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RISE AND FALL OF SKELETAL MUSCLE SIZE OVER THE ENTIRE LIFE SPAN

2007; Wiley; Volume: 55; Issue: 7 Linguagem: Inglês

10.1111/j.1532-5415.2007.01228.x

1532-5415

Ilse M.P. Arts, Sigrid Pillen, Sebastiaan Overeem, Helenius J. Schelhaas, Machiel J. Zwarts,

Frailty in Older Adults

To the Editor: Sarcopenia, the age-related decline in muscle mass, is an important physical feature of growing old. It is thought to result from a complex interplay between neurological, metabolic, hormonal, nutritional, and physical activity–related changes.1 The loss of muscle mass and strength is associated with a greater tendency to fall, dependency, nursing home admission, and mortality.2,3 Sarcopenia can be partially reversed using resistance training interventions, even in the very old.4 Previous studies examined the effect of aging on muscle size predominantly in elderly subjects. In the current study, the rise and fall of muscle size across the entire life span was established by measuring muscle diameter in subjects aged 0 to 90 using ultrasound. A cross-sectional study was conducted in which the thickness of the biceps brachii and quadriceps femoris muscles were measured in 194 healthy volunteers (103 male subjects). Age ranged from 2 months to 90 years, with at least six male subjects and six female subjects per decade. All elderly subjects were still physically active and functionally independent and walked without a walking aid. The local ethics committee approved the study, and informed consent was obtained from all participants or their parents. Ultrasound was performed using a phased-array real-time scanner with a 7.5-MHz transducer (Sonos 2000 Phased Array Imaging System, Hewlett-Packard, Andover, MA). The patient was put in a supine position with the arms and legs extended and the muscles completely relaxed. A generous amount of contact gel was used to minimize the required pressure of the transducer on the skin. Transverse ultrasound images were made at the muscle belly (biceps brachii: two-thirds of the distance from the acromion to the antecubital crease; quadriceps femoris: halfway along the line from the anterior superior iliac spine to the superior aspect of the patella).5 Muscle thickness was measured with electronic calipers placed at the superficial fascia and at the underlying bone. To determine the relationship between age and muscle thickness, a nonlinear regression analysis was performed (SPSS version 12.0, SPSS Inc., Chicago, IL). Because of the asymmetrical curve-shaped distribution, a third-order polynomial regression equation was used to describe the data (y=b0+b1x1+b2x2+b3x3). The thickness of the biceps and quadriceps muscles as a function of age and sex are shown in Figure 1, together with the regression equations. The data were distributed according to a asymmetrical parabola. Up to age 20, muscle thickness increased steeply; between 25 and 50, peak muscle thickness was reached; and thereafter, sarcopenia set in. Starting between the first and second decade, men had thicker muscles than women. This sex difference was most pronounced in the biceps muscle, although men also had a steeper decrease in muscle thickness than women. Between age 40 and 90 quadriceps muscle thickness declined 50% in men and 30% in women, and biceps muscle thickness declined 30% in men and 20% in women. Ultimately, muscle thickness in men and women decreased to a size comparable with that of children in the first decade. Age-related changes in thickness of the biceps muscle (A) and the quadriceps muscle (B). Overlayed lines represent the fitted polynomial model according to the following equations (y=b0+b1x1+b2x2+b3x3): biceps muscle thickness, male=0.858+(0.118 × age)−(1.915 × 10−3× age2)+(8.546 × 10−6× age3), coefficient of determination (R2)=0.82, P< .001; biceps muscle thickness, female=1.142+(0.064 × age)−(1.130 × 10−3× age2)+(5.508 × 10−6× age3), R2=0.51, P< .001; quadriceps muscle thickness, male=1.812+(0.171 × age)−(2.996 × 10−3× age2)+(1.295 × 10−5× age3), R2=0.67, P< .001; quadriceps muscle thickness, female=2.081+(0.133 × age)−(2.744 × 10−3× age2)+(1.521 × 10−5× age3), R2=0.41, P< .001. This study showed the striking prevalence and severity of muscle loss due to aging; the thickness of the quadriceps muscle of a 90-year-old man is similar to that of a 5-year-old child. All studied subjects were healthy and physically active, even up to the highest ages, indicating that sarcopenia in the general population will be even more pronounced. Loss of muscle mass is the largest contributing factor to the decline in strength with age and is caused mainly by a decrease in muscle fiber size in combination with reduced number of muscle fibers.6 In addition, the number of muscle fibers per unit of muscle mass decreases.6 A reduction in motor axons is the primary cause of this fall in muscle density.7 Decreased muscle density leads to a further decline in muscle strength than explained by atrophy alone, because muscle thickness may be relatively spared because of fatty and fibrous muscle infiltration.6,8 Examining the results and taking into consideration that adults have to bear more weight than their 5-year-old counterparts, it is striking that 90-year-old men are still able to walk independently. Financial Disclosure: All authors certify that there are no financial interests that may inappropriately influence our actions. There are no conflicts of interest. Author Contributions: Arts and Pillen: acquisition, analysis, and interpretation of data and manuscript preparation. Overeem, Schelhaas, and Zwarts: analysis and interpretation of data and manuscript preparation. Sponsor's Role: No sponsor was involved in preparation of this letter.