What is a Muscle?

Do you know what a muscle is?

muscle anatomy

Muscles are more intricate than we often realize. While we might think of them as one solid thing, they are actually far more nuanced than that. Their structure is what makes them so functional. Would it surprise you if I said that muscles are mostly connective tissue?

Muscle structure

Let’s break down a muscle. Picture a whole muscle that you’re familiar with. Let's say the biceps brachii in the arm. This “whole muscle” is really a group of what are called muscle fascicles wrapped in a layer of fascia [link to What is muscle fascia? post] called the epimysium. You could picture a muscle a bit like a grapefruit. The grapefruit peel is like the fascia that wraps around the muscle fascicles. The muscle fascicles are like the pulpy sections of the grapefruit.

We can go even deeper into layers of muscle structure, however. Each muscle fascicle is composed of a bundle of muscle fibers and each of those bundles is wrapped in a layer of fascia which is called the endomysium. We could compare this to an individual wedge or section of a grapefruit where a bunch of pieces of pulp are held together by a wrapping of tissue. Each of these muscle fibers is itself a group of muscle cells that is wrapped in fascia. At the smallest level, a muscle cell is made up of two proteins: actin and myosin.

Muscle function

What makes muscle work or “contract” then? Muscle contraction happens at the level of individual muscle cells. The two types of proteins, actin and myosin, are arranged in a row. When the nervous system sends a signal that a contraction is needed, calcium is released and this triggers the two proteins to be attracted, or pulled toward one another. At a cellular level, this is what happens when a muscle contracts.

Integration

This helps us understand how muscles and muscle tissue are not separate from the rest of the body. While we may talk about “individual” muscles for the purposes of breaking down our understanding of the body into manageable chunks, movement happens with the whole body and is affected by the whole body. A single muscle is inseparable from the fascia that is interwoven within its layers. A single muscle, because of its connecting fascia, is also inseparable from the “individual” muscles that surround it. We should keep this in mind as we deepen our understanding of muscles and movement.