The most important contribution of this study was the simultaneous evaluation of seven physical performance measures among Japanese men aged 80 years or older who were able to walk independently, revealing the reference values for these measures in this study population. Following previous studies, we also evaluated the correlations between cognitive function and physical performance [5,7,8,11,12,13] and between age and physical performance [6,9,10,11]. To the best of our knowledge, the present study is the first to examine correlations among multiple physical performance measures, finding various levels of correlation. We found negative correlations between age and all performance measures except FRT, and age was not correlated with MMSE. MMSE was found to be weakly correlated with FRT and unrelated to the other six performance measures.
Cognitive status is considered a predictive factor for healthy life span among older adults—especially in terms of physical activity and walking level [24–26]. Matsueda et al.  reported a significant relationship between walking level and cognitive status evaluated by the MMSE among patients with hip fracture: Significant differences were observed in mean MMSE by walking status among people in their 80s (dependent walking status: 5.9 points, partially dependent walking status: 16.5 points, independent walking status: 23.7; p < 0.0001). In the present study, all participants were able to walk independently, and 95% (114/120) were categorized as not cognitively impaired using the MMSE. Taken together, our findings along with those from previous reports indicate that cognitive status can be viewed as an important predictor of maintaining physical activity levels among men aged 80 years or older. However, we did not find significant correlations between MMSE and most of the examined physical performance measurements. This is in contrast to the many studies that have reported significant correlations between cognitive function and aspects of physical performance, such as GS [12,13], QS , and walking speed [5,27]. These conflicting results may be explained by the fact that these previous studies included participants with various degrees of physical performance ability, whereas the present study evaluated only participants who were able to walk independently and therefore had a relatively high level of physical performance. Therefore, most participants in our study could be expected to also have relatively high MMSE scores because of their high level of physical performance. This focus on only participants with high levels of physical performance and cognitive ability may have made it difficult to detect a significant association between MMSE and the physical performance measurements.
With regard to MS, GS and QS have been reported to decline with aging [6,9,11,28]. Likewise, we found weak negative correlations between aging and both GS and QS. Seino et al.  reported the mean GS as 27.7 ± 5.9 and the mean weight-adjusted ratio as 0.49 ± 0.11 for those aged 80–84 years; for those aged ≥ 85 years, these researchers reported these values to be 23.2 ± 5.3 and 0.45 ± 0.10, respectively. In addition, a few previous studies [28,29] have evaluated QS using the same arthrometer used in this study. Ishii et al.  previously reported that the median MS/BW ratio for QS was 3.3 kgf/kg for patients with posterior cruciate ligament-retaining TKA, 3.4 kgf/kg for patients with posterior cruciate ligament-substituting TKA (median age: 81 years), and 4.6 kgf/kg for controls (median age: 83 years). For Japanese men aged 80–89 years (n = 86), another study reported the median MS/BW ratio for QS to be 5.9 kgf/kg . The values for both GS and QS found the present study were consistent with these previous findings. We believe that the index values of both GS and QS reported in the present study can be used as target values for adults aged under 80 years to maintain independent walking ability as they age.
One-leg balance is an important predictor of injurious falls in older persons , and a systematic review has demonstrated that it can also be a predictive tool for frailty among community-dwelling older adult populations . For individuals aged > 75 years, Seichi et al.  proposed a cutoff of 6 seconds for average OLS time to screen older adults for medical interventions or training programs. Seino et al.  reported mean OLS values of 26.0 seconds for those aged 80–84 years and 21.9 seconds for those aged 85 years or older. Harato et al.  reported 8.7 seconds as the mean OLS among patients graded as Kellgren–Lawrence grade IV  before TKA. Taking these previous results into account, the participants in the present study can be considered to have maintained relatively good one-leg balance and to be at low risk of falls.
The FRT is commonly used as an indicator of dynamic balance in older adults. Kamide et al.  concluded that older age and female sex were negatively associated with FRT value and that height and two-arm reach were positively associated with FRT value. In this study, because FRT was the only indicator of the seven performance measures that was not correlated with age, it was speculated that FRT was influenced more by other factors, such as physique, height, and two-arm reach, than by age. A recent meta-analysis of data from 20 published studies reported that the weighted mean (standard error) of FRT in these previous studies was 27.2 (0.9) cm, which provides a reasonable standard for interpreting FRT performance among community-dwelling older adults . Considering the findings of these previous studies, the median FRT value of 27 cm found in the present study can be considered to indicate that the study participants maintained satisfactory dynamic balance.
With regard to walking ability, previous studies [5,6,9,27,36] have focused on walking speed. Recently, Grande et al.  summarized the existing evidence concerning the associations of slow gait speed with cognitive decline and dementia, drawing on 39 previous studies. Maximum gait speed may be the best walking-ability marker for cognition among older adults . Seino et al.  reported mean values of 1.16 m/second for usual gait and 1.73 m/second for maximum gait among community-dwelling people in Japan aged 80–84 years; for those aged 85 years or older, these values were 1.11 m/second and 1.65 m/second, respectively. Amano et al.  identified sex, age, and Kellgren–Lawrence grade  as factors influencing walking ability such as 5-m walk test and TUG test outcomes. In the present study, both walking time and walking speed fell within the ranges presented in previous reports [2,5,9,27,36,38]. We believe that our results of relatively high values for MMSE and walking speed, even among men aged 80 years or older may support the previously reported positive correlations between cognitive level and physical performance [27,37].
The TUG test is a commonly used screening tool to assist clinicians in identifying patients at risk of falling. This test is recommended for the routine screening for risk of falls in the guidelines published by the American Geriatric Society and the British Geriatric Society . However, a systematic review of diagnostic accuracy  suggests that a single assessment tool such as the TUG test should not be used to identify community-dwelling older adults at increased risk of falls. Whether there is a significant correlation between TUG test score and falling remains a controversial topic. However, the TUG test might be helpful in detecting cognitive impairment: The mean TUG test time has previously been reported as 13.2 seconds among people in their 80s without MCI . Taniguchi et al.  reported a mean TUG test score of 9.0 ± 4.1 seconds for community-dwelling older adults in their 80s. The average TUG test values found in the present study were higher than these previous findings. This discrepancy may be explained by the fact that the participants in the present study might be at low risk of falling because few participants with cognitive impairment were included.
BUA is recognized as a parameter associated not only with bone density but also with bone architecture and elasticity . Ultrasonographic measurements of the os calcis have been shown to predict the risk of hip fracture among older women with equal accuracy to dual-energy x-ray absorptiometry of the hip . Ishii et al.  reported a significant improvement in BUA 5 years after TKA surgery, concluding that this improvement might be explained by increased activity levels after surgery, which might increase mechanical loading of the calcaneus among older adult patients. Yanagimoto et al.  used BUA to demonstrate that the amount of walking was positively correlated with bone quality in older adults. In addition, Yung et al.  reported that weight-bearing exercises had positive effects on calcaneal bone properties, as assessed using quantitative ultrasound (including BUA). We believe that men aged 80 years or older who are able to walk independently may maintain good bone quality because of their activity levels.
The present study revealed significant correlations of varying levels among all measured performance variables. Because walking independently requires people to adjust their center of gravity while maintaining good body balance, all physical performance measures in this study should be effectively linked to accomplish this task. If even one of the seven types of physical performance was not well-functioning, we believe older adults might not accomplish independent walking. In fact, a previous report  found that even grip strength, which is often thought to be unrelated to walking ability, affects the level of activities of daily living for people aged 80 years or older. In terms of the different strengths of the correlations among the performance measurements, it seems reasonable that walking speed was strongly correlated with the other variables. It also seems reasonable that participants with a fast 5-m NWS would also have a fast 5-m MWS, leading to a positive correlation. Likewise, a fast 5-m MWS was linked to a short TUG time, resulting in a negative correlation between these measures. However, it is difficult to determine the reasons for the differences between weak and moderate correlations among performance measurements in the present study. These differences should be clarified using a larger number of cases in multicenter studies in the future.
This study had several limitations. First, the participants who took part in the study suffered from some skeletal dysfunction caused by degenerative joint and/or cartilage disease. However, because they did not need the support of others to walk, the impact of their disease status on their physical performance seemed to be negligible or small. Conducting our study in this clinical setting also meant that some equipment that is not usually available in research could be used to evaluate QS and bone quality. This allowed us to add two rarely explored physical performance measurements to this study. Second, the total number of participants was lower than the samples used in previous studies of community-dwelling older adults [6,9,16]. However, the number of participants who were men aged 80 or older was comparable to the samples of this population subgroup used to evaluate physical performance in previous studies, where the number participants in this age group has ranged from 9 to 68 [8,14,28,30,31]. Third, we did not consider other factors that may impact the physical performance measures, such as age, sex, and race. Finally this was a single-center study, which may limit the generalizability of the results. Verification of the validity of our results through research at multiple facilities is expected in the future.
Despite these limitations, this is a valuable report showing reference values for the highest number of physical performance measurements to date (previous work has examined a maximum of six physical performance measurements [5,6]), as well as the correlations among them, for 120 men aged 80 years or older who were able to walk independently in one of the most rapidly aging nations in the world. The reference ranges found in the present study can be used by older adults who have not yet reached the age of 80 years and their health care providers as target values to facilitate the maintenance of independent walking.