Loss of bone tissue and bone strength occurs with aging, and is accelerated in menopausal women [1
]. Osteoporosis, a condition marked by low bone mass, is three times more common in women compared to in men [2
]. Low bone density can result in hip fractures [3
]. A study reported that among adults who could previously perform activities of daily living (ADL) independently, only 36% returned to independence, 27% survived but required ADL assistance, and 37% died 6 months to 2.5 years following a hip fracture [4
]. Therefore, it is important for older women to maintain bone health to prevent fractures.
Muscle mass also reduces with aging [5
], and age-related loss of bone and muscle mass occurs almost simultaneously [7
]. Muscle mass is related to bone mass in postmenopausal women [8
]. However, studies have reported inconsistent findings on the association between muscle mass and bone mineral density in older women. One study reported that muscle mass is related to bone mineral density in older adults [9
], whereas another study reported that age-related muscle loss does not seem to be involved in bone mass loss [10
It is helpful for health professionals to understand the characteristics of muscle mass in older women diagnosed with low muscle mass, in order to maintain their bone health and reduce the risk of fractures. The aim of this study was to investigate the characteristics of muscle mass in older women who have low bone mass. Furthermore, we investigated the characteristics of muscle strength and physical performance in these women, because it is possible that muscle mass is related to muscle strength, which influences physical performance [11
Participants’ characteristics are presented in Table 1
. Age and height were not significantly different between the three groups. Weight and BMI were significantly lower in the very low bone mass group than the low bone mass group (p
< 0.001 and p
= 0.001, respectively). The prevalence of diseases was not significantly different between the three groups. In the low bone mass group, one of the participants with orthopedic disease had a fracture.
A comparison of SMI, muscle strength, and physical performance between the normal, low, and very low bone mass groups is shown in Table 2
. Arm SMI, leg SMI, and appendicular SMI were significantly lower in the very low bone mass group compared to those of the low bone mass group (p
= 0.034, p
= 0.011, and p
= 0.009, respectively). These findings suggest that muscle mass decreases when bone mass decreases. Meanwhile, grip strength and knee extension strength were not significantly different between the three groups. In addition, usual and fastest gait speed and TUG were not significant different between the groups. These findings indicate that it is possible that physical performance does not decrease when bone mass decreases. The rates of sarcopenia were 0, 5.4, and 3.6% in the normal, low bone mass, and very low bone mass groups, respectively. The rates of sarcopenia were not significantly different between the three groups.
The aim of this study was to compare the characteristics of SMI, muscle strength, and physical performance among older women with normal, low, and very low bone mass. There were several major findings in this study. First, SMI was low in the very low bone mass group compared to the low bone mass group. Second, muscle strength was similar for all groups. Finally, physical performance of all groups was similar. The arm, leg, and appendicular SMI were significantly lower in the very low bone mass group compared to those in the low bone mass group. Taniguchi et al. [19
] reported that participants with osteoporosis had lower appendicular SMI compared to those without osteoporosis. In addition, some studies reported that bone mineral density was correlated with SMI [20
] and lean muscle mass [21
]. Our results support the findings of these studies and suggest that older women with low bone mass have low muscle mass. A study by Kaji [23
] reported that age-related loss of muscle mass appears to occur earlier than bone mass loss due to aging. Taken together, decreased muscle mass might be associated with loss of bone mass. Significant differences in any of the SMI values in the normal bone mass group were not observed, unlike those in the low and very low bone mass groups. The proportion of participants with normal bone mass was only 8% in the present study. This indicates that the sample size for the normal bone mass group was too small to detect a significant difference between the low and very low bone mass groups.
Regarding muscle strength, grip strength and knee extension strength were not significantly different between the groups (Table 2
), suggesting that muscle strength does not decrease when bone mass decreases. The age-related rate of reduction of muscle mass is different from that of muscle strength [24
]. Moreover, resistance training with an elastic band increased knee extension strength but did not change SMI in older women [25
]. Muscle strength was related to not only muscle mass, but also muscle quality [24
]. These findings indicate that changes in SMI do not necessary reflect changes in muscle strength. Therefore, muscle strength was not changed in spite of the decrease in SMI. Participants in this study might be a relatively healthy population. Therefore, muscle strength was not reduced in spite of muscle mass loss by maintenance of muscle quality. A previous study reported that grip strength was not significantly different between older adults with osteoporosis and those without osteoporosis [19
]. In addition, there were no associations between muscle strength, such as knee extension torque and grip strength, and bone mineral density in older women [26
]. Our results are similar to the findings of these studies. Meanwhile, other studies reported that grip strength is associated with higher bone mineral density [22
], which does not concur with the findings of the present study. A previous study reported that knee extension strength is associated with bone mineral density at the femoral neck site but not at the lumbar spine site in postmenopausal women [28
]. These findings imply that muscle strength is associated with only certain bone sites. In the present study, calcaneus bone mass was measured. Therefore, all three groups had similar grip strength and knee extension strength when grouped by T-scores for the calcaneus site.
Regarding physical performance, there were no significant differences in either gait speed or TUG between the groups (Table 2
). In previous studies, these physical performance indicators were also not correlated with bone mineral density [29
] in older women. Meanwhile, a study reported that gait speed is associated with bone mineral density and found an association between grip strength and gait speed in postmenopausal women [27
]. In the present study, muscle strength was similar for all groups. Muscle strength is associated with gait speed in older women [13
]. Therefore, although the reason for the discrepancy between our results and those of a previous study [27
] is not clear, our results seem to be appropriate because of the similar levels of muscle strength between the three groups.
In sarcopenic older women, the risk of osteoporosis was 12.9 times greater when compared to non-sarcopenic women [31
]. The rate of osteoporosis was higher in the group of older women with sarcopenia than that in the group of older women without sarcopenia [32
]. These findings suggest that low bone density and sarcopenia are related to each other. However, there was no association between bone mass and sarcopenia in the present study. The rate of sarcopenia has been reported to increase with advancing reduction of bone mineral density [32
]. The prevalence of sarcopenia, which was determined according to criteria published by the European Working Group on Sarcopenia, was 22.1% in Japanese older women [33
]. In the present study, only 4.2% of participants were evaluated with sarcopenia, and the rate was very low when compared to the previous study [33
]. Participants recruited in the present study might be from a relatively healthy population, which may be the reason why there was no association between bone mass and sarcopenia in the present study.
In the present study, there are some limitations. First, this study used a cross-sectional design, and it was difficult to estimate causality between muscle mass and bone mass. Further longitudinal research is required to investigate causality. Second, the study recruited a small sample of older women with normal bone mass compared to sample sizes of low and very low bone mass groups. Therefore, there might be beta error when comparing between the normal group and the low bone mass and very low bone mass groups. Further research is required with a larger sample size to clarify the association of low bone mass with decreased skeletal muscle mass between the normal bone mass group and the low and very low groups. Third, participants recruited in this study were relatively healthy. It is unclear whether the findings apply to less healthy populations; similar research is necessary. Despite these limitations, this study is meaningful, as the findings suggest that muscle mass could be used as an indicator of bone mass loss in older women and that maintaining muscle mass could prevent low bone mass.