This study aimed to determine the association between head lifting strength and pharyngeal residuals using VFSS. The results showed that the number of repetitions of maximum head lifting was independently correlated with NRRSv and NRRSp, even after adjusting for age and sex. This study represents the first evidence of a link between head lifting strength and the NRRSv and NRRSp, indicators of pharyngeal residuals.

Association between Head Lifting Strength and Pharyngeal Residuals

The primary muscles involved in raising the head include the suprahyoid and infrahyoid muscle groups involved in swallowing, as well as the sternocleidomastoid muscle, an auxiliary respiratory muscle [23]. In particular, the suprahyoid muscle group play a role in pulling the hyoid bone and larynx anteriorly and superiorly. Furthermore, these muscles contribute to the relaxation and opening of the upper esophageal sphincter (UES) [24,25,26]. Head lifting exercises have been reported to increase the anterior excursion of the larynx and the anteroposterior diameter of the upper esophageal sphincter opening [27]. If the suprahyoid muscle weaken, the movement of the hyoid and larynx will become insufficient to open the esophageal inlet, resulting in pharyngeal residue and aspiration [13]. This suggests that our method provides a valid approximation of suprahyoid muscle strength, even though it does not measure maximal strength directly. It should be noted that the assessment of head-lifting strength in this study differs from conventional measures of maximal muscle strength, such as maximum voluntary contraction, which evaluates single-instance maximal force generation. Instead, our method employs endurance-based parameters —namely, the number of repetitions for maximum head lifting and the time to maintain maximum head lifting. Rather than absolute strength, these measures primarily reflect muscular endurance. However, previous research has established a strong correlation between endurance performance and maximal muscle strength [28, 29]. Therefore, although our method does not directly measure single-instance maximal muscle strength, it serves as a reasonable proxy for suprahyoid muscle strength in clinical settings. As a result, the number of repetitions of maximum head lifting emerged as an independent predictor of pharyngeal residuals. Future research may benefit from considering the number of repetitions of the maximum head lifting as an important evaluation item when assessing patients with suspected dysphagia. However, it has been revealed that the sternocleidomastoid muscle fatigues before other muscles during head lifting exercises [30]. Therefore, careful consideration is necessary when interpreting the results of this study.

Differences in the Number of Repetitions of Maximum Head Lifting and the time To Maintain Maximum Head Lifting

In the adjusted model, the number of repetitions of maximum head lifting was independently associated with NRRSv and NRRSp. In contrast, the time for which the head is maintained in a raised position for as long as possible was independently related to NRRSv only. A reduced range of hyoid and laryngeal elevations is a risk factor for aspiration and pharyngeal residuals [31]. Therefore, the hyoid and laryngeal elevations are important aspects of swallowing. Both hyoid and laryngeal elevations during swallowing are isotonic movements. The maximum number of head lifting repetitions represents an isotonic movement, whereas the time to maintain maximum head lifting is characterized as an isometric movement [26]. Thus, the number of repetitions of maximum head lifting, an isotonic movement pattern similar to the swallowing movement, may be more sensitive in detecting swallowing dynamics and dysphagia. In this study, both the number of repetitions of maximum head lifting and the time taken to maintain maximum head lifting showed significant correlations with pharyngeal residue, as measured by NRRSv and NRRSp. However, in multiple regression analysis, only the time to maintain maximum head lifting was not a significant predictor of NRRSp. Despite this, the overall trend of association between these head-lift measures and pharyngeal residue was relatively similar. Therefore, we plan to increase the sample sizes in future studies to validate these associations and to enhance the statistical power to detect the independent contribution of each head-lift parameter to pharyngeal residue.

Association between jaw-opening Force and Pharyngeal Residuals

In this study, jaw-opening force was not significantly associated with NRRSv or NRRSp. It has been shown that jaw-opening force is related to the cross-sectional area of the geniohyoid muscle [32]. Additionally, a study examining the relationship between jaw-opening force and pharyngeal residue in the vallecula and pyriform sinuses using the fiberoptic endoscopic evaluation of swallowing test reported a significant difference in jaw-opening force between groups with and without pharyngeal residue in these areas [14]. However, in previous studies, pharyngeal residue was assessed at three levels (trace or none, mild, and severe), and only severe cases were considered as having pharyngeal residue, focusing on participants with substantial pharyngeal residue.

In contrast, this study used the NRRS, a continuous scale, to assess pharyngeal residue. The median amount of pharyngeal residue among participants was low, with NRRSv at 0.03 (IQR, 0.00–0.23) and NRRSp at 0.01 (IQR, 0.00–0.26). These differences in the patient population may have influenced the results of this study.

Mouth opening involves the suprahyoid muscles, specifically the anterior belly of the digastric muscle, the geniohyoid muscle, and the mylohyoid muscle, as well as the lateral pterygoid muscle [13]. In contrast, head lifting involves the suprahyoid muscles, the infrahyoid muscles, and the sternocleidomastoid muscle [23]. These movements share common muscle groups, and in fact, this study confirmed a moderate correlation between jaw-opening force and head-lifting strength. However, jaw-opening force was measured under isometric conditions, which may not fully capture the dynamic muscle contractions required during swallowing. This methodological differences, along with the differences in specific muscle involvement, may have contributed to the lack of significant association between jaw-opening force and pharyngeal residue.

Nevertheless, given the limited existing evidence, this remains a hypothesis. Further research is needed to clarify the relationship between individual swallowing-related muscle strength and swallowing function.

Limitation

This study had several limitations. First, the study was conducted at a single institution, necessitating caution when generalizing the findings. Second, the safety of the evaluation method was not ascertained. Third, the sample size of 50 participants may have resulted in relatively wide confidence intervals, potentially affecting the precision of our estimates and the generalizability of the findings. Fourth, this study focused on hospitalized older patients with suspected dysphagia, which may limit the generalizability of the findings to older patients in outpatient settings. Additionally, the underlying diseases in hospitalized patients could have affected the assessment of head lifting strength. Fifth, the same evaluator who assessed head lifting strength also measured the NRRS, meaning that blinding was not achieved. However, to mitigate potential bias, the NRRS was calculated by two individuals, and the average value was used. Sixth, the method of evaluating head lifting strength used in this study does not strictly distinguish between muscle strength and endurance, and it is thought that it includes elements of endurance. However, the maximum number of repetitions and the duration of maintenance— both reflecting muscle strength—provide a clinically feasible and non-invasive approach, which represents a notable strength of our study. We aim to address these limitations by continuing data collection, enabling the addition of disease-specific subgroup analyses, and considering their application in clinical evaluation on a case-by-case basis.