Skeletal muscle mass declines by approximately 1% per year after the age of 30 years, with an overall decline of 20–30% between the ages of 20 and 80 years. Although this age-related loss of muscle mass had been considered as a physiological aging, previous studies revealed that excessive muscle decline in older adults was associated with adverse health outcomes. Based on these backgrounds, the concept of sarcopenia was proposed to distinguish pathological muscle loss from normal aging. Sarcopenia is now defined as age-related muscle loss and dysfunction. Since the European Working Group on Sarcopenia in Older People proposed diagnostic criteria in 2010, several definitions have been developed and updated by consensus groups including those in Asia (Asian Working Group for Sarcopenia). While all criteria include skeletal muscle mass, muscle strength, or physical function, the combinations and priorities among these criteria differ, and a consensus has yet to be established. This review aims to summarize the conceptual and operational evolution of sarcopenia definitions and to compare the differences of definitions among consensus groups. Moreover, we discuss the measurement methods and its limitations related to skeletal muscle mass, muscle strength, and physical function.

This article discusses the concept of muscle quality, a relatively new term in physiological and medical research that has gained prominence since 2006. Initially defined as specific tension (muscle strength per unit muscle mass), the concept has expanded with technological advancements. Early research in 1990’s defined muscle quality as the ratio of muscle strength to lean mass. The relationship between aging and muscle quality is complex, with studies showing inconsistent results across genders and measurement methods. Since 2000, advanced imaging techniques (MRI, CT, ultrasound) have expanded muscle quality assessment to include muscle composition, particularly intramuscular fat infiltration and extracellular matrix changes. Muscle quality can be evaluated through various methods including MRI/MRS, CT, ultrasound imaging, dilution methods, bioelectrical impedance, and specific tension measurements. Each method has advantages and limitations regarding accessibility, cost, and reproducibility. In sarcopenia guidelines, the Global Leadership Initiative on Sarcopenia (GLIS) recently proposed that specific tension (force-to-size ratio) should be included in sarcopenia’s conceptual definition, while morphological features like fat infiltration should not. However, standardized measurement protocols and cutoff values remain challenging to establish.

This review focuses on body mass index (BMI) to clarify the characteristics of sarcopenic obesity from the perspective of muscle fibers (energy intake), capillaries (energy transport), and mitochondria (energy production). Fast-twitch muscle fibers are more susceptible to aging than slow-twitch fibers. The number of capillaries and mitochondrial function decrease as muscle atrophy progresses with age. However, these age-related changes are offset by changes in muscle fiber size. Interestingly, there is a proportional relationship between muscle fiber size and capillary number regardless of BMI level or age. There is a high degree of plasticity in mitochondrial structure and function in response to metabolic demands. Therefore, further research is needed to determine whether age-related mitochondrial changes are inevitable or can be prevented by exercise. In conclusion, while age-related atrophy and functional decline in fast-twitch muscle fibers maintain a balance with capillaries and mitochondria, BMI levels do not significantly affect this relationship. These findings suggest a need to re-evaluate our understanding of sarcopenic obesity, particularly the notion that excess fat exacerbates sarcopenia.

Traditionally, skeletal muscle health in older people has been focused primarily on the loss of muscle mass. However, there are several aspects of age-dependent progressive changes associated with aging, including skeletal muscle fibrosis, decreased muscle cell density, and intermuscular adipose tissue infiltration. While resistance training has been known to be effective in maintaining or increasing skeletal muscle mass, it is important to understand its effectiveness in addressing the different aspects in skeletal muscle health with aging. This review outlines methods for assessing various changes in skeletal muscle in older people and summarizes the evidence for the effectiveness of resistance training in improving skeletal muscle health. In terms of the effectiveness of resistance training for skeletal muscle mass, an alternative approach that is more feasible for older adults has been shown to be effective in addition to standard resistance training methods such as high-intensity and being performed to exhaustive state. On the other hand, scientific evidence on the effectiveness of resistance training in reducing inter- and/or intramuscular adipose tissue and increasing skeletal muscle cell mass remains to be accumulated. This review aims to identify these gaps in the research findings and highlights the need for individualized and multifaceted assessments and approaches to skeletal muscle in older people.