In the present study, we evaluated the prevalence of LoS and sarcopenia. Furthermore, we assessed the knee extension strength, balancing ability, and sarcopenia in relation to LoS. The total number of participants was 312 (75.5%) for those diagnosed as LoS and 31 for those diagnosed as sarcopenia (9.81%). Knee extension strength and OLS test between the no LoS and LoS groups, knee extension strength was significantly lower in the LoS group. Particularly in males, sarcopenia patients could be assumed to have LoS, but non-sarcopenia patients may or may not have LoS.
A characteristic feature of this study was the collection of participants with age ranging from 50 to 89 years by randomly sampling from the resident register. Compared to conventional research on residents that recruits active volunteers, this research was designed to create a cohort that more accurately reflects common residents. In addition, another unique feature of this study was that we gathered approximately 50 participants for physical examinations in each age group and sex, and as a result, the age and male-female ratio of participants aged 50-89 years were uniformly distributed. This uniform distribution is advantageous for performing accurate statistical comparisons between sexes or age groups.
The prevalence of LoS in this study increased with age, with approximately half of the participants aged 50-59 years and over 90% aged 70-79 years and 80-89 years who were diagnosed as LoS. There were approximately 43,376,300 patients with LoS (75.5% of 50-89 years), 29,195,593 patients (50.8%) with LoS Stage 1, and 14,180,716 patients (24.7%) with LoS Stage 2 in Japan when weighted by the Japanese population. In previous studies [5-8] and in this study, there are differences in the composition ratio for age and sampling method of subjects; thus, cannot simply compare the rate of prevalence for LoS in this study with results from previous studies. However, the prevalence of LoS in this study was comparable to the prevalence of LoS described in a study by Yoshimura et al. [5] that calculated the rate of prevalence by age and sex. On the other hand, when comparing the ratio of participants who were diagnosed with LoS by each criterion in the LoS risk test, the ratio in this study was greater than in the study by Yoshimura et al in terms of those with difficulties in maintaining a one-leg stand from a 40-cm-high seat during the SUT. In contrast, the ratio of participants who obtained a TST score >1.3 was less than what was reported by Yoshimura et al. There was no significant difference between both studies in terms of the results in 25-qGLFS. The reason for the difference in results for each criterion in diagnosing LoS is unknown. However, there are reports that suggest that the average step count increases with size of the city [24] and that those who exercise at a moderate or higher intensity are greater in urban areas than in rural areas [25]. We suspect that the results of the SUT in this study were inferior to other studies due to decrease in the muscle strength of knee extension as a result of the comparatively small amount of daily physical activity in residents of rural areas.
When comparing the physical function between the no LoS and LoS groups, the knee extension strength was significantly lower in both male and female participants with LoS aged 50-59 years, and female participants aged 60-69 years. From this fact, we believe that some measures to prevent or improve LoS may require exercise to increase the muscle strength of the lower limbs.
In terms of the prevalence of sarcopenia in the general elderly population of Japan, one report described that the prevalence of sarcopenia according to age group stratifications of 60-64, 65-69, 70-74, 75-79, and ≥80 years were 0.5, 0.0, 4.3, 11.2, and 27.0%, respectively [26]. The prevalence of sarcopenia was higher when comparing each age group with past reports. In the present study, the total prevalence of sarcopenia was 9.81%, which was slightly higher than in previous studies. There were approximately 4,031,275 patients with sarcopenia (7.02% of 60-89 years) in Japan when weighted by the Japanese population.
While there have been reports that describe no sex difference in the prevalence of LoS and sarcopenia [6, 26, 27], there have also been reports that describe a higher prevalence of sarcopenia in males compared to females [28]. Significant associations between sarcopenia and LoS were found in both sex based on the results of the ROAD study, which is a study of Japanese residents aged 60 and over [15]. On the other hand, the results of this study showed that males might have an association between sarcopenia and LoS, while females might not. There are several possibilities as to why this difference may have occurred. First, the sample size may have been insufficient. The results for females could be clarified with a larger sample population. However, other causes should also be considered. The cause of LoS, which is more common in females, might have been implicated. For example, osteoporosis and knee osteoarthritis are common in females [6]. Walking speed, which could not be assessed in our study, may contribute to a more severe diagnosis of sarcopenia for females than for males. Moreover, sex differences of the muscle mass cut-off in the diagnostic criteria for sarcopenia might have been affected.
There are several limitations in this study. First, although the research design reduces the sampling bias by adopting random sampling from the resident register, we may not have been able to control for all potential biases as a result of the low participation rate. Furthermore, this study may not reflect the regional characteristics of urban residents, because it is a sampling of one town. Subjects were randomly sampled from the resident registry of the town, but the decision to participate in this study was left to the sampled subjects. Therefore, we conducted a sensitivity analysis on the association between sarcopenia and LoS, assuming that there was heterogeneity in the participation rate depending on whether or not the participants felt impaired in terms of their mobility. While the overall participation rate (32%) remained constant, we changed the participation rates of each cluster, which were LoS (+) / Sarcopenia (+), LoS (+) / Sarcopenia (-), LoS(-) / Sarcopenia(+), and LoS(-) / Sarcopenia(-), to 20%, 30%, 40%, and 50%. These participation rates resulted in an adjusted odds ratio of 4.1 between sarcopenia and LoS, which was larger than the crude odds ratio of 3.4. Conversely, when we changed the participation rates of the above-mentioned clusters to 50%, 40%, 30%, and 20%, the adjusted odds ratio was 4.1, which was also larger than the crude odds ratio of 3.4. Assuming the impact of heterogeneity in participation rates due to the small sample size, it is suggested that we were likely underestimating the relationship between sarcopenia and LoS. Secondly, the diagnosis of sarcopenia is underestimated because we could not measure walking speed in this survey. This has a severe impact on the results of analysis for association between sarcopenia and LoS. Therefore, we performed a sensitivity analysis for sarcopenia misclassification as an additional analysis, and the results are presented in Table 5. The crude odds ratio for sarcopenia/LoS was 3.4 in this study. Because participants with LoS are expected to have low walking speed, the false-negative rate of sarcopenia for LoS-positive participants is likely to be higher than that of LoS-negative participants. Accordingly, the adjusted odds ratio would be greater. Sarcopenia may prove to be a predictor of LoS with the introduction of an appropriate diagnostic procedure for sarcopenia.
Table 5. Sensitivity analysis for misclassifications of sarcopenia
Sensitivity of sarcopenia diagnosis in LoS (+)
|
Sensitivity of sarcopenia diagnosis in LoS (-)
|
|
100%
|
90%
|
80%
|
70%
|
60%
|
50%
|
100%
|
3.4
|
3.0
|
2.7
|
2.3
|
2.0
|
1.6
|
90%
|
3.8
|
3.4
|
3.0
|
2.6
|
2.2
|
1.8
|
80%
|
4.4
|
3.9
|
3.5
|
3.0
|
2.6
|
2.1
|
70%
|
5.1
|
4.6
|
4.1
|
3.5
|
3.0
|
2.5
|
60%
|
6.2
|
5.5
|
4.9
|
4.3
|
3.6
|
3.0
|
50%
|
7.8
|
7.0
|
6.1
|
5.3
|
4.5
|
3.7
|
Note: Values represent adjusted odds ratio between sarcopenia and locomotive syndrome.
Abbreviations: LoS, locomotive syndrome.