Quadriceps Lipid Content Has Sex-Specific Associations With Whole-Muscle, Cellular, and Molecular Contractile Function in Older Adults

Abstract

Increased adiposity is associated with reduced skeletal muscle function in older adults, but the mechanisms underlying this relationship remain unclear. To explore whether skeletal muscle properties track with adiposity, whole-muscle, cellular, and molecular function were examined in relation to adiposity measured at various anatomical levels in healthy older (60-80 years) men and women. Although women had greater absolute and relative body and thigh fat than men, quadriceps muscle attenuation, an index of intramuscular lipid content, was similar between sexes. At the whole-muscle level, greater quadriceps attenuation was associated with reduced knee extensor function in women, but not men. In women, decreased myosin heavy chain I and IIA fiber-specific force was associated with higher intramuscular lipid content, which may be explained, in part, by the reduced myofilament lattice stiffness found in myosin heavy chain IIA fibers. Longer myosin attachment times in myosin heavy chain I fibers from men and women were associated with greater amounts of adipose tissue, suggesting that fat deposits lead to slower myosin-actin cross-bridge kinetics. Our results indicate greater quantities of adipose tissue alter myofilament properties and cross-bridge kinetics, which may partially explain the adiposity-induced decrements in single-fiber and whole-muscle function of older adults, especially women.

Keywords: Adiposity; Human; Muscle fiber; Myosin heavy chain.

Figure 1.

Relationships that (left) knee extensor function in older men (n = 19) and women (n = 24) and (right) specific force in myosin heavy chain (MHC) I and IIA single muscle fibers in older men (n = 18) and women (n = 21) had with intramuscular lipid content. *p ≤ .05; **p < .01.

Figure 2.

Relationships between (left) myofilament lattice stiffness (parameter A from sinusoidal analysis) in myosin heavy chain (MHC) I and IIA single muscle fibers and intramuscular lipid content and (right) specific force and myofilament lattice stiffness in older men (n = 18) and women (n = 21). *p < .05; **p < .01; ^δp = .07.

Figure 3.

Relationships between myosin attachment time in myosin heavy chain (MHC) I and IIA single muscle fibers and adiposity in older men (n = 18) and women (n = 21). IMAT = intermuscular adipose tissue. *p < .05; ^δp = .06.