Variable physical functions in individuals in response to sensory stimuli

This study was approved by the Medical Ethics Committee of the National Center for Geriatrics and Gerontology, Obu, Japan (Approval Number: 1466) and was conducted according to the tenets of the Declaration of Helsinki. Written informed consent was obtained from all study participants. The data were anonymized for analysis.

This pilot study included the general healthy population; thus, we recruited participants who regularly attended gyms aged 20–70 years without any physical disorders. The exclusion criteria were visual impairment that may affect color perception, presence of complications or oral diseases that may affect taste perception, presence of cognitive or motor impairment that may intervene with the study procedures, and allergies to any of the test foods used for taste stimulation.

In the experiments, each of the six stimulant tests including the control were conducted in 1 day and for 6 successive days for taste stimuli and for color stimuli, totaling 12 days. The order of the stimuli was randomized, and the ordinal coefficient of the stimuli was the same (Fig. 1).

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Three physical function measurements were selected for ease of evaluation, minimal mutual interference, and their role in motor function. Muscle strength was measured twice in both arms using a grip dynamometer (Digital Grip Dynamometer; Takei Science Instruments Corp., Niigata, Japan), and the scores were averaged. Flexibility was assessed by measuring the level of extension when each participant bent forward from a sitting position, using a digital long-seat anteflexion meter (Long-Seat Anteflexion Meter; Takei Science Instruments Corp.). Finally, balance was evaluated by measuring the trajectory of the center of gravity (trunk movement during a wobble) while the participants stood on a stabilometer (Gravicoder GW-10; Anima Corp., Tokyo, Japan) for 30 s with their eyes open. Higher values indicated better muscle strength and flexibility, whereas lower values indicated fewer trajectory fluctuations and suggested better balance.

In this study, no specific pre-test restrictions were set because we wanted to assess physical changes in daily life. For the comparison of the differences between stimuli, we considered that the condition of the examinee would affect all stimuli results obtained in one session equally. Therefore, no individual condition was expected to affect the specific stimuli results on that day.

The average score of the values obtained from the physical examination for each of the six taste stimulus was calculated, similar to previous studies that used the mean value [24, 25]. One-way Repeated Measures ANOVA with Bonferroni correction was used for the analysis of the repeated bias and ordinal effect. The best and worst scores on the physical tests among the six stimuli were defined as the maximum and minimum scores, respectively. Data are presented as mean ± standard deviation. The change ratio was defined as the change in the maximum effect compared to that in the minimum effect. In this study, only the degree of change, not the direction of change, was examined; thus, the absolute values were used for calculating the rate of change (change ratio = [{maximum score − minimum score}/minimum score] × 100). Data are presented as median (interquartile range [IQR]). Friedman’s test was used to evaluate significant differences among three or more groups. When a significant difference was observed, the Wilcoxon signed-rank test was performed to examine which combination of the three groups showed a significant difference. Bonferroni correction was applied to all multiple comparisons. Data management and statistical analyses were performed using IBM® SPSS® Statistics version 27.0 (IBM Corp., Armonk, NY, USA).

Raw data were generated at the Innovation Centre for Translational Research at the National Center for Geriatrics and Gerontology. Data generated during this study are available from the corresponding author upon reasonable request.

A total of 41 participants were recruited, including 10 men and 31 women (mean age: 43.3 ± 10.1 years). Given the imbalanced sample composition, sex-based analyses apart from the overall analyses were performed.

To evaluate practice effect, which could affect the result, day-by-day comparison was performed (Table 1). A significant difference (P < 0.05) was observed in flexibility with color stimuli between days 1 and 4 (P = 0.045), days 1 and 5 (P = 0.039), days 1 and 6 (P = 0.047), days 2 and 4 (P < 0.01), days 2 and 5 (P < 0.01), and days 2 and 6 (P < 0.01); and in grip strength with taste stimuli between days 1 and 6 (P = 0.026) and days 3 and 6 (P = 0.021).

The ordinal variation of the results, from the first to sixth tests, was analyzed (Table 2). A significant difference (P < 0.05) was observed in flexibility with color stimuli days 1 and 2 (P < 0.01), days 1 and 3 (P < 0.01), days 1 and 4 (P < 0.01), days 1 and 5 (P = 0.018), and days 1 and 6 (P < 0.01); balance with color stimuli between days 1 and 4 (P = 0.039); and grip strength with taste stimuli between days 1 and 6 (P = 0.011).

The impact of each stimulus on the physical examination outcomes (Table 3) was examined. The percentage of each stimulus that resulted in the maximum and minimum scores for each participant was evaluated (Sup.1 and 2). Friedman’s test revealed that no stimuli resulted in significantly better or worse physical evaluation outcomes. In addition, sex-specific analyses did not reveal any stimuli with larger or smaller effects in men or women.

The maximum and minimum scores on all physical function tests were compared. For color stimulations, the median (IQR) of the maximum and minimum scores were 1.36 kg (0.94–1.80) for grip strength, 3.08 cm (2.33–3.92) for flexibility, and − 1.34 cm² (− 1.82 to 1.07) for balance. For taste stimulations, the results were 1.27 kg (1.03–1.67) for grip strength, 2.25 cm (1.88–3.42) for flexibility, and − 0.100 cm² (− 1.44 to 0.53) for balance.

Figure 2 shows the overall change ratios of the maximum and minimum scores for each physical test. In the analysis of total samples, the rates of change in muscle strength, flexibility, and balance scores upon color stimulation were 5.68% (4.14–8.07%), 8.52% (5.11–13.39%), and 30.60% (26.81–36.18%), respectively. For taste stimulation, the rates of change in muscle strength, flexibility, and balance scores were 4.96% (3.67–7.89%), 6.11% (4.37–8.86%), and 28.92% (21.38–34.01%), respectively. One participant had a rate of change of ≥ 40% in the flexibility tests upon taste stimulation. In the balance tests, there were six and five cases with a rate of change of > 40% upon color and taste stimulation, respectively.

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No significant differences in physical function were observed for any of the stimuli (Table 3). In addition, the percentages of stimulation with the best and worst results for each participant (Sup.1 and 2) showed no significant differences. In the sex-specific analysis, no significant stimuli were found in either men or women. Based on these results, it can be inferred that the response of physical body functions to stimuli varies among individuals. Our results support the conflicting findings of previous studies, showing that no specific stimulus effects tend to occur [20‐22]. Regarding the cause of these results, we believe that different participants responded differently to the stimuli and thus had varying reports. In addition, as shown in Sup. 1 and 2, the stimulants that affect the body the most and least are different for each individual. Therefore, this study shows the possibility of providing personalized and appropriate stimuli to affect an individual’s physical functioning. Specifically, these results suggest that while appropriate stimulation may be effective in improving physical function, inappropriate stimulation can impair physical function. In addition, in the present study, even in the same participant, different types of stimuli performed well depending on the type of exercise; for example, the best color for grip strength was different from the best color for flexibility in the same participant, suggesting differences in the underlying mechanisms. Future studies should include brain imaging to elucidate these associations.

Next, the extent of changes in physical function in response to various stimuli was evaluated. As mentioned previously, the cognitive processes involved in interpreting color and taste are becoming clearer. Exposure to color increases blood flow in the prefrontal cortex [26] and changes brain connectivity in response to color stimulation [16, 27]. Exposure to taste activates the anterior cingulate cortex, insular cortex, and ventral striatum via the gustatory and orbitofrontal cortices, thereby influencing motor behavior [28]. Although the mechanisms are gradually becoming clearer, there has been limited analysis of the actual changes; therefore, we examined the differences in actual color and taste stimulation. To elucidate the causes of differences in the effects of color and taste stimuli on physical function, as in this study, further exploration of the changes in neural function in response to stimuli is an appropriate next step. Regarding the differences in the physical assessment results, the balance outcome tended to be altered compared with grip strength and flexibility outcomes (Fig. 2). Balance plays an essential role in various physical activities and sports; however, we could not determine that the balance function was the most affected. To the best of our knowledge, no previous study has examined the neural correlates of changes in balance in response to sensory stimuli. Further studies are required to elucidate the mechanisms by which sensory stimulation affects balance.

Furthermore, the results of our study showed that responsiveness to stimuli varied among the same participants, and in some cases, individuals were extremely responsive to the stimuli. For example, the taste stimulus changed flexibility by > 40% in one participant, whereas similar ratios of change were observed in six cases using visual stimuli. Furthermore, a > 40% change ratio was observed in the balance tests in five cases upon taste stimuli. Another key to future research may be the “responders,” those who are strongly responsive to stimuli. A more detailed analysis of these highly respondent participants, comparing them with those with weak responses, may contribute to elucidating the mechanisms of changes in physical function caused by stimuli.

This study has some limitations. The present study only reported the range of change, and it cannot be concluded whether the change is clinically significant. Therefore, it is crucial to analyze actual neural network alterations using functional brain imaging and relate the findings to the physical changes evoked by the stimulations in this study.

In this study, the numbers of male and female participants differed. A comparison between sexes is important, as many previous reports have indicated that the effects can differ according to sex [29, 30]. More participants with an equal distribution of sex should be included and evaluated to analyze sex-related differences more accurately.

Regarding the study environment, all tests were performed under the same conditions in the same room, within an indoor sports gymnasium with constant light, and all participants wore the same type of goggles, which were designed to keep all fields of vision in the same color. However, the color saturation and brightness were not measured using a colorimeter. Future studies should address these limitations.

In addition, the effects of taste samples may not be representative of the taste effects. For example, coffee may affect body function as a component, rather than as a taste sensation. Furthermore, although the same dose was used for all participants in this study, it is necessary to evaluate the differences in the sensitivity of individuals because taste preferences vary among individuals. Future studies should define taste components and sensitivity ranges and perform analyses stratified by sensitivity levels.