects of prolonged loud reading on trained and untrained subjects. Subjects were eight young women singers, and eight young women with limited musical experience. Each subject underwent videostroboscopic examination prior to and following 1 h of prolonged loud reading. The pretest and posttest videotaped samples were randomized and presented to three experienced judges, who evaluated various aspects of laryngeal appearance and vibratory characteristics. Analyses of group data revealed that untrained subjects showed a small but significant increase in amplitude of vocal fold excursion following the experimental task. No significant differences were noted in the trained singer group. When individual variation was analyzed, it was found that most subjects did not show many changes from pretest to posttest. It was concluded that a l-h loud-reading task was not sufficient to induce notable laryngeal alterations     

The Frequency of Perceived Stress, Anxiety, and Depression in Patients with Common Pathologies Affecting Voice

The study's objectives were to investigate (1) the frequency of perceived stress, anxiety, and depression for patients with common voice disorders, (2) the distribution of these variables by diagnosis, and (3) the distribution of the variables by gender. Retrospective data were derived from self-report questionnaires assessing recent stress (Perceived Stress Scale-10), anxiety, and depression (Hospital Anxiety and Depression Scale) in a cohort of new patients presenting to a voice clinic. Data are presented on 160 patients with muscle tension dysphonia (MTD), benign vocal fold lesions, paradoxical vocal fold movement disorder (PVFMD), or glottal insufficiency. Pooled data indicated that average stress, anxiety, and depression scores were similar to those found for the healthy population. However, 25.0%, 36.9%, and 31.2% of patients showed elevated stress, anxiety, and depression scores, respectively, compared to norms. Patients with PVFMD had the most frequent occurrence-and patients with glottal insufficiency had the least frequent occurrence of elevated stress, anxiety, and depression. Stress and depression were more common with MTD than with lesions, whereas reverse results were obtained for anxiety. More females than males had elevated stress, anxiety, and depression scores. The data are consistent with suggestions that stress, anxiety, and depression may be common among some patients with PVFMD, MTD, and vocal fold lesions and more common for women than men. However, individual variability in the data set was large. Further studies should evaluate the specific role of these conditions for selected categories of voice disorders in susceptible individuals.     

Strobophotoglottographic transillumination as a method for the analysis of vocal fold vibration patterns

The purpose of this exploratory study was to determine if laryngeal transillumination in combination with stroboscopy (strobophotoglottography; SPGG) is useful for (1) the visualization of vocal fold vibration (VFV) opening patterns, (2) the localization of initial vocal fold opening in horizontal glottal thirds (anterior, midmembranous, and posterior), (3) determination of the temporal correspondence of the so-called electroglottography (EGG)-knee and initial vocal fold separation, and, finally, (4) automatized quantitative measurements of glottal area function within endoscopic images. With stroboscopic transillumination, initial inferior vocal fold separation was detectable during the "closed" phase, where the vocal folds were still closed in the upper portion and therefore initial inferior vocal fold separation could not be visualized with usual laryngoscopy techniques. In the horizontal plane within similar fundamental frequencies in modal voice registers in two male subjects, localization of initial glottal opening depended on the voice types used (soft, normal, or pressed phonation). We found zipperlike posterior-to-anterior openings, initial midmembranous openings, initial anterior openings, as well as simultaneous initial opening of all three portions in the two healthy male adults examined. This technique proved to add temporal and spatial information to vocal fold opening patterns and extends our examination techniques to the very beginning of vocal fold opening at the inferior portion. Simultaneous electroglottogram tracking and comparison with bidirectionally illuminated stroboscopic images revealed a time-locked correspondence of the EGG-knee with the aforementioned initial inferior vocal fold separation. Bidirectional illumination combined with digital color extraction techniques allowed for image separation of subglottally and supraglottally illuminated structures. This facilitated vocal fold contour detection and automatized image processing, for example, for determination of glottal area function, and is considered to be a further step to objective automatized quantitative measurements within endoscopic images.     

Paradoxical vocal fold motion: Presentation and treatment options

Paradoxical vocal fold motion is a rare disorder in which adduction of the folds occurs on inspiration. The disorder presents with signs of airway obstruction and often airway distress, so proper diagnosis by the otorhinolaryngologist is critical to subsequent management. We present a retrospective review of 10 patients with the diagnosis of paradoxical vocal fold motion seen over a 6-year period. Eight patients were females, and 6 required an acute airway intervention at presentation; 3 patients eventually underwent tracheotomy for respiratory decompensation. Six patients had a prior diagnosis of asthma, and this was determined to contribute to their respiratory status. Five patients were treated with botulinum toxin and 2 with flexible nasolaryngoscopic biofeedback, which improved the outcome. A review of the literature confirms a female predominance of patients presenting with paradoxical adduction and airway distress, often with a history of asthma and psychopathology. Our experience with botulinum toxin and biofeedback suggests that these procedures are viable treatment options in the management of patients with this disorder.     

An Investigation of a Modal-Falsetto Register Transition Hypothesis Using Helox Gas

This study concerned the effect of the first subglottal formant (F1') on the modal-falsetto register transition in males and females. Phonations using air and a helium-oxygen mixture (helox) were used in a comparative study to tease apart possible acoustic and myoelastic contributions to involuntary register transitions. Recordings of the first subglottal formant and its accompanying bandwidths, and the lower and upper shift point marking the outer boundaries of abrupt register transitions, were obtained via a neck-mounted accelerometer, and analyzed using spectrograms and power spectra on a K-5500 Sona-Graph. The four subjects had their hearing masked bilaterally with speech level noise to increase the likelihood of involuntary register transition via minimized auditory feedback. In three of the four test subjects registration was surmised to be primarily a laryngeal event, as evidenced by the similar frequency dependency of voice breaks in both air and helox. It may be hypothesized that subglottal resonance influenced register transition in the fourth subject, as voice breaks rose with helox-induced phonation; however, this result did not reach statistical significance. Therefore, in this experiment subglottal resonance was not found to have a significant influence on register transition as originally hypothesized.     

Interrelation of mandibular laryngeal functions

The purpose of the experiment was to explore relations between jaw and laryngeal functions. The general question was whether laryngeal adduction was affected by jaw opening or by jaw biting. Twelve untrained, vocally healthy male and female adults participated as subjects. Subjects produced repeated tokens of /uh/ in each of 12 experimental conditions involving combinations of 3 jaw openings (10 mm, 25 mm, 40 mm), 2 jaw biting pressures (10 kPa, 200 kPa), and 2 fundamental frequencies (conversational and high). For each token, laryngeal adduction was estimated from the electroglottographic closed quotient. The most straightforward results were that (1) laryngeal adduction increased as jaw opening increased at the conversational pitch, for all subjects, independent of biting pressure, and (2) laryngeal adduction increased as biting pressure increased, at the conversational pitch, for males, independent of jaw opening. Other relations between estimated laryngeal adduction and jaw manipulations were more complex, varying with fundamental frequency and gender. Speculations are made about possible biomechanical and neurological explanations for the findings.