There’s often a lot of confusion around what muscles actually do. I frequently hear students at workshops speaking about muscle action in a way that suggests the only type of muscle action they’re aware of is the kind where muscles shorten to create movement (a concentric contraction). While that’s not false, it is an oversimplification. And, understanding the different ways in which muscles can function is important for truly wrapping our heads around what constitutes muscle contraction in our yoga practice. So in this article, we’ll unpack the concept of muscle contractions.
Types of muscle tissue
First, let’s just be clear that when we say “muscle”, what we specifically mean is skeletal muscle. Remember that we actually have three kinds of muscle tissue in the body. There is cardiac muscle tissue, which is only found in the heart. We also have smooth muscle tissue, which is found in the walls of internal organs like the intestines, and in other places like our eyes, where involuntary movement is needed. But in yoga, when we’re talking about how muscles work, what we’re talking about is skeletal muscle.
How skeletal muscles contract – the basics
Let’s start by defining the term “contract”. In anatomical language, when we’re talking about skeletal muscle contraction, the word contract just means the muscle is activated, sometimes referred to as “firing”. It does not necessarily mean that the entirety of the muscle shortens! This is important to understand right from the beginning.
Keep in mind that when we think of a muscle, we tend to think of the entire muscle. But, that muscle is actually made up of thousands of cells. When the nervous system signal is sent to a muscle, it is only a portion of the cells that receives the signal. That signal then causes the proteins (actin and myosin) inside of the cells to become attracted to one another. Essentially, on a cellular level, there is a shortening. But, that doesn’t necessarily mean, in the larger picture, that the overall muscle itself is shortening. Instead, contraction means that there is an increase in tension in the muscle. As we’ll see, shortening, lengthening, or staying the same length are all possibilities in a muscular contraction at the macro level.
Skeletal muscle functions
Skeletal muscles can contract (activate or fire) to:
- Create movement (bones moving)
- Resist movement
- Stabilize bones in place
In order to do these functions, skeletal muscles can change two things:
1) Their tone – Change in tone means there is a change in tension
2) Their length – This includes lengthening as well as shortening at the macro level
There are anatomical terms to describe each of these situations.
In this case, what changes anatomically, is the tone of the muscle. The length doesn’t change! Functionally this type of contraction is used in the body to stabilize things. In yoga we can think of a high plank. It obviously takes muscular effort to be in this posture. But, we’re not doing a movement. Muscles in our body are doing isometric contractions all the time even if we don’t think we’re doing anything.
Isotonic concentric contraction
In an isotonic concentric contraction, the muscle length changes. More specifically, the overall length of the muscle gets shorter during a concentric contraction. Functionally this kind of muscle contraction moves our bones at joints to create movement. In yoga, think of lifting your arms to reach up at the beginning of a sun salutation. In order to move the humerus at the shoulder joint, muscles have to contract and shorten.
Isotonic eccentric contraction
In this type of muscle contraction the muscle length also changes. But, the overall length of the muscle gets longer during an eccentric contraction. Functionally our muscles do this kind of contraction to resist movement. In yoga, think of your hamstrings resisting the pull of gravity as you slowly fold forward. Without this type of contraction, you would move into your forward fold rather rapidly.
Unpacking concentric contraction and eccentric contraction
Muscles are typically activated when they have a resistance to work against. The base level of resistance that we are working against is gravity, which is what causes our body and various body parts to weigh something. Now this may seem obvious, but it’s often forgotten when we have been on the path of learning that a muscle creates a certain action.
A simple example
Let’s start with an example of how you may be tricked into thinking that a muscle creates an action. Let’s say I’m standing upright and I have my arm out to the side. In anatomical language, my arm is in an abducted position at the shoulder joint. The movement itself to lift it to that position is called aBduction of the shoulder joint.
So, my arm is in an aBducted position and now let’s say I want to lower my arm back down to my side. This action is called aDduction. You might easily assume that since it is a movement, there must be a muscle that makes it happen. Most likely, you would assume that a muscle which does adduction of the shoulder joint would cause my arm to return to my side from an abducted position. But, both of these assumptions would be wrong.
Working with gravity
In order for my arm to return to my side, I would not have to contract any muscle. In fact, I would let a muscle relax and GRAVITY would bring my arm, relatively quickly, back to my side. If however, I wanted to slowly lower my arm to my side, I would take advantage of a muscle contracting and lengthening at the same time. This is an example of an isotonic eccentric contraction.
Surprisingly, it is the abductor of the shoulder joint that controls the adduction we’re talking about in this example. Think about that for a moment. The aBductor, in this case the deltoid muscle, contracts and lengthens at the same time. The action of the deltoids slows the momentum of allowing our arm to fall back to our side by resisting the pull of gravity. Although the motion is not created by a muscle which we would label as an adductor, such as latissimus dorsi, we would still call the movement of letting our arm fall back to our side from an aBducted position, aDduction. This is important to understand. The position we are in changes how gravity affects our body parts, and therefore affects which muscles need to activate to do an eccentric contraction or even a concentric contraction.
Why do we need to know about how muscles work?
The most common question that I get from students is a version of: how do I do X pose. Muscles are the dynamic structures that create or prevent movement in our body. So, the answer to that question always involves muscles. The better we understand not just which muscles create a particular movement, but also which muscles might prevent a movement, the more information we’ll have for puzzling through our experience in yoga practice. So let’s take these concepts into an example from yoga practice.
Let’s do a triangle pose
When we do any yoga posture, there are of course many different muscles engaging at the same time. But, for simplicity’s sake, let’s follow a single muscle within a single posture to further understand the different types of muscle contractions that we find in yoga. As we walk through this example, keep in mind that isotonic contractions imply that movement is happening. Therefore, as we move into and out of a yoga posture, there will be multiple muscles doing isotonic eccentric and isotonic concentric contractions. When we are static, in the ‘state of the asana’, we are more likely to experience isometric contractions of multiple muscles.
Moving into the posture
Imagine doing trikonasana (triangle) on the right side, meaning that our right leg is out in front. And then, let’s focus in on a muscle that you are most likely familiar with, the hamstrings. As we extend our body and begin to lower our right hand to the floor near the right foot, we are technically doing a mixture of abduction and flexion of the right hip joint which is externally rotated in this posture.
During the movement of lowering down, as you have probably experienced yourself, the hamstrings lengthen. The word lengthen here implies that the two ends of the muscle, where they attach to bones, are getting further away from one another. As we lower down, gravity and body position will take us to the floor rather quickly unless we choose to control the speed. In order to control the speed, the hamstrings do an isotonic eccentric contraction. You’ll recall, that means that the muscle is getting longer while also maintaining tension or contracting.
Holding the posture
Once we are in the posture we are not really moving. Instead, muscles are contracting to help stabilize our body and maintain the posture itself. At this point the hamstrings are no longer changing in length, but there is still tension in them. Tension, or contraction without movement, you’ll recall is an isometric contraction.
Moving out of the posture
When we are ready to come back up out of the posture, our hamstring length will start to change. If the hamstrings lengthened on the way into the posture then they must be getting shorter when we start to come up. As they contract and get shorter, they pull on the pelvis to bring it back to its original position. It’s in this situation where the hamstrings are getting shorter and contracting simultaneously, that we find the isotonic concentric contraction.