Fat Saps Muscle

The accumulation of fat within skeletal muscle, as happens with obesity, diminishes muscle performance.

By | November 1, 2015

CALF CROSS-SECTIONS: Fatty regions of human leg muscle, outlined in red, differ between lean (left) and obese (right) subjects. HADI RAHEMI, SIMON FRASER UNIVERSITY/MANCHESTER METROPOLITAN UNIVERSITY

EDITOR'S CHOICE IN PHYSIOLOGY

The paper
H. Rahemi et al., “The effect of intramuscular fat on skeletal muscle mechanics: Implications for the elderly and obese,” J R Soc Interface, 12:20150365, 2015.

Adipose accumulation
Human muscle is normally around 1.5 percent fat, but can reach up to 11 percent adiposity among the elderly and more than 5 percent in people with obesity, a condition tied to muscle weakness. To figure out how fat affects muscle performance, Hadi Rahemi of Simon Fraser University in British Columbia, Canada, developed an in silico model of a human calf muscle.

Fat’s effects
Working in collaboration with his advisor, James Wakeling, and with mathematician Nilima Nigam, Rahemi confirmed that increased fat impaired muscle performance by stiffening the tissue.

Exceeding expectations
Wakeling says the impact of excess fat was greater than he had expected. “If you put 5 percent fat into muscle, you’d imagine that you’re going to lose 5 percent of the contractile part of the muscle. But when you look at muscle performance, it’s reduced by far more than 5 percent,” he says. “When you put fat into muscle, it changes the way that the whole tissue responds. . . . It actually has to work against the fat when it is contracting.”  

Quality, controlled
The study “attempts to uncouple the separate effects of connective tissue stiffness, fiber [structure], and fatty tissue level [and] dispersion,” says Dave Tomlinson, a postdoc at Manchester Metropolitan University in the U.K. “Interestingly, the model . . . demonstrates that intramuscular fat independently lowers muscle quality.”

Because researchers don’t yet understand precisely how fat is distributed throughout human muscles, “the models we’ve got at the moment are conceptual,” says Wakeling. Working to create more-lifelike simulations using human MRI data is their next step.

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