How do muscles work?
We looked at the anatomy of a skeletal muscle in our article, What Is A Muscle? As a reminder, in addition to skeletal muscles which are the kind of muscles that move bones, we have two other kinds of muscle tissue: cardiac muscle and smooth muscle. Cardiac muscle tissue is found in the heart. Smooth muscle tissue is found in places where involuntary action is needed, for example, in our arteries and visceral organs. When we delve further into types of muscular contractions in this post, we’ll be referring to the types of contractions that are primarily relevant to skeletal muscles. So, we have an idea of what a skeletal muscle is, but how does it do its work?
The action of a muscle is called a contraction. On a cellular level, when muscle activity is needed, the nervous system sends a message to a muscle cell or cells and tells them to act, to contract. Not all of the cells in a single muscle contract at the same time to create the action called for by the nervous system. Only the number of muscle cells actually needed for the action will contract at one time.
Types of muscle contractions
Our muscles can do different types of contractions depending on the kind of muscle activity that the body needs. Muscles can move bones to create movement, of course, but they can also act to resist movement, or stabilize an area of the body. These different kinds of activity require different kinds of muscle contractions.
The word ‘tonic’ means tone. Tonic contractions then are simply the contractions that maintain the resting tone of our muscles.
The word ‘isometric’ means same length. Isometric contractions occur when the tension or tone of the muscle changes, but the length of the muscle stays the same. For example, if we pick up a rock and carry it around in our hand out in front of us, several muscles are doing an isometric contraction to resist gravity and keep the rock steady in our hand.
Isotonic contractions — concentric and eccentric contractions
The word ‘isotonic’ means same tone. Isotonic contractions occur when the tension of the muscle stays the same, but the length of the muscle changes. There are two types of isotonic contractions. In an isotonic concentric contraction the muscle gets shorter while contracting. This is the idea of muscle contraction that we are usually most familiar with. For example, my biceps brachii will do an isotonic concentric contraction when I lift a glass of water from the table and bring it toward my mouth.
There is another type of isotonic contraction, however. We also experience isotonic eccentric contractions. In this type of contraction the muscle is getting longer while the tension remains the same. For example, if I bend over to pick something up from the floor, my hamstrings will do an isotonic eccentric contraction. They will get longer as they resist gravity to slow my momentum and allow me to reach to the floor with some control.
Have a play with identifying muscle contractions in your own body. As you’re moving about your day, think about which muscle actions are required to do the tasks you’re doing. Which muscles are doing isotonic concentric contractions? Which muscles are doing isotonic eccentric contractions? Are there also muscles doing isometric contractions in that same action?