In this study, we investigated which inspection methods were superior for the assessment of motor function including muscle strength and physical performance, in other words, the muscle quality. The result indicated that CSA showed the most correlation with muscle strength, whereas CTV showed the most correlation with physical performance.
The concept of sarcopenia has considerably changed over time, and recently, the EWGSOP2 has stated that the importance of measuring muscle quality is expected to grow as a defining feature of sarcopenia; however, there is no universal consensus on the assessment method for routine clinical practice [8]. Patients with dynapenia and sarcopenia are considered to be at a high risk of falling [3]. Further, in the present study, patients with poor function and sarcopenia had a significantly high risk of falling, which we believe reflected their reduced motor function. There is a physical state in which muscle mass is maintained but with reduced functioning and muscle quality. McGregor et al. [24] referred that not only changes in muscle mass but also other factors underpinning muscle quality including composition, metabolism, aerobic capacity, insulin resistance, fat infiltration, fibrosis, and neural activation may play a role in the decline in muscle function and impaired mobility associated with aging.
To date, muscle quality is determined by evaluating intramuscular fat infiltration using highly sensitive measurement devices such as CT and MRI [25]. Moreover, in the recent years, muscle quality has been evaluated using ultrasound; however, there is no consensus for any of these assessments. There are few reports wherein CT of the mid-thigh has been used to evaluate the relationship of CSA and CTV using motor function. The present study was performed to evaluate the muscle quality by CTV and its relationship with motor function.
In the evaluation of simple muscle strength such as grip and quadriceps femoris muscle strength, we found the strongest correlation with CSA in all subjects. Furthermore, in the evaluation of physical performance test that combined physical movement (single-leg standing, walking speed, stand-up test, TUG, SPPB, and two-step test), the highest correlation was observed with CTV, and some test showed significant stronger correlation with CTV suggesting that physical performance could be evaluated using CTV.
Lang et al. [26] reported that a low CTV of the thigh indicated a higher risk of fall and hip fracture; the results of the present study are consistent with these results. In normal and low-mass group, there was no decrease in CTV, whereas the poor-function and sarcopenia groups showed a significant decline in CTV. Therefore, we believe that this indicated that reduced motor function was caused by the reduced muscle quality in patients with poor function and sarcopenia. In this study, we classified patients into four groups according to muscle mass and motor function; this classification method including an evaluation of muscle quality may be useful in assessing functional status in the aging population.
Ikemoto-Uezumi et al. [17] compared tissue in the vastus medialis muscle of patients with osteoarthritis and reported that intramuscular adipose tissue (IMAT) and an increased proportion of interstitial tissue were observed. Thus, a decline in CTV with an increased proportion of IMAT was associated with intramuscular fat infiltration [27]. Similarly, in sarcopenia, increased IMAT is observed [16], and owing to changes in muscle tissue, CTV is decreased. Reportedly, increased IMAT measured by MRI is a prognostic factor of gait ability [28], and IMAT is increased in young adults with reduced physical activity [29]. Therefore, reduced physical performance and activity could lead to increased IMAT.
As a minimally invasive tool, CT is considered as a gold standard to measure muscle mass; however, it is not used in general practice owing to the high costs, poor portability, and requirement of an experienced operator [15]. In patients with cancer, reduced muscle mass is an independent predictor of immobility and mortality [30]. Kasai et al. [31] reported that among the muscles in the mid-thigh CT, CSA decreased with age mainly in quadriceps femoris. Schweitzer et al. [32] reported that on MRI, the best estimates for skeletal muscle were shown in the thigh. CT of the femur was a useful method to evaluate muscle mass for the entire body [33].
Compared with other test equipment, DXA showed no change with age among Asian women [11]. Recently, we reported significant associations of thigh CT CSA with muscle strength [34].
For BIA, changes associated with the physical condition such as fluid balance and body temperature fluctuated during the day, resulting in reduced accuracy of BIA [35]. CT could be a better form of assessment because of its accuracy, reproducibility, and objectivity; it also enables the simultaneous measurement of the muscle mass and quality [36]. CT is useful in diagnosing the severity of sarcopenia by simultaneously evaluating CSA and CTV. Regarding the amount of radiation exposure, as only a single slice is scanned, we consider that there is no problem in terms of the radiation levels [10]. In addition single slice leg CT scanning takes only about five minutes.
There were limitations to the present study. First, the sample size was relatively small. Second, this was a cross-sectional study and changes caused by natural progress or therapeutic intervention could not be evaluated. Further study is required to determine whether or not the changes occurred as a result of exercise therapy, nutritional guidance, and pharmacotherapy. Third, this study did not include large-scale data of CSA and CTV from each age group of the general population without sarcopenia, and therefore, the healthy control information was weak. Thus, analysis using a large-scale epidemiological study is required.
As noted in the EWGSOP2, our results showed that CT may be used to diagnose sarcopenia more accurately and understand the pathology in greater detail [8].