The tongue’s role in oropharyngeal swallowing is extensive and essential to normal swallowing. The tongue performs significant functions in the oral preparatory, oral transit, and pharyngeal phases of swallowing, including major contributions to bolus manipulation and transport. Acute or progressive neurogenic disorders frequently result in a swallowing disorder, or dysphagia [1–4]. If a patient’s neurogenic disorder results in tongue weakness, common dysphagia symptoms that may result include poor bolus manipulation, reduced mastication, oral pocketing of the bolus, reduced propulsion of the bolus posteriorly in the oral cavity, premature spillage of the bolus into the pharynx, and diminished bolus propulsion within the pharynx [5]. Dysphagia resulting from any of these problems can compromise an individual’s safety, hydration, and/or nutrition, and inhibit his/her quality of life [6, 7].

The range of maximum isometric tongue strength for normal individuals has been documented by several studies [8–18]. Fewer studies have documented tongue strength during swallowing [10, 11, 17]. Although only a few studies have examined tongue strength during the swallowing of a limited number of bolus volumes and consistencies, the data from these studies imply that the maximum isometric tongue strength of normal individuals is much higher in comparison to their tongue strength utilized during swallowing. That consistent finding has led to the notion of a “tongue strength reserve” which is an indication that individuals have more tongue strength than is required for oral phase swallowing [11].

Furthermore, some studies have demonstrated that men have significantly greater maximum tongue pressures than women [8, 16, 17]. Other studies demonstrated trends in the same direction, but the difference did not reach levels of statistical significance [10, 18]. Studies have also revealed significantly decreased maximum tongue strength in older adults [10, 11, 16, 17]. Despite the decline in maximum strength, some investigations have shown that tongue strength demands during swallowing do not differ significantly between men and women or throughout adulthood [10, 11, 17]. It stands to reason that if tongue strength demands during swallowing remain constant between the sexes and throughout adulthood, whereas tongue strength is lower for women and diminishes with age, then women and older persons, especially older women, may have a reduced tongue strength reserve. Neurologic disorders can further weaken the tongue [16]; therefore, it would seem that as age increases (especially in women), the likelihood of having dysphagia related to tongue weakness would increase. Moreover, as age increases, the incidence of neurologic disease increases [2]. Therefore, older adults not only have a higher incidence of neurologic diseases and disorders, they are also more susceptible to a swallowing impairment as a result of the disease or disorder.

However, as part of their investigation on tongue function in normal individuals, Youmans and Stierwalt [17] calculated the percentage of maximum tongue strength used during swallowing. According to their results on the proportion of total tongue pressure used during swallowing, there were no significant differences between gender and age groups; this does not support the notion of a decreasing strength reserve. Their results did indicate that maximum tongue strength decreased significantly with age and that maximum swallowing pressure did not change significantly as a function of age, which appears to support a decreasing tongue strength reserve at face value; however, the tongue pressure during swallowing did vary (although it did not reach a statistically significant level) in proportion to maximum tongue strength. Because of the discrepancy, the authors suggested further investigation into the idea of a decreasing tongue strength reserve with age because of its important implications.

The identification of a tongue strength reserve that is reduced significantly over a person’s lifetime would help to identify those who are most at-risk for oropharyngeal dysphagia related to tongue weakness. This, in turn, could facilitate an expedient diagnosis of oropharygeal dysphagia. Likewise, it would allow for earlier intervention with these individuals who might benefit from a strengthening protocol to improve tongue function and ultimately improve swallowing [2, 16, 19].

The primary purpose of this study was to examine the normal, maximum isometric tongue strength possessed by individuals across gender and adulthood and compare it to the amount of tongue strength used during the swallowing of various volumes and consistencies to determine whether an attenuating tongue strength reserve exists across age and/or gender. An additional purpose of this study was to investigate normal swallowing physiology as a necessary first step to understanding disordered tongue pathophysiology. At present, only a few studies have studied tongue strength during swallowing, and they sampled only a limited number of bolus and volume combinations [10, 11, 17]. This investigation will examine the following questions: Does tongue strength vary as a function of age or gender? Do swallowing demands vary as a function of age or gender? Do swallowing demands vary as a function of bolus volume or consistency? Are there interactions among these variables?

Method

The procedures, instrumentation, and dependent measures in this study are similar to those used in the investigation by Youmans and Stierwalt [17] which was influenced by Robbins and others [11]. There are, however, key differences in the bolus volumes and consistencies that were administered. Prior to initiating this investigation, all proposed procedures were submitted to the Institutional Review Board and approval was obtained for conducting this research.

Participants

Ninety-six participants (48 males, 48 females; 20–79 years of age) participated in the investigation. Stratified sampling was used to ensure that each age/gender group was balanced. Eight men and eight women from each decade participated in the study. The subjects were then categorized into younger (20–39), middle, (40–59), and older (60–79) age groups. Consequently, each age group contained 32 participants (16 men and 16 women). Table 1 contains a description of the participants.

Table 1 Description of the participants
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All of the participants reported that they were in good to excellent general health and had no history of swallowing, speech, respiratory, neurologic, or other related disorders that could impair swallowing or inhibit proper tongue functioning, including oral surgery (beyond routine dental surgery) and cancer. In addition, each participant underwent an oral-peripheral examination by an experienced speech-language pathologist to ensure that the anatomy and physiology of his/her oral mechanism was within normal limits.

Instrumentation

The Iowa Oral Performance Instrument model 2.0 (IOPI) [20] was used to obtain all tongue strength scores. The IOPI is a handheld pressure transducer that was developed to assess hand and tongue strength and endurance. An air-filled “tongue bulb” is attached to the device via an 11.5-cm connecting tube. The tongue bulb is approximately 3.5 cm long and approximately 4.5 cm in diameter. The device allows the measurement of peak tongue pressures which are captured on an LCD display. Pressure is measured in kilopascals (kPa). The calibration of the IOPI was checked weekly, as recommended by the manufacturer, to ensure reliable and accurate measurement. The IOPI has been used in numerous studies, including all of the aforementioned studies on tongue strength, and has been shown to have excellent test–retest reliability [17].

Procedures

All testing took place in a small, well-lit, quiet environment. Subjects were seated in a straight-back chair in an upright position for the duration of the study. Care was taken to eliminate auditory and visual distractions. The participants were first asked to read and sign an informed consent document and answer questions about their health history, current health status, and demographic information. An oral mechanism examination was then conducted.

Once these procedures were completed and the participants were deemed candidates for the study, instructions were given for tongue bulb placement. The instructions were as follows: “Place the air-filled bulb (the blue part) in your mouth. Just the bulb (the blue part) should be in your mouth. It should rest on top of the middle of your tongue. This is where the tongue bulb should be during all of the following tasks.”

Once the participants demonstrated proper tongue bulb placement and acknowledged comprehension of the instructions, the examiner gave them instructions regarding the maximum tongue strength task. The instructions were as follows: “Place the tongue bulb in your mouth where I taught you to put it. When you are ready, press your tongue to the roof of your mouth and squeeze as much air out of the tongue bulb as possible. Press as hard as you can for about two seconds and then relax.”

Once the participants acknowledged comprehension of the instructions, the task was completed three separate times with a 30-s break between trials. Maximum effort was encouraged by the examiner during each trial. After each trial, the maximum isometric pressure (MIP) that was displayed on the LCD display was recorded on paper by the examiner.

Next, the instructions for the mean peak anterior tongue pressure during swallowing (MSP) task were provided. The instructions were as follows: “Now, I want to see how much tongue strength you use during swallowing. I will give you small amounts of liquids and solids. I want you to place them in your mouth, place the tongue bulb in your mouth in the place where I taught you to put it, and swallow as normally as possible. In other words, I do not want you to push extra hard with your tongue when you are swallowing; just let your tongue do what it normally does when you swallow.”

Once the participants acknowledged comprehension of the instructions, the examiner directed them to practice swallowing two sips of water with the tongue bulb in place to acclimate to the bulb while swallowing. Once the participants demonstrated appropriate task completion and comprehension, they were given a series of liquids and solids to swallow while their tongue strengths were being measured. The items to be consumed were administered in the following order: 5 milliliters (ml) of thin liquids, 10 ml of thin liquids, 15 ml of thin liquids, 5 ml of nectar-thick liquids, 10 ml of nectar-thick liquids, 5 ml of honey-thick liquids, 10 ml of honey-thick liquids, and 5 ml of purees. Thin liquids consisted of bottled water. Nectar-thick and honey-thick liquids consisted of “Novartis Resource”-thickened apple juice. Nectar-thick liquids were reported to be 84% water content and honey-thick liquids were reported to be 83% water content per the manufacturer. Prethickened liquids were used to ensure a reliable consistency of thickened liquids. The puree consistency consisted of apple sauce. Three trials of each volume/consistency combination were elicited and recorded on paper by the examiner. It should be noted that the items consumed by the participants were not presented in a randomized manner. Because of this, it is possible that tongue function on preceding boluses could have affected tongue function on subsequent boluses in some systematic and unforeseen way.

Dependent Measures

The maximum isometric pressure (MIP) variable refers to the maximum tongue strength that the participant was capable of producing. The maximum tongue strength elicited by each of the participants was displayed on the IOPI and transferred to paper by the examiner. Three trials were recorded for each participant, and the highest score was considered their MIP.

The mean swallowing pressure (MSP) variables are the mean of the anterior tongue pressures generated by each participant during swallowing across three trials. The peak tongue strength used during swallowing was displayed on the IOPI and transferred to paper after each swallowing trial. The three trials at each volume/consistency level were averaged to calculate the MSP variables. In total, eight MSP variables were calculated (one at each volume/consistency level).

The percentage of maximum tongue pressure used during swallowing (PMPS) variables refer to the proportion of each subject’s maximum strength generated when different volumes/consistencies were swallowed. The PMPS variables were derived by dividing each participant’s MSP variables by the MIP variable and multiplying by 100. A PMPS variable was calculated for each of the MSP variables; therefore, eight PMPS variables were computed for each participant.

Data Analysis

Descriptive statistics were computed for all of the variables. A two-way analysis of variance (ANOVA) was computed to determine the main effects of age and gender on the MIP variable as well as their interaction. A repeated-measures multivariate analysis of variance (RM-MANOVA) was computed to analyze the main effects and interactions of the independent variables on the dependent variables (MSP and PMPS). The within-subjects independent variable was bolus type and the between-subjects independent variables were age and gender. A Tukey HSD procedure was used for post-hoc analyses. An α level of 0.05 was set. All computations were made using SPSS version 13.0 (SPSS Inc., Chicago, IL).

Reliability

Interjudge reliability was confirmed by having another examiner transfer the numbers produced on the IOPI blindly and simultaneously to the primary examiner during 10% of the samples. Exact agreement was calculated between the two judges’ numbers for 11 participants. One hundred percent of the two judges’ scores matched. The interjudge reliability was established to be excellent.

Results

Maximum Isometric Pressure

The results of the ANOVA indicated that there were no significant differences based on gender for the MIP variable (F 1,90 = 1.35; p = 0.25). In addition, there was no interaction effect between age and gender on the MIP variable (F 2,90 = 0.451; p = 0.64), indicating that there was no differential effect according to age and gender. There was a significant main effect for age (F 2,90 = 11.88; p < 0.0001); furthermore, post-hoc tests indicated significant differences between the younger and older groups (p < 0.0001) and between the middle and older groups (p = 0.008). There was no significant difference between the younger and middle groups (p = 0.19). Figure 1 illustrates the data distribution via a scatterplot. Figure 2 includes a graphic description of the means and standard errors for each age/gender group.

Fig. 1
figure 1

Scatterplot of maximum isometric pressures (MIP) according to age in 96 participants. r = −0.41; r 2 = 0.17; p < 0.0001

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Fig. 2
figure 2

Means and standard error values of the maximum isometric pressure (MIP) variable according to age and gender groups. White bar = male; black bar = female

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Repeated-Measures Analysis of Variance Multivariate Omnibus Tests

The results of the RM-MANOVA between subject multivariate omnibus tests (using Wilks’ Lamda) were significant for age (F 4,178 = 4.64; p = 0.001) and gender (F 2,89 = 4.86; p = 0.01), but the interaction was not significant (F 4,178 = 1.61; p = 0.17). Within-subjects multivariate omnibus tests were significant for bolus (F 14,77 = 2.12; p = 0.02) and for a bolus/age interaction (F 28,154 = 1.58; p = 0.04), but not for interactions between bolus and gender (F 14,77 = 0.64; p = 0.83) or for a bolus/age/gender interaction (F 28,154 = 1.35; p = 0.13). Univariate analyses were then analyzed for significant data followed by pairwise comparisons of significant univariate tests.

Mean Swallowing Pressure

The results of the RM-MANOVA indicated that there was no significant main effect for age (F 2,90 = 2.52; p = 0.09) nor was there a significant interaction between age and gender on MSP (F 2,90 = 1.33; p = 0.27); however, there was a significant main effect for gender (F 1,90 = 5.51; p = 0.02). Women had a significantly stronger mean MSP (mean = 41.53) compared to men (mean = 34.52). Figure 3 includes means and standard errors for the MSP variable for each of the groups.

Fig. 3
figure 3

Mean and standard error values of the maximum swallowing pressure (MSP) variable according to age and gender groups. White bar = male; black bar = female

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In addition, the MSPs differed significantly based on bolus type (F 1,90 = 17.38; p < 0.0001). Figure 4 depicts the values for the MSP scores across bolus type. Pairwise comparisons yielded the following results: 5 ml of thin liquids differed significantly from all other boluses except 10 ml of thin liquids; 10 ml of thin liquids differed significantly from all other bolus types except 5 ml and 15 ml of thin liquids; 15 ml of thin liquids differed from all other bolus types except 10 ml of thin liquids; 5 and 10 ml of nectar-thick liquids, 5 and 10 ml of honey-thick liquids, and pureed solids differed significantly from all three volumes of thin liquids, but did not differ significantly from any other bolus type/volume. A bolus/age interaction was also significant during omnibus testing (F 2,90 = 6.81; p = 0.002); a visual representation of the MSP variable across bolus types by age group is depicted in Fig. 5. Follow-up analyses yielded significantly greater MSP scores for participants in the younger group than participants in the older group for 5-ml honey, 10-ml honey, and puree boluses in the absence of other significant differences.

Fig. 4
figure 4

Mean and standard error values for maximum swallowing pressure (MSP) across bolus types: 5 ml thin, 10 ml thin, 15 ml thin, 5 ml nectar, 10 ml nectar, 5 ml honey, 10 ml honey, 5 ml puree

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Fig. 5
figure 5

Mean and standard error values for maximum swallowing pressure (MSP) across bolus types by age group: 5 ml thin, 10 ml thin, 15 ml thin, 5 ml nectar, 10 ml nectar, 5 ml honey, 10 ml honey, 5 ml puree

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Percentage of Maximum Tongue Strength Used During Swallowing

Similar results were observed for the PMPS variable. The results of the RM-MANOVA indicated that there was no significant main effect for age (F 2,90 = 0.02; p = 0.98), nor was there a significant interaction between age and gender on PMPS (F 2,90 = 2.00; p = 0.14). However, there was a significant main effect for gender (F 1,90 = 9.60; p = 0.02). Women had a significantly higher PMPS (mean = 62.88) compared to men (mean = 49.65). Figure 6 includes means and standard errors for the PMPS variable for each group.

Fig. 6
figure 6

Mean and standard error values of the percent maximum swallowing pressure (PMPS) variable according to age and gender groups. White bar = male; black bar = female

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In addition, the PMPS values differed significantly based on bolus type (F 1,90 = 17.55; p < 0.0001). Figure 7 depicts the values for the PMPS scores across bolus type by age group. Pairwise comparisons yielded the following results: 5ml of thin liquids differed significantly from all other bolus types except 10 ml of thin liquids; 10 ml of thin liquids differed significantly from all other bolus types except 5 and 15 ml of thin liquids; 15 ml of thin liquids differed from all other bolus types except 10 ml of thin liquids; 5 and 10 ml of nectar-thick liquids, 5 and 10 ml of honey-thick liquids, and pureed solids differed significantly from all three volumes of thin liquids in the absence of any other bolus type/volume. A significant bolus/age interaction was also observed during omnibus testing (F 2,90 = 4.59; p = 0.01); however, follow-up analyses for a bolus/age interaction on the PMPS variable yielded no significant differences. A visual representation of the PMPS variable across bolus types by age group is depicted in Fig. 8.

Fig. 7
figure 7

Mean and standard error values for percent maximum swallowing pressure (PMPS) across bolus types: 5 ml thin, 10 ml thin, 15 ml thin, 5 ml nectar, 10 ml nectar, 5 ml honey, 10 ml honey, 5 ml puree

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Fig. 8
figure 8

Mean and standard error values percent maximum swallowing pressure (PMPS) across bolus types according to age group: 5 ml thin, 10 ml thin, 15 ml thin, 5 ml nectar, 10 ml nectar, 5 ml honey, 10 ml honey, 5 ml puree

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Discussion

There were two general purposes for this study. One was to improve our understanding of the normal physiology of the anterior tongue during nonswallowing tasks and while swallowing several bolus volumes and consistencies for future comparisons with the physiology of persons with disordered swallowing. A second purpose was to investigate how normal persons vary across age and gender in terms of their maximum isometric tongue strength and the strength they typically use during swallowing to determine whether there is a diminished strength reserve in older persons as previous studies have suggested [10, 11].

Maximum Isometric Pressure

The maximum isometric pressure (MIP) scores (the greatest strength produced by the anterior tongue against the tongue bulb with maximal effort) obtained by the participants revealed that the younger group was significantly stronger than the older group. This result mirrors those of other studies [10, 11, 16, 17] and appears to be a robust finding. In most of the previous studies, the significant differences in MIP scores based on age were between the youngest and oldest groups [16, 17] or groups that had a relatively large age difference between them [10, 11]; however, in this study, the middle group also scored significantly higher than the older group. A significant difference between the middle and oldest age groups indicates a significant reduction in maximum tongue strength in the participants who were over 60 years old, even when compared to those in their 40s and 50s. It is evident from inspecting Figs. 1 and 2 that a decline in MIP scores elicited by participants occurs with age. The tongue, which is made up predominantly of muscle, appears to decrease in strength as part of the natural aging process, much like other muscles in the body [8, 11, 17]. As Deshenes [21] pointed out, between the ages of 50 and 80 years, humans undergo significant muscle mass/fiber loss. Overall, however, the observed tongue strength decline with age appeared to be gradual. The correlation coefficient for MIP regression as a function of age was significant but not large (r = −0.41; r 2 = 0.17; p < 0.0001). These data explain more of the variance than the previous investigation by Youmans and Stierwalt [17] (r = −0.25; r 2 = 0.06; p = 0.02) and the findings by Crow and Ship [8] for females (r = −0.27; r 2 = 0.07; p = 0.07); however, the result is similar to the findings of Crow and Ship [8] for males (r = −0.38; r 2 = 0.14; p < 0.001) and the correlation between age and MIP found by Clark et al. [22] (r = −0.46; p < 0.0001).

Our finding of no significant difference between the MIPs of men and women is less robust across studies. As previously mentioned, some earlier studies found a significant difference in MIP between men and women [8, 16, 17], whereas others did not [10, 18]. Although the younger and older groups of women demonstrated weaker aggregate MIP scores than men, the middle group of women elicited strength scores than were higher than the male group. Thus, it appears that there is a common trend across studies that men are generally stronger than women in terms of tongue strength as with other muscles of the body; however, the difference is neither large and, therefore, may not be clinically relevant, nor is it universally present as observed in this middle group of participants.

Mean Swallowing Pressure

The results for the mean swallowing pressure elicited by the anterior tongue during swallowing (MSP) obtained by the participants indicated that MSP neither differs significantly as a process of age nor as an interaction between age and gender. These results are similar to results from previous studies [10, 11, 17]. However, the result that women had significantly higher MSPs than men is not supported by previous studies [10, 17]. In this sample of participants, women generally swallowed boluses with more tongue force than men during the oral phase of swallowing. Figure 3 illustrates that in this sample of subjects, men and women had similar MSPs in the younger group, but their MSPs differed in the middle and older groups. The implications of this will be discussed below.

Although a trend of increasing MSP pressures existed with increasing bolus volumes and consistencies (as depicted in Fig. 4), the only statistically significant differences were between thin liquids and all other bolus types and the 5 and 15 ml of thin liquids. Therefore, although it appears that additional tongue force is required to propel posteriorly boluses of increasing volume or viscosity in general, the differences are small and may be subclinical. One other study that investigated tongue pressure differences as a function of bolus volume and viscosity concluded that a significant tongue strength increase did not exist due to bolus volume changes [23]. However, the study did observe significantly increased tongue force during increased bolus viscosities; this result mirrored that of another previous investigation [17]. Likewise, the older group demonstrated significantly less tongue strength than the younger group while swallowing both the honey and the puree boluses. As can be seen in Fig. 5, there appears to be a slight general decline in MSP across age, but, more strikingly, there appears to be a reduction in the variability of MSPs across bolus types/volumes as age increases, which explains the significant age/bolus interaction.

Percent Maximum Swallowing Pressure

The results for the percentage of maximum isometric tongue strength used during swallowing (PMPS) demonstrate that PMPS does not differ significantly based on age or an age/gender interaction. This supports the results of the Youmans and Stierwalt [17] investigation, which is the only other study that looked at the difference in this variable with respect to age and gender. However, the result that women had significantly higher PMPS scores than men did not support their findings. Figure 6 illustrates that women generally used a higher percentage of maximum tongue strength while swallowing than men in this sample of participants, and the difference increased across the age groups. Whereas the younger males in this study tended to use less of their maximum strength to swallow than males who were older, the females used more of their maximum strength to swallow in the older groups.

Similar to the MSP results for bolus, although a trend of increasing PMPS scores with increasing bolus volumes and consistencies existed (as depicted in Fig. 7), the only statistically significant differences were between thin liquids and all other bolus types and between the 5 and 15 ml of thin liquids. This finding supports earlier research on volume and consistency [17, 23]; however, those investigations did not examine an array of volumes or bolus types as the current study did. Based on our findings, it appears that although additional force is required with increasing volume or viscosity in general, the differences were small and may not be clinically relevant.

The notion of a diminishing tongue strength reserve with age has been promoted in the literature [10, 11] to explain the data from those studies that MIP decreased significantly with age, whereas MSP did not vary significantly with age. If maximum tongue strength capacity shrinks while requirements for swallowing remain the same across age, it would seem that a person’s strength reserve decreases with age. This is a logical argument given the data from those investigations; however, the Youmans and Stierwalt study [17] and the current study added another variable that could serve to refute or at least modify this notion. In both studies, MIP decreased significantly with age and MSP did not differ significantly with age; however, PMPS did not differ significantly with age. The PMPS variable is dependent on both the MIP and the MSP variables. It stands to reason that if a decreasing MIP (denominator) exists with a constant MSP (numerator), then you would have an increasing PMPS variable as a result. This is not the case, however. The reality is that MIP did act as expected, but MSP did not. Although MSP did not differ significantly across age, it did change. In other words, it was not constant. When collapsed across gender, MSP also decreased with age. Hence, because both MIP and MSP decreased across age groups, albeit at different rates, PMPS scores did not increase at statistically significant rates. Therefore, the findings from the Youmans and Stierwalt [17] study and the current study do not support the notion of a diminishing tongue strength reserve with age; rather, they support the notion of decreasing tongue strength and, to a lesser extent, decreased tongue strength demands during oral phase swallowing with age.

Interestingly, in this sample of subjects, women did exhibit a significantly higher PMPS than men. Men and women did not significantly differ with respect to the MIP variable, but they did vary significantly on the MSP variable. Therefore, using similar logic, if the MIP (denominator) does not differ and the MSP (numerator) is significantly higher, then the PMPS should also be significantly higher. Furthermore, it appears that the men in our study had decreasing PMPS scores with age and the women had increasing PMPS scores with age. Based on the data from the current study, women have a decreased tongue strength reserve when compared to men due to their increased demands on tongue strength during swallowing. As mentioned, this finding did not support the findings from the only other study to examine this variable [17]; therefore, a replication of these data is warranted to support or refute these findings.

There are potential clinical implications of the findings. Although the mean strength reserve decreased with age in our sample of participants, it did not reach a level of significance. Based on these results, older persons do not appear to have a significantly increased risk of oral dysphagia than younger persons due to their decreased tongue strength requirements during swallowing. However, the women in our sample did have a significantly reduced tongue strength reserve compared to the men in our study. If these results are valid, they could point to an increased risk of oral phase dysphagia following a neurologic incident for women compared to men. This information could be useful to the clinician evaluating dysphagia and could promote earlier identification and treatment of oral phase dysphagia. Prior to implementing this knowledge clinically, further investigations into the differences in the tongue strength reserves between men and women should be conducted to determine the validity of these findings. In addition, the incidence of oral dysphagia in men versus women should be further investigated to determine—if these data are indeed correct—whether women do in fact have an increased risk of oral phase dysphagia than men following a neurologic incident.

Of additional potential clinical relevance is the finding that the participants in this study used significantly less tongue strength for thin liquids than for nectar-thick liquids, honey-thick liquids, and purees, but that the more viscous consistencies did not differ significantly from each other despite the volume of the bolus. This means that if clinicians are choosing which diet to prescribe for a given patient, tongue weakness should not be a limiting factor unless the decision is between thin liquids and a more viscous consistency. In other words, the patient does not need to use much additional strength as viscosity increases beyond thin liquids. Rather, variables such as the specific type of oropharyngeal dysphagia with which the person presents, including oral and pharyngeal phase breakdowns, the presence or absence of aspiration, and patient reaction to the varying liquid viscosities, should be considered when deciding about diet modifications.

Finally, the findings from this study will add to the database on normal tongue strength and normal tongue strength during swallowing using the IOPI, the latter of which is limited at present. Although the assessment of tongue strength has been a longstanding component to dysphagia evaluations, the assessment has traditionally been subjective in nature [17, 24–26]. The addition of an objective, quantifiable method for evaluating tongue strength that is reliable and valid, such as the IOPI, makes it possible to establish ranges of normal tongue strength and tongue strength during swallowing.

This information can be used for future comparisons of the tongue strength of persons with dysphagia during nonswallowing and swallowing tasks. This knowledge could potentially improve our ability to diagnose oral phase dysphagia more efficiently.