Since the World Health Organization (WHO) proposed the concept of "healthy life expectancy" in 2000, there has been growing interest not only in how to extend medical life expectancy but also in how to maintain a higher quality of life and how to prevent the need for long-term care (care prevention). This is important from the viewpoint of shortening the gap between medical life expectancy and healthy life expectancy, which is a matter of great importance to many elderly people. Japan and many other countries will soon become super-aging societies. At that time, sarcopenia, which is considered to be a major factor in the decline of physical function among the elderly, and the frailty it causes will become a huge barrier to maintaining a healthy life expectancy.
Generally speaking, human muscle mass and strength gradually decrease with age and other factors.1) Decreased muscle strength results from decreased muscle mass, which in turn leads to impaired balance, falls, trauma and mobility difficulties, which then may lead to a disability that results in the need for nursing care. Furthermore, muscle weakness leads to a decrease in walking speed, which leads to decreased activity and energy expenditure, resulting in poor nutrition. This decline in muscle mass and strength is known as sarcopenia, and the term "primary sarcopenia" refers to age-related sarcopenia, while sarcopenia arising from other causes, such as activity, disease or nutritional status, is referred to as secondary sarcopenia.2) Since this article discusses age-related loss of muscle mass and strength, for our purposes, “sarcopenia” will refer to primary sarcopenia.
The concept of sarcopenia was first proposed by Rosenberg in 1989,1) and initially referred to age-related loss of muscle mass. Balagopal et al. report that in general, human beings lose 1–2% of muscle mass annually after age 50, and at age 70, humans have 25–30% less skeletal muscle area and 30–40% less muscle strength than they had in their 20s.3) Although everyone may experience a decrease in skeletal muscle mass as a result of aging, sarcopenia is suspected when the degree of muscle loss deviates from the range of aging phenomena and muscle strength declines. This muscle weakness is considered a major factor in frailty,4) and early detection of sarcopenia is important in order to prevent this frailty.
Since muscle mass varies widely by race, sex, lifestyle and eating habits, there are currently no unified diagnostic criteria for sarcopenia. However, the diagnostic criteria that are generally used were originally proposed by the European Working Group on Sarcopenia in Older People (EWGSOP),2) and have cutoff values adjusted according to race. Since the EWGSOP defines sarcopenia as a loss of muscle mass regardless of age, accompanied by a loss of muscle strength or physical function, a diagnostic algorithm that removes the age cutoff value is now used.5)
The Japanese Association on Sarcopenia and Frailty defines sarcopenia as "a decrease in skeletal muscle mass and a decrease in muscle strength or physical function seen in old age"4) and has recommended the use of the Asian Working Group for Sarcopenia (AWGS) diagnostic criteria6) since 2020. The AWGS diagnostic criteria use dual-energy X-ray absorptiometry or bioelectrical impedance analysis to establish cutoff values.
On the other hand, there is a concept similar to sarcopenia in Kampo (Traditional Japanese) medicine: kidney deficiency, which indicates muscle weakness. It is thought to refer to weakness of the muscles of the lower half of the body. In Kampo medicine, abdominal examination7) may be based on the quantity/strength of the rectus abdominis muscles. Weakness in this area is known as weakness of the lower abdominal region (WLAR). WLAR is widely known to be more common in people with weak legs and a weak lower body, and in the elderly.8)9)
Figure 1 shows an abdominal image created using Aquarius NET Server (TeraRicon, Incorporated, Durham, NC, USA) to illustrate WLAR. The examination method is to compare the resistance of the midsection of the abdomen a few fingers’ width above and below the umbilicus. If the resistance below the umbilicus is weaker than that above, the WLAR finding is considered positive. In other words, the WLAR finding is an examination of lower abdominal muscle weakness that may reflect lower body muscle strength.
If there is a significant correlation between lower abdominal muscle mass and lower body muscle mass, WLAR findings might be used as a screening tool to diagnose sarcopenia. WLAR is an extremely simple method that uses only the hands to make a determination. The technique is easy to learn and has been incorporated into student practice in our department10); it is even simpler than the currently used SARC-CalF (strength, assistance in walking, rising from a chair, climbing stairs – calf circumference) assessment.6) If WLAR findings are proven to be useful in screening for sarcopenia, the WLAR method will be of great advantage in early detection and intervention, which are crucial in the treatment of sarcopenia and frailty. Therefore, we decided to investigate the relationship between lower abdominal muscle mass and lower body muscle mass. As a criterion for lower body muscle mass, Hamaguchi et al. propose a method that establishes cutoff values for Asian people using the psoas muscle index (PMI), which is calculated from the subject’s height and bilateral iliopsoas (psoas) muscle area at the 3rd lumbar subvertebral level measured by computed tomography (CT) scan.11) Under Hamaguchi’s method, falling below the cutoff value set for the appropriate gender could be an indicator of sarcopenia. In Japan, this method is widely used not only in hepatobiliary surgery but also in gastroenterological surgery,12) respiratory surgery,13) gynecology14) and other fields.
In the present study, we used CT images to measure the cross-sectional area of the iliopsoas and rectus abdominis muscle-related indices, and investigated which abdominal measurement indices are relevant to a diagnosis of sarcopenia.
For details on the concepts of Kampo medicine, please refer to the Supplement.