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Comparisons among young and older men and women with regard to muscle size have been made in several small studies starting in the 1980s. The results showed that healthy women in their 70s had a 33% smaller quadriceps cross‐sectional area as obtained by compound ultrasound imaging compared with women in their 20s [2]. Using the same methodology and age groups, healthy older men had a 25% smaller quadriceps cross‐sectional area [3]. In a study investigating thigh composition using five computed tomography (CT) scans of the total thigh, smaller muscle cross‐sectional areas were observed in older men compared with younger men even though their total thigh cross‐sectional area was similar. The older men had a 13% smaller total muscle cross‐sectional area, 25.4% smaller quadriceps, and 17.9% smaller hamstring cross‐sectional area [4]. Using magnetic resonance imaging of the leg anterior compartment, muscle area was measured in young and older men and women [5]. The older persons had a smaller area of contractile tissue, 11.5% less in women and 19.2% less in men, compared with the young persons. These data, obtained by different body composition technologies, clearly showed a smaller muscle size in older persons compared with young persons. The observed differences in muscle size between age 20 and age 70 suggested a loss of skeletal muscle mass of about 0.26–0.56% per year.

Figure 2.1 Differences in fat‐free mass and lean mass using different body composition methodologies between men of different age groups. BIA = bioelectrical impedance; DXA = dual‐energy x‐ray absorptiometry.

Source: Based on references [7, 8].

The amount of non‐muscle tissue within the muscle was also assessed using five CT scans of the thigh in 11 older men and 13 young men [4]. Older men had 59.4% more non‐muscle tissue within the quadriceps and 127.3% within the hamstring muscle. In a similar study, the amount of non‐muscle tissue in older men was 81% higher in the plantar flexors as compared with young men [6]. Thus, apart from the smaller muscle size in old age, these studies suggested that the composition of the muscle also changed with aging, leading to less “lean” muscle tissue in old age.

With the greater availability of body composition methods such as bioelectrical impedance and dual‐energy x‐ray absorptiometry (DXA) over time, cross‐sectional data on muscle size in large study samples including a broad age range have been collected. Examples of these studies using lean mass from DXA (the non‐bone, non‐fat soft tissue mass) and fat‐free mass from bioelectrical impedance, presented by 10‐year age groups of men, are presented in Figure 2.1 [7, 8]. Older age groups had a lower total body fat‐free mass, lower total body lean mass, and lower arm and leg lean mass. Figure 2.2 presents the differences in muscle size between 10‐year age groups in men and women. With increasing age group, the data suggested a lower whole‐body lean mass and leg lean mass as assessed by DXA [9], a smaller arm muscle cross‐sectional area (from anthropometric measures [10]), and a smaller calf muscle cross‐sectional area (from peripheral qualitative CT [11]). These cross‐sectional data derived from samples from Italy, Australia, India, Japan, and the United States consistently suggested a decline in muscle size with aging. These data also suggested a steeper decline in muscle size with aging in men compared with women.

Figure 2.2 Differences in muscle cross‐sectional area and lean mass using different body composition methodologies between men and women of different age groups. DXA = dual‐energy x‐ray absorptiometry; CT = computed tomography; Anthrop. = anthropometry, using arm circumference and triceps skinfold.

Source: Based on references [9–11].

Cross‐sectional data from a sample of 72 women aged 18–69 years suggested a strong correlation between age and the amount of low‐density lean tissue as assessed by a CT scan of the mid‐thigh. The density of muscle tissue as assessed by CT is indicative of the amount of fat infiltration into the muscle [12]. Higher age was associated with greater amounts of low‐density lean tissue (correlation coefficient = 0.52 [13]). This result again suggested a greater fat infiltration into the muscle with the increasing age.

These cross‐sectional data, however, should be interpreted carefully as cohort and period effects, and not aging per se, may have caused the observed differences in muscle size and muscle composition between the age groups. For example, well‐known cohort differences in body height, a strong determinant of muscle size, may partly explain the lower muscle mass in older persons compared with younger persons. In addition, period differences in lifestyle (e.g. sports participation, diet, and obesity status) and job demands may have differentially affected muscle size and muscle composition between age groups. Therefore, prospective data are needed within the same individuals to investigate the true change in muscle mass with aging.