obstructive sleep apnea/hypopnea syndrome (OSAHS) is characterized by repetitive episodes of upper airway (UA) closure during sleep. The contraction of UA dilator muscles is critical in the maintenance of UA patency, since it is the only force counterbalancing the collapsing effects resulting from an increase in tissue weight, an increase in UA collapsibility, or alteration of UA shape/dimension, emphasizing the importance of mechanical effectiveness of these muscles and their propensity to develop fatigue (10).

So far, in the UA of patients with obstructive sleep apnea (OSA), adaptive changes of muscle properties, neural drive, and sensorimotor function have been investigated by a variety of neurophysiological approaches, including histochemical, physiological, and electrophysiological studies (12, 13, 36, 38, 43, 44, 46). These findings were consistent with alteration of the mechanical efficacy of UA dilators as a consequence of myogenic or neurogenic changes of UA in response to adaptive training effect. Collectively, factors such as UA shape (34), UA muscle (UAM) physiological and histochemical properties (36, 43), muscular tissue damage (or remodeling) (46, 50), neurogenic injury (or adaptation) (36–40), UA fluid content (51), and mechanical properties of surrounding soft tissues are thought to interfere with the mechanical effectiveness of UA stabilizing muscle contraction and concur to influence the net mechanical effect of the neuromuscular activation process (30).

In the context of an increase in UAM activity with alteration of mechanical effectiveness, these factors may contribute to the development of fatigue of UA dilators that could further worsen their ability to maintain UA patency during sleep. Short-term activation of genioglossus (GG) is accompanied by an ≈50% decrease in GG endurance (42). In vitro studies demonstrated that, in patients with OSA, GG fatigue was observed and improved following continuous positive airway pressure therapy (4). The role of fatigue in the worsening of obstructive breathing disorders throughout the night is further supported by the positive relationship between the decline in UA stability following negative UA pressure trials during wakefulness and the increase in apnea index from the first to the last quarters of the night (14).

Considering that inadequate UA dilator muscle functions (i.e., weakness and fatigue) are prone to contribute to UA instability, reliable investigations of UAM mechanical properties are pivotal in the pathophysiology of OSA. Thus, assessing to what extent tongue muscle impairment is associated with UA instability as a function of OSA severity and patient anthropometric characteristics is a topic of interest. In fact, tongue mechanical properties have been assessed mainly using protrusion protocol with conflicting results, as some but not all studies have documented an increase in fatigability of tongue muscle in patients with OSA (3, 12, 22, 24). However, there is limited information regarding the importance of tongue fatigue on OSA pathophysiology since, overall, participating patients had severe OSA. In this regard, comparison of UAM fatigue between subjects with different sleep apnea status is needed to establish the physiological importance of this UAM feature in OSA, another issue related to the specific movement used to characterize tongue fatigue. Given that GG muscle includes a continuous array of fibers with varying orientation and mechanical properties, the protrusion and elevation maneuvers obviously recruit different arrays of tongue muscle fibers, with a dominant role of intrinsic and dorsally oriented horizontal fibers of extrinsic tongue muscle for protrusion and with anterior and vertical fibers of the GG for elevation (11, 15). Therefore, it is reasonable to question whether tongue mechanical features could be varyingly altered by protrusion/elevation of the tongue in patients with OSA.

Therefore, the aim of this study was to assess the mechanical performance (strength, endurance, fatigability, and strength stability) of tongue muscle using elevation and protrusion tasks in patients with a wide range of severities of obstructive sleep apnea events. We hypothesized that tongue mechanical properties would differ between the two tasks and that tongue function may be more altered in patients with less severe OSA disease.

Normal data distribution (Shapiro-Wilk test) and homogeneity of variances (Levene<s test) were computed. Anthropometric and sleep characteristics were examined using a one-way ANOVA with heterogeneous variances between groups. The Tukey-Kramer post hoc procedure was performed to establish the statistical significance of differences among groups when appropriate. The statistical analyses of tongue mechanical data were performed using a two-way mixed model. A fixed factor was defined as the comparison among groups according to severity of OSA, BMI, or age, and the other fixed factor was linked to the measurements taken before and after the fatigue task with an interaction term between fixed factors. The latter was analyzed as a repeated-measure factor using an unstructured covariance matrix, which was also used to investigate tongue strength and CV data changing over time during fatiguing tasks. Associations between OSA severity, anthropometrics, and tongue function data were assessed with Spearman<s rank correlations. When necessary, statistical analyses were adjusted for AHI, BMI, or age to further control for the impact of them on dependent tongue mechanical data. When appropriate, some variables were log-transformed to fulfill the model assumptions and report P values that were based on these transformations. As to the fatigue curve, an interval-censored analysis was performed with a Weibull distribution to investigate the distribution of subjects suffering fatigue at each time interval. Analyses were also completed, with subjects gathered according to BMI and age, with cutoff values of 30 kg/m −2 and 60 yr, respectively. The latter value was chosen based on previous results reporting a weaker strength observed in subjects with age >60 yr when compared with younger subjects ( 28 ). All analyses were conducted using the statistical package SAS version 9.4 (SAS Institute, Cary, NC), and the significant level was set to 5%.

In addition to the time to task failure described above ( criteria 1 ), two other criteria were established to assess tongue endurance. One ( criteria 2 ) consisted of the time elapsed until the subjects could not maintain 80% of the baseline MVC force for ≥1 s in two consecutive attempts. The third criterion ( criteria 3 ) was defined as the time elapsed until failure of three consecutive submaximal attempts (70% of MVC) to reach 95% of the target force for at least half of the required contraction time. The acquisition of data according to the last criteria was accomplished via smoothing the spikes of force with a fixed smoothing value. The development of these three criteria was based on our previous experience with UA fatigue protocols supporting the need to develop refined analysis criteria. Endurance measured according to the last two criteria was reanalyzed by a second investigator. Intrarater and interrater agreement were within 10 s for 90 and 85% of the trials, respectively, and the intra-rater/scorer difference did not exceed the difference observed between groups, so that data accuracy was considered as strong. Since the real time for development of fatigue was within a time interval earlier or equal to the last MVC attempt, it is not possible to identify the exact timing of fatigue occurrence (according to criteria 1 ). Therefore, a cumulative distribution curve was built to examine the percentage of subjects who developed fatigue at each time interval (fatigue curve).

The assessment of tongue fatigue induced by the tasks was quantified by the changes in MVC, MMF, and CV throughout the trial. Fatigability was represented by the slope of the changes in strength overtime. The fatigue index was determined by calculating the ratio of the difference in strength or stability throughout the fatigue procedure to their respective baseline values.

Strength data for protrusion and elevation contraction in forms of force (N) and pressure (kPa), respectively, were recorded and digitized using the Labchart software during the experiment. Mean maximal force/pressure (MMF) was determined for the last 4-s segments of each MVC value throughout the endurance trial. This duration was chosen to bypass the initial ≤1-s segment for force elevation before the target level was reached ( Fig. 1 C ). Instability of force/pressure developed during MVC was assessed by its coefficient of variation (CV) corresponding to the ratio of standard deviation of the force/pressure values to MMF.

Fig. 1. A : representative example of recording of maximal voluntary tongue contraction force (MVC) measurements for tongue elevation. In this case, the MVC achieved by the subject was 44 kPa (kilopascals). B : display of the repetitive isometric contraction task with the recording of MVC during and after the protocol. Endurance time according to criteria 1 and recovery time criteria are also presented. In this case, endurance time was 6 min, whereas recovery time was 4 min. C : display of mean maximal force (MMF) within 4 s and coefficient of variation (CV) within 4 s in 1 MVC effort. MMF and CV were obtained after the trial (CV = SD/mean). D : illustration of the area under the curve for one of the contractions generated by MVC or submaximal efforts.

For each task, a session of familiarization regarding tongue protrusion/elevation maneuvers preceded the trial so as to minimize the learning effect. During familiarization, subjects were asked to perform warmup maximal efforts for several attempts, followed by complete relaxation. Then, maximal voluntary tongue contraction force (MVC) was assessed for each task. Subjects were asked to perform at least three MVCs to obtain <10% difference between upper and lower values. MVC was derived as the peak value obtained during each attempt separated by at least 1-min intervals ( Fig. 1 A ). During fatigue protocols, subjects were instructed to complete sessions of sustained (5 s) submaximal (70% of MVC) isometric contractions repetitively (until exhaustion), with strong verbal encouragement to keep the pressure steady. Subjects had a visual feedback of the real-time force output and target force level from the computer screen throughout the trial. A 5-s rest was given between each contraction. Furthermore, subjects were asked to perform brief (5 s) MVC every 50 s (every 5 efforts) to measure changes in MVC overtime. Endurance time ( criteria 1 ) was assessed by time to task failure, which represented the time elapsed from the beginning of the fatigue protocol up to the time subjects were not be able to develop 80% of baseline MVC on two consecutive maximal effort attempts. This was followed by a recovery session where control MVCs were repeated every 2 min until MVC had reached 90% of the baseline MVC value ( Fig. 1 B ).

Experiments were conducted at the Institut Universitaire de Cardiologie et de Pneumologie de Québec Clinical Research Center. Measurements started at 9 AM. Protrusion and elevation protocols were completed in a randomized crossover study design. Each subject was asked to produce repetitive isometric contractions of tongue muscle until exhaustion, using protrusion and elevation movements with the dedicated equipment for baseline measurements of force and completion of the fatigue protocol. A 30-min resting period separated the two arms of the protocol.

The Iowa Oral Performance Instrument (IOPI) device is a portable, handheld device that uses an air-filled plastic tongue bulb connected via a catheter to a pressure transducer providing the pressure inside the bulb in kilopascals (kPa). The IOPI bulb was positioned between the tongue and the anterior part of the hard palate for the tongue to push up against. The bulb is connected to a catheter inserted into a 3-mm id rigid tube placed between the incisors, allowing for measurements of the pressure developed by the tongue while pushing on the hard palate through the IOPI<s pressure-transducer circuitry ( 1 ). The test-retest reliability of the results provided by this technique is high (Cronbach-α 0.98 with elevation, coefficient of variability 6.3% with protrusion) ( 25 ). Compared with the protrusion maneuver, tongue performance using the IOPI device differed not only by the direction of tongue movement (and then the primary recruited muscles), but also by strength measurements units, which were force (N) and pressure (kPa) for protrusion and elevation displacements, respectively.

During the protrusion maneuver, the fitting of the head position by a chin support and forehead plate was carefully checked, so as to standardize the position of the tongue, the mandibular, and the plate connected to force transducer. The position of the head was kept unchanged throughout the trial to maintain good reliability and consistency of measurements. With head position fixed, subjects were asked to bite on the stem, which was inserted into a rubber tube (20 mm id) for protection of incisors. The tongue was affixed on the plate placed just behind the incisors ( 24 ), with protrusion force transmitted to the force transducer. Protrusion force was measured in Newtons (N).

Twelve normal plus mild (including normal subjects and mild OSA) subjects and 28 (moderate-severe) patients with OSA were recruited, whose status was confirmed by a conventional sleep study. Subjects were ≤75 yr old (18–72 yr), had regular sleep habits, and were free of sleep debt (insomnia, sleep deprivation), free of history/symptoms of heart failure and neurologic disease, had no treatments with sedatives, tranquillizers, antidepressive drugs, or narcotics, and alcohol consumption of <0.5 g alcohol·kg −1 ·day −1 . Patients with OSA were not treated and were free of nocturnal hypoventilation. All subjects reported no specific use of tongue muscle that might influence airway and tongue muscle strength, such as singing or wind instrument playing for at least the past 5 yr. This protocol was approved by the Institutional Ethics Review Board of the Pneumologue Institut Universitaire de Cardiologie et de Pneumologie de Québec, and written consent was signed by each subject.

Fig. 3. A and B : cumulative occurrence of fatigue at the end of each recording interval according to obstructive sleep apena (OSA) severity in the whole study group for tongue elevation and protrusion task, respectively. ●, Control group, n = 12; □, moderate group, n = 17; ▲, severe group, n = 11. C and D : cumulative occurrence of fatigue at the end of each recording interval according to BMI in OSA subjects for elevation and protrusion task, respectively. ■, Obese subjects, n = 12; □, nonobese subjects, n = 28.

Using a 60-yr cutoff value, the effect of age on tongue mechanical features was assessed in two age groups (young: 47.1 ± 9.0 yr; old: 64.5 ± 3.6 yr). BMI and AHI were not significantly different between the two groups. A significant effect of age on tongue function was observed after adjusting for BMI and AHI, with older subjects presenting shorter endurance time ( criteria 2 ; 222 ± 137 vs. 392 ± 223 s) and longer recovery time (360 ± 210 vs. 200 ± 114 s) than the younger group [ F (1,24) = 8.69, P = 0.007; F (1,24) = 7.34, P = 0.02]. These differences were observed only for protrusion task.

Among nonobese subjects with or without OSA ( n = 28), total muscle work for protrusion was negatively correlated with AHI, whereas it was positively correlated with AHI among obese subjects ( n = 12) ( Table 4 and Fig. 2, A–C ). For the elevation task, BMI was positively correlated with total muscle work and endurance time ( criteria 1 ) in obese subjects, whereas for protrusion, positive correlations were found between tongue strength (MVC at baseline, MVC after fatigue, and MMF at baseline) and neck circumferences in all subjects. ( Table 5 ).

Using a 30 kg/m −2 BMI cutoff, moderate and severe patients with OSA were divided into obese ( n = 10) and ( n = 18) nonobese groups with BMI values of 26.4 ± 2.6 and 31.9 ± 1.2 kg/m −2 , respectively [ F (1,25.9) = 8.78, P = 0.01]. Nonobese patients were significantly younger (52.0 ± 12.6 yr) than obese ones (60.7 ± 6.4 yr) [ F (1,25.9) = 5.87, P = 0.02]. AHI was not significantly different between the two groups. After adjusting for age and AHI, there was a tendency for a shorter endurance time ( criteria 1 ) in nonobese OSA subjects (elevation: 433 ± 252 s; protrusion: 443 ± 240 s) than in obese subjects (elevation: 546 ± 210 s; protrusion: 534 ± 180 s), regardless of type of task [ F (1,22.9) = 3.83, P = 0.06]. MMF after protrusion fatiguing task tended to be lower in nonobese patients with OSA (19.2 ± 7 N) when compared with obese patients (27.4 ± 7 N) [ F (1,18.9) = 3.76; P = 0.07]. Muscle total work was significantly lower in nonobese (elevation: 7,850 ± 5,254 kPa/s; protrusion: 3,882 ± 2,525 N/s) than in obese OSA (elevation: 11,281 ± 4,658 kPa/s; protrusion: 5,997 ± 2,562 N/s) for both elevation and protrusion tasks [ F (1,26) = 5.86, P = 0.02; F (1,26) = 7.29, P = 0.01]. Noteworthy is that the above findings were not observed when obese and nonobese subjects were split among the whole study population ( n = 40).

For both tasks, endurance time ( criteria 2 and 3 ) was negatively correlated with CV measured at baseline for elevation ( r s = −0.430, P = 0.01) and protrusion ( r s = −0.330, P = 0.04), respectively. After subjects were split according to the median AHI value (as a cutoff value) of 18.5 events/hour, tongue endurance time ( criteria 1 ) for elevation task and muscle total work for protrusion were negatively correlated with AHI supine in the less severe OSA group, whereas conversely, endurance time ( criteria 2 and 3 ), strength (MVC after fatigue, MMF at baseline), muscle total work, and CV increment overtime for both tasks were positively correlated with AHI in patients with more severe with OSA ( Table 3 ).

Despite highly reproducible values obtained at baseline, CV was found to increase during fatiguing tasks. Increment in CV with time was higher during the elevation task [ F (15,402) = 1.74, P = 0.04]. Furthermore, the increase in CV with time was observed in the three groups [ F (15,337) = 7.46, P < 0.0001], but the degree of increment varied between groups. Looking at the elevation task, a significant OSA group effect was observed regarding CV increment overtime (CV △/pre% ), with the smallest variability observed in moderate OSA subjects [ F (28,384) = 1.54, P = 0.04].

Type of task was found to influence endurance time ( criteria 2 ), with a longer endurance for protrusion than for elevation [ F (1, 23.9) = 7.06, P = 0.01]. In addition, sleep apnea status influenced endurance. After adjusting for BMI and age, the lowest muscle total work for protrusion task [ F (2,35) = 4.84, P = 0.01 ] and the shortest endurance time ( criteria 3 ) [ F (2,22.3) = 3.64, P = 0.04] were observed in moderate OSA subjects regardless of type of task. The difference was most obviously observed between patients with moderate OSA and the normal plus mild OSA group. No difference in fatigability or fatigue index was observed between tasks or groups.

DISCUSSION

This study was designed to explore tongue mechanical performance (strength, endurance, and fatigue) using two types of fatiguing task (elevation and protrusion) among subjects with different sleep apnea status. Our results indicated that tongue elevation task was more prone to fatigue and that baseline force stability was related to tongue endurance for both tasks. Tongue muscle dysfunction was observed only in moderate patients with OSA, whereas BMI and age also presented disparate effects on tongue mechanical properties.

Effect of Type of Task on Tongue Performance The development of fatigue during tongue elevation has not been investigated in patients with OSA. In general, longer endurance time and lower slope of strength stability decrement were observed in protrusion thanin elevation, indicating a greater capacity for subjects to sustain a relatively stable and efficient force for protrusion throughout the trial. This can be attributed to the difference in recruited tongue muscle fibers and their distinct synergistic contraction mechanisms for different directions of movements (15, 27). Notably, tongue upward bending (elevation with tongue tip) results from two synergistic mechanisms, unilateral contraction of the peripherally located and longitudinally oriented sheath, combined with bidirectional contraction of core intrinsic fibers. Although we were not able to estimate what their respective contribution was to tongue mechanical fatigue in each task, it should be pointed out that the simultaneous bidirectional contractions of intrinsic (transversus and verticalis) lingual muscles are involved primarily in tongue anterior protrusion, with a secondary role played by the extrinsic (GG) muscle. Concerning tongue elevation, it involves mostly midline GG fibers (anterior, vertical), with a secondary role played by the longitudinalis, verticalis, and tranversus fibers, which serve to stiffen, broaden, and bend the tongue (11, 15). Thus, during tongue elevation movement, the complexity of contraction with two synergistic mechanisms may account for its lower endurance, since the anterior portion of GG and intrinsic lingual muscles (transversus fibers) do not differ significantly in the percentage of slow muscle fibers (41). From another point of view, the differences in tongue mechanical performance between these two tasks may not be accounted for only by differences in muscle fiber recruitment but also by differences in adaptive muscle training resulting from their attempt to overcome the UA negative pressure and tissue weight during sleep apnea events. It is reasonable to propose that the repeated high-intensity phasic contraction of UA muscles occurring during the course of UA collapse represents a powerful training effect for UA muscles. Such higher “training” intensity imposed on severe or obese patients with OSA may result in longer/stronger adaptive processes in terms of strength gain. In this regard, such adaptive strength gain should be specific to protrusive direction, as the tongue muscle is nightly “trained” to dilate the UA (11). This could account for the findings that strength differences between severe and moderate patients with OSA were prominent for protrusion compared with elevation (MVC difference: 15.2% for protrusion vs. 5% for elevation) as well as between obese and nonobese patients with OSA (MVC difference: 25% for protrusion vs. 10.3% for elevation). This “training specificity” for tongue protrusion yields implications in the development of UAM training therapy (7).

Effect of OSA Status on Tongue Mechanical Properties Inadequate UA dilator muscle mechanical effectiveness, including insufficient (weak) or inefficient (instable) contractile force and fatigue, may predispose to UA instability. Oropharyngeal muscle exercise training has recently been found to improve OSA (16, 29), implying a potential role for UAM dysfunction in OSA pathogenesis. Indeed, one study revealed a negative correlation between maximal tongue protrusion force and AHI (23). Our results differ somewhat from those reported in earlier studies, including subjects with generally less severe OSA and who were found to have on average lower tongue endurance than normal, if any (3, 12, 21). In contrast, we found endurance and total work to be lower in moderate patients with OSA, with no difference of tongue strength between groups. The discrepancy with previous studies might relate to differences in the targeted populations, their absence of grouping according to OSA severity, and distinct methods and tasks used to induce fatigue (i.e., repetitive vs. sustained contraction, submaximal vs. maximal intensity). Previously, we found that the proportion of type II muscle fiber in GG was increased in patients with OSA, without a link between fiber-type composition and apnea severity (46). In the present study, the difference in tongue endurance observed between subjects with different OSA status may relate to the muscular and/or neurogenic adaptation process, which could impose disparate effects on UAM function. Poor performance in the moderate OSA group could relate to muscle damage resulting from OSA disease (20, 21, 47), as suggested by the negative correlation we observed between tongue function and OSA severity in subjects with AHI <18.5 n/h. Conversely, in patients with more severe OSA (AHI >18.5 n/h), tongue strength and endurance increased with OSA severity, which is in accord with the absence of tongue dysfunction in the severe OSA group. Thus, it can be hypothesized that neuromuscular adaptive processes are prone to occur in these patients and could account for the lower tongue fatigability in more severe OSA cohorts. Such hypothesis is supported by our previous findings regarding the larger and smaller proportion of type IIA and IIB fibers, respectively, presented in severe patients with OSA (AHI: 36.2 ± 17.7 n/h) (43). In this regard, changes in strength stability overtime during a fatiguing task are interesting physiological features since they reflect motor control or cortical-muscular coherence of UAM contraction (49). Thus, the increased CV value corresponded to lower force stability, the extent of CV increment varying among subjects with different OSA status. Interestingly, force stability measured at baseline correlated with endurance time for both tasks. This is consistent with a previously proposed concept that alteration in sustained control of the tongue output level might alter muscle endurance (48). Noteworthy is that the impairment of steady force could be ascribed to neural mechanisms such as decreased number of motor units and increased variability of discharge (2, 32). These could be altered by the “motor unit remodeling” adaptation process in OSA, as evidenced by neurogenic adaptations, including increased activation, earlier firing, increased sprouting of the XII motoneurons, and increased central motor conductivity of GG (13, 40, 52). In our study, baseline CV decreased with OSA severity among patients with more severe OSA, with higher force stability at baseline being observed in severe OSA.

Effect of BMI and Age Obesity is a common feature of OSA; its effect on UAM function has not been explored extensively in vivo among sleep apnea cohorts. In the present study, nonobese patients with OSA had lower muscle total work, with a tendency for shorter endurance time (criteria 1) regardless of type of task. Furthermore, as opposed to obese patients, they were prone to have a higher fatigue rate according to endurance time (criteria 1) and a lower capacity to generate maximum force for protrusion (Fig. 3). No effect of BMI status was observed in tongue function among all subjects, including normal plus mild. Our findings are consistent with those of in vitro studies where electrical stimuli were used to assess GG contractile properties that found a significant increase in GG fatigability in nonobese patients with OSA, whereas GG endurance in obese patients was indistinguishable from normal (5). Notably, we found total tongue muscle work to decrease with OSA severity in nonobese subjects. Conversely, in obese patients with OSA with higher resistance to fatigue and slightly greater strength, sleep-related UA collapse may be due to the impeding effects of the surrounded deposition of fat tissue rather than to a mechanical impairment of UA dilator muscles (6, 18), since obesity per se did not influence GG structure or function in terms of fiber type distribution in patients with OSA (5). From another perspective, in the moderate and severe OSA groups, obesity appeared to be protective for tongue function decrements. This finding may be due to the fact that in obese patients with OSA the lack of cranio-facial anatomic abnormalities (that are frequently seen in nonobese OSA) could prevent tongue positioning at a mechanically disadvantaging position that would favor the development of fatigue. Additionally, unsaturated fatty acids in obese patients could contribute to improve muscle function (19). Moreover, greater tongue volume and muscle mass associated with their overweight status could also contribute to the positive relationship observed between OSA severity and tongue protrusion function (strength and total work). This is also supported by the positive correlation between neck circumferences and tongue strength that we found in all subjects. In previous studies, the association of aging and tongue weakness observed in healthy subjects was considered to be linked to sarcopenia (24, 25), a loss of muscle mass and reduction in tongue muscle fiber diameter (8, 26). Unexpectedly in our study, no evidence of strength weakness was found in old subjects, except for an impact of age on tongue endurance time (criteria 2) and recovery time independently of sleep apnea status. Therefore, it may be speculated that aging per se could lead to increased UAM fatigue that could further alter the ability to maintain UA patency during sleep, as demonstrated by an increased susceptibility to UA obstruction with age (28). This interpretation, however, has to be taken with caution due to the relatively small number of subjects studied.

Methodological Strengths and Limitations One strength of the present study is the specific design of the fatiguing protocol. In opposition with previously used sustained isometric contractions, tongue muscle activity during sleep apnea events is characterized primarily by dynamic contractions as opposed to solely static efforts. In this consideration, we believe that the repetitively sustained submaximal fatiguing contractions would be more functionally relevant to sleep apnea pathophysiology. Regarding the high variability of data between subjects, we cannot rule out the effect of individualized motivation. Although all subjects were provided with the same standardized instructions, followed by strong verbal encouragement, subjective factors, including the person<s motivation, competitive spirit, and tolerance for pain undoubtedly influenced the amount of force they could develop and, consequently, the endurance time. On the other hand, cautious pretrial familiarization attempts were systematically included in the protocol for subjects to be highly accustomed to the alternative performance of submaximal and maximal efforts. Considering the high test-retest reliability of the proposed tongue mechanical assessment techniques (Cronbach-α 0.98 with elevation, coefficient of variability 6.3% with protrusion) (25), we are confident that intrasubjects< variability did not alter our ability to observe differences in the measurements with the proposed sample size. Besides, distinct parameters, including endurance time (with 3 different criteria), muscle total work, and fatigue curve, were proposed to evaluate the onset of tongue fatigue. Considering that endurance time depicted according to criteria 1 (applied during the trial) could be influenced by individuals< motivation, the other two criteria with more demanding requirements (i.e., add time limits for the maintenance of strength) were created as post hoc analysis to account for this bias. Furthermore, one might argue that muscle fatigue could have occurred anytime during the last session of the trial, rather than at the point of the last MVC attempt; although it is not possible to precisely identify the real time of fatigue occurrence, “fatigue curve” was built to further quantify the percentage of subjects who developed fatigue at the end of each recording interval. Finally, since endurance time can be influenced by the amount of loading during contraction (42), we completed measurements of the muscle total work that is a highly sensitive and reliable parameter for assessing limb muscle fatigue in chronic obstructive pulmonary disease patients (31). Total work provides a robust and comprehensive marker of tongue function, which integrates both strength and endurance, thus representing a more reliable parameter with better reproducibility and feasibility than parameters used in previous studies.

Clinical and Physiological Implications Although this study did not answer the question regarding the critical level of tongue dysfunction for sleep apnea pathophysiology, assessing the role of UAM fatigue as an underlying contributory mechanism is important. Its therapeutic implications include selection of the potential responders for treatments aimed at improving UAM contraction efficiency (i.e., hypoglossal electrical stimulation, UAM training). It is noteworthy that our results highlighted the protective effect of obesity on tongue functional alteration in patients with OSA, demonstrating distinct pathogenic mechanisms underlying OSA development among obese and nonobese patients. Therefore, to identify the UAM dysfunction associated with OSA and to recognize potential responders to the above-described therapies, tongue mechanical evaluation might be more appropriately used among nonobese patients with OSA. Moreover, based on the principle of UAM training (9), it is possible that the UAM training therapy can reduce the propensity to fatigue by improving the functional mechanical efficiency of UAM contraction, i.e., the central motor control of strength in terms of stability or accuracy (neuroplasticity), rather than pure strength gaining, as observed by the prediction role of baseline CV on tongue endurance in the present study. This has been documented for the masseter muscle, whose mechanical performance improved following low-intensity motor task training (17). These results are consistent with the decrease in AHI observed after 1 wk of tongue task training (35).