**1. Introduction**

Trunk muscles, especially the paravertebral muscles (PVM), play an important role in supporting the spinal column [1]. The trunk muscles, which include the erector spinae (ES), multifidus (MF), and psoas major (PM), are reported to provide spinal stability during both moving and static states [2]. A decrease in trunk muscle volume or quality, due to sarcopenia [3] and fatty infiltration, along with aging, leads to spinal problems such as low back pain [4] and spinal sagittal imbalance [5]. Therefore, the importance of assessing trunk muscles, especially for the elderly in clinical settings, has attracted attention in recent years.

Traditionally, the quantitative assessment of trunk muscles is performed by measuring the cross-sectional area (CSA) of PVM using magnetic resonance imaging (MRI) or computed tomography (CT) [6–9], and the functional assessment of trunk muscles is performed by measuring back muscle strength (BMS) [10,11]. However, quantitative assessment of

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**Citation:** Salimi, H.; Ohyama, S.; Terai, H.; Hori, Y.; Takahashi, S.; Hoshino, M.; Yabu, A.; Habibi, H.; Kobayashi, A.; Tsujio, T.; et al. Trunk Muscle Mass Measured by Bioelectrical Impedance Analysis Reflecting the Cross-Sectional Area of the Paravertebral Muscles and Back Muscle Strength: A Cross-Sectional Analysis of a Prospective Cohort Study of Elderly Population. *J. Clin. Med.* **2021**, *10*, 1187. https:// doi.org/10.3390/jcm10061187

Academic Editor: Joel T. Cramer

Received: 22 February 2021 Accepted: 1 March 2021 Published: 12 March 2021

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trunk muscles using MRI or CT is not routinely performed owing to the high cost and time requirements [12].

Bioelectrical impedance analysis (BIA) is a non-invasive examination technique that determines body composition by measuring the electrical resistance (bioimpedance) of living tissues [13]. In recent years, it has frequently been used in clinical settings as a guiding tool for fluid managemen<sup>t</sup> and identification of the optimal method for patients undergoing dialysis [14–16]. BIA has been widely used to determine appendicular skeletal muscle mass (ASM) for diagnosis of sarcopenia [17]. In the limbs, which are mainly composed of muscle, bone, and fat, the muscle mass calculated by BIA is considered to be reliable [18].

Moreover, BIA has been used to calculate trunk muscle mass (TMM–BIA) and, recently, a decline in TMM–BIA has been associated with low back pain and poor quality of life [12]. However, it has been unclear whether the TMM–BIA reflects actual muscle mass, since the trunk also contains organs. Only one study [19] has reported a correlation between TMM– BIA and the CSA of PVM on MRI; however, due to the small sample size in that study, the accuracy of TMM–BIA could not be adequately investigated. Thus, the purpose of this study was to verify whether TMM–BIA correlates with the quantitative and functional assessments traditionally used for trunk muscles.

#### **2. Materials and Methods**

#### *2.1. Ethics Approval*

This study used data obtained from the Shiraniwa Elderly Cohort (Shiraniwa) study [20]. The study protocol was approved by the Institutional Review Board of Osaka City University Graduate School of Medicine (No. 3484). All methods were performed in accordance with the Declaration of Helsinki and the Ethical Guidelines for Medical and Health Research Involving Human Subjects in Japan. Written informed consent was obtained from each participant.

#### *2.2. Study Population*

The Shiraniwa study is a prospective cohort study that investigates sarcopenia, locomotive syndrome, frailty, and spinal sagittal imbalance among elderly people (aged 65 years or more) living in suburban areas of Japan and recruited by community notices and bulletin boards within our hospital. The inclusion criteria of the subjects were as follows: able to visit the hospital for the survey, able to walk independently, and willing to participate in annual surveys for 5 years. In total, 458 people applied voluntarily and were sent consent forms and self-administered questionnaires. After written consent was obtained, 409 participants (164 males, 245 females; mean age, 73.5 years; SD, 5.4 years) were finally included in the Shiraniwa study. In this analysis, we obtained the data from the first-year survey of the Shiraniwa study and excluded participants who could not undergo MRI or who had metal implants for spinal fusion surgery in their trunk (Figure 1).
