This study aimed to demonstrate the relationship between BMD and sarcopenic indices in community-dwelling healthy older women. HGS was weaker in subjects with low BMD at the LS and FN than in subjects with normal BMD. The 6 MWS was not different in subjects with or without low BMD at the LS but slower in subjects with low BMD at the FN.
Sarcopenia is a condition characterized by progressive and generalized loss of skeletal muscle mass and function [16], which was officially classified as a disease state in 2016 under the International Classification of Diseases, Clinical Modification (ICD-10-CM) code (M62.84) [17]. The guidelines by various international working groups suggest different cutoff values for the diagnosis of sarcopenia, but all agree on using muscle mass and function to diagnose sarcopenia [18]. Sarcopenia has been reported to be a risk factor for falls [19] and fractures [20] in older adults, and there are several studies on the relationship between sarcopenic indices and low BMD, which represent fracture risk, although the results are inconsistent. Some studies have reported that low muscle mass is related to low BMD [21, 22], whereas others have reported no relationship between muscle mass and BMD [8, 23]. Muscle strength, a sarcopenic index, has been reported to have a positive relationship with BMD [24, 25], but physical activity showed little correlation with BMD [24].
Our study showed that HGS was positively associated with BMD at both the LS and FN, but 6 MWS was only associated with BMD at the FN. This is consistent with previous findings that showed that HGS was related to bone mass at the LS and hip in subjects with a similar mean age of 63.6 years old [26]. Some studies have explained the relationship between muscle and bone. The concept of bone a muscle unit crosstalk involves molecules such as myokines released by skeletal muscle, which affect bone, and osteokines secreted by osteoblasts, which in turn affect muscle cells [5]. Another explanation is that muscles and bones have common factors, including hormonal changes, low levels of physical activity, and chronic inflammation [27]. However, in our study, no relationship was observed between muscle mass and BMD.
Contrary to our results, some studies have shown a positive correlation between muscle mass and BMD. One study reported that BMD was positively associated with muscle mass in older women with a mean age of 74 years, but not with physical activity, walking speed, and TUG (Timed Up and Go test) {Bijlsma, 2013 #54}. Nonaka et al. also reported that muscle mass was significantly lower in subjects with low bone mass, but muscle function estimated by the HGS, TUG, gait speed, and knee extension strength were not different in subjects with or without low bone mass {Nonaka, 2020 #68}. The main different part of that study was that the mean age of the population was 74 years, which is much older than the subjects in our study, and only 8% of the subjects had normal BMD. In a recent study, the decline in muscle strength was much more rapid than the loss of muscle mass, suggesting that muscle quality first declines [28]. The mean age of the subjects in our study was 66 years, and there were no sarcopenic subjects according to the Asian Working Group for Sarcopenia guidelines [29]. We suspect that the subjects were relatively younger and healthier than those in other studies, which is why BMD showed no correlation with muscle mass, but a positive relationship with muscle strength. Moreover, another study on Korean women reported that HGS correlated with BMD at age 60–69, but not at ages over 70 [30]. This also implies that there is a possibility that muscle mass and function can change through aging, but not linearly.
This study had some limitations. First, this was a cross-sectional study conducted in a single hospital with only a few subjects. Therefore, it is difficult to conclude whether these results can be generalized. Second, we could not determine the causal relationship between bone loss and a decrease in muscle mass and function. Finally, all participants in this study were relatively healthy, and none were categorized as having sarcopenia. Nevertheless, this point strengthens the results of our study. For subjects diagnosed with sarcopenia, it is certain that doctors and patients would pay attention to the risk factors of frailty and fractures and would have more opportunities to measure BMD. We suggest that sarcopenic indices such as HGS can be a helpful method to screening patients with low bone mass, which accounts for the risk of fractures in relatively healthy community-dwelling older women even before the subjects have not yet been diagnosed with sarcopenia. Our study is also in line with the fact that muscle strength can reflect the status of bone loss and can change before the muscle mass decreases. We suggest evaluating muscle function using HGS in older women. For subjects with low HGS, it would be better to measure the BMD status and need more attention for fractures, even though they are not yet diagnosed with sarcopenia. Further studies are required to confirm these findings and determine how low grip strength influences fracture risk.