Study: Single muscle far more complex than previously believed

3 min read

New research from Harvard’s Concord Field Station has shown that the common perception of a muscle as a single functional unit is incorrect and that different sections within an individual muscle actually do different work.

The finding enriches a gradually emerging picture of a single muscle as a far more complex unit than traditionally believed. Research in the late 1970s and early 1980s showed that the amount of work a muscle does can vary along a lone muscle fiber. The most recent research is the first to document a range of activity within a single muscle of a live animal, in this case a helmeted guinea fowl.

The new understanding of muscle function has the potential to influence the way muscles are treated everywhere, from academic laboratories to rehabilitation clinics, according to postdoctoral fellow Timothy Higham, who has worked on the project for a year.

“People typically look at one part of a muscle and assume the muscle is uniform,” Higham said. “This [finding] will force people to look at the muscle in greater detail.”

The new data not only advance science’s understanding of muscle function, Higham said, but could also be valuable in treating injuries. With the knowledge that different parts of a particular muscle do different work, knowing which part is injured can potentially guide treatment.

The research, published in the journal Biology Letters, was done by Higham, Concord Field Station Director Andrew Biewener, and James Wakeling of Simon Fraser University in British Columbia. The helmeted guinea fowl used in the study are a type of ground-dwelling bird, that, though capable of flying, are accomplished runners. Higham said that they make ideal subjects for research that involves collecting data while a bird walks and runs on a treadmill.

To measure muscle action, researchers implanted tiny electrodes and special crystals in the birds’ calf muscle, called the gastrocnemius. Both of the implants were so small that there was no measurable difference in how the birds walked with or without them, Higham said. The electrodes measured the electrical impulse reaching the muscle fibers, while the crystals, which send and receive sound waves, measured how much the muscles contracted, indicated by how long a sound took to go from one crystal to the other. Researchers also measured the force exerted at the tendon that joins the muscle to the foot.

The study showed that the lower part of the calf muscle did less work than the upper part of the calf muscle. Researchers also studied the differences in the work done by two parts of the calf muscle, called the medial and lateral gastrocnemii, which are considered separate muscles anatomically. The two muscles span the same joints so researchers thought they’d do about the same amount of work. That was not the case, however, with the study revealing a much larger difference than expected.

The biggest surprise, however, was how much the work varied within a single muscle, larger at times than the difference between the two separate muscles.

Biewener said the study confirms that parts of a muscle can specialize in particular movements. Though conducted on one set of muscles in one kind of bird, Biewener said the study’s results should be broadly applicable, as the bird’s calf muscle functions much as a human calf muscle does.

The emerging picture of muscle function, which also includes the long-known difference between powerful fast-twitch and high-endurance slow-twitch fibers, is one with more subtlety than previously understood, Biewener said.

“It is clearly the case that … muscles are more complex than previously appreciated,” Biewener said.