Sarcopenia, a form of muscle atrophy associated with aging and advanced disease, occurs as part of the natural aging process involving involuntary loss of skeletal muscle mass and functionality. The diagnosis of sarcopenia requires clinical findings of low muscle mass and either low muscle strength or poor physical performance. Several techniques including MRI, CT, DXA and BIA are available for measurement of muscle mass [11]. MRI and CT have high validity but are complex and costly. DXA and BIA are simple and inexpensive, and more realistic clinical methods. European Working Group on Sarcopenia in Older People and Asian Working Group for Sarcopenia have adopted limb muscle mass measured by DXA and BIA as the diagnostic criteria for sarcopenia [6, 12]. Previous studies have shown that DXA is low cost, low radiation exposure (< 1 µSv for whole-body scans), and reliable for research setting [8, 13, 14]. BIA is a non-invasive and low-cost tool used to determine body composition by measuring the electrical resistance of living tissues [15, 16]. However, BIA, differently from the other body composition tools, does not actually measure a specific body composition. Because body composition assessment from BIA relies on a calibration equation developed using a reference method such as DXA, CT or MRI. As well, this method has limitations due to the chemical composition of fat-free mass (i.e., water, proteins, glycogen, and minerals) owing to considerable inter- and intraindividual variability as a consequence of changes in fat-free mass occurring with growth, maturation, aging and disease states. Therefore, it is likely that DXA is the most suitable method for diagnosing sarcopenia in terms of cost, invasiveness, and accuracy.
Association between sarcopenia and lumbar spine disorder has been studied. Tanishima et al. reported that the ODI score in local residents with low back pain was significantly higher in sarcopenia group than in pre-sarcopenia group and normal group [17]. In contrast, Tsuji et al. reported that low back pain was not associated with sarcopenia [18]. There remains a controversy regarding association between low back pain and sarcopenia. As a reason for that diagnostic criteria of sarcopenia include limb muscle mass, not trunk muscle mass. It means that diagnosis of sarcopenia does not reflect a loss of trunk muscle mass, which is thought to be more important for lumbar spine.
Recent studies have shown relation between paravertebral muscle atrophy and several spinal pathologies [19–22]. Takahashi et al. reported that a decrease in paravertebral muscles in patients with osteoporotic vertebral fractures was significantly related to low back pain and delayed union after fracture onset [21]. Ranger et al. showed that paraspinal muscle CSA was associated with disability from low back pain [22]. The current study showed that DXA-derived trunk muscle mass (TMM) was significantly correlated with CSA of back muscles. From the viewpoint of invasiveness, cost and facilities required for the examination, CT or MRI which is ideal to evaluate the muscle CSA, is not suitable for large-scale research studies or clinical screening. Alternatively, DXA is a straightforward and reliable method for large cohort studies.
We measured CSAs of not only paraspinal muscles but also gluteus maximus, and DXA-derived TMM was strongly correlated to CSA of gluteus maximus in our data. For many years, it has been shown that bending forward is two-part movement involving both the spine and the pelvis. In extension from the fully flexed position, the movement is reversed, so that trunk extension is achieved through the cooperative contraction of the hip extensors including gluteus maximus and muscles of the back [23, 24]. These findings suggested that gluteus maximus plays an important role in maintaining sagittal spinal alignment as well as a key muscle of hip extensor. Bao et al. also reported the close relationship between gluteal muscles and sagittal malalignment [25]. As DXA-derived TMM is an indicator of gluteus maximus volume, longitudinal measurements of TMM may help early detection and early prevention of spinal disorders with sagittal malalignment.
The current study has several limitations. Participants were young and middle-aged healthy volunteers, not including elderly population 70 years or older. We did not undergo quantitative evaluation such as fat infiltration because fat infiltration of trunk muscles is not significant for people under 70 years of age. It remains unclear whether the current results could be applied to elderly population. To measure trunk muscle CSA using MRI more accurately in the elderly, histograms showing the signal intensity for excluding fatty infiltration from the ROIs of each muscle CSA should be generated using digitalized image processing software. Furthermore, the current study did not include individuals with serious low back pain and spine disorders, because the aim of this study is to obtain the data from healthy individuals. The ODI and RDQ scores of all participants were quite low. Additional studies involving subjects with spine pathologies are needed to clarify the differences between healthy volunteers and spine patients. Lastly, there is a belief that muscle strength and muscular endurance are clinically more important than muscle size, and muscle volume and mass may not major predictors of muscle strength and physical performance. Wang et al. reported that muscle density may represent a more clinically meaningful surrogate of muscle performance than muscle size [26]. Future studies should focus on analyzing the quality of muscle as well as quantity of muscle.