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Skeletal muscle is the major constituent of lean body mass and essential for the body's locomotor function. Women have less muscle mass (and more body fat) than men and are therefore not able to exert the same absolute maximal force as men. The difference in body composition between the sexes is evident from infancy but becomes most marked after puberty (when boys experience an accelerated growth spurt) and persists into old age. During early adulthood until approximately the fourth decade of life, muscle mass is relatively stable, both in men and women, but then begins to decline, and the rate of loss is slower in women than in men. In this review we discuss the underlying mechanisms responsible for the age-associated sexual dimorphism in muscle mass (as far as they have been elucidated to date) and highlight areas that require more research to advance our understanding of the control of muscle mass throughout life.

The purpose of this study was to determine whether brisk walking improves multiorgan (liver, muscle, adipose tissue) insulin sensitivity in older women. Ten nonobese older women (age: 66.7 ± 1.5 yr, mean ± SE) completed two 2-stage hyperinsulinemic-euglycemic clamp procedures [insulin infusion rate stage 1: 10 mU/m2body surface area (BSA) per min; stage 2: 50 mU/m2BSA per min] in conjunction with stable isotope-labeled glucose and palmitate tracer infusions: one in the morning after a single, ∼1-h bout of brisk treadmill walking, the other after an equivalent period of rest in the late afternoon of the preceding day. We found that basal glucose rate of appearance (Ra) into plasma was not different after rest and after exercise (17.3 ± 0.8 and 17.1 ± 0.4 μmol/kg fat-free mass per min, respectively). The insulin-mediated decrease in glucose Ra during stage 1 of the clamp was also not different after rest and exercise (82.2% ± 3.4% and 77.7% ± 2.1%, respectively), but glucose rate of disappearance (Rd) during stage 2 of the clamp was significantly greater ( P < 0.05) after exercise than rest (88.0 ± 5.9 and 78.4 ± 6.5 μmol/kg fat-free mass per min, respectively). There were no differences in palmitate Ra during basal conditions or insulin infusion after exercise and after rest. Therefore, we conclude that a single bout of brisk walking for ∼1 h improves muscle insulin sensitivity but has no effect on liver and adipose tissue insulin sensitivity in older women.

men have more muscle than women, and several attempts have been made to determine the physiological mechanisms responsible for this phenomenon by measuring the rates of muscle protein synthesis (MPS) and breakdown (MPB) without much luck so far. Except for one group of investigators who reported

Men have more muscle than women, but most studies evaluating sex differences in muscle protein metabolism have been unable to discern sexual dimorphism in basal muscle protein turnover rates in young and middle-aged adults. We hypothesized that the anabolic response to nutritional stimuli (i.e., amino acids and insulin) would be greater in young/middle-aged men than women. We therefore measured the rates of muscle protein synthesis (MPS) in 16 healthy individuals [8 men and 8 women, matched for age (mean ± SE: 37.7 ± 1.5 yr) and body mass index (25.2 ± 0.7 kg/m2)] after an overnight fast (plasma insulin ∼5 μU/ml and plasma phenylalanine ∼60 μM) and during a hyperinsulinemic-hyperaminoacidemic-euglycemic clamp (plasma insulin ∼28 μU/ml; plasma phenylalanine ∼110 μM; plasma glucose ∼5.4 mM). The rates of MPS were not different between men and women (ANOVA main effect for sex; P = 0.49). During the clamp, the rate of MPS increased by ∼50% ( P = 0.003) with no difference in the increases from basal values between men and women (+0.019 ± 0.004 vs. +0.018 ± 0.010%/h, respectively; P = 0.93). There were also no differences between men and women in the basal concentrations of muscle phosphorylated AktSer473, AktThr308, mTORSer2448, and p70s6kThr389 or in the hyperinsulinemia-hyperaminoacidemia-induced increases in phosphorylation of those signaling elements ( P ≥ 0.25). We conclude that there are no major differences in the rate of MPS and its intracellular control during basal conditions and during hyperinsulinemia-hyperaminoacidema between young and middle-aged adult men and women.