what muscles do

Sometimes I am amazed by what yoga teachers learn in terms of “anatomy”. Like so many of us know something about the psoas muscle, or we throw around body parts like the sacro-iliac joint, piriformis, rotator cuff etc. without knowing things like how muscles work and what muscles do in the body. I am as guilty as anyone, although, finally I am learning.

When we think of muscle, we are generally referring to skeletal muscles (the other 2 types are smooth muscle and cardiac muscle, which move involuntarily in the heart or other organs). This is a general discussion of how skeletal muscles work, and a vocabulary to help you understand and feel their workings. There are people who are more expert on the details and exact science of each muscle group, and we might go deeper in depth with this material later. This discussion is intended to give you a functional and overall view of how you think about movement and muscle.

When I ask yoga teachers what muscles do the usual answer is 3 things: shorten, relax and stretch. So when we think of getting stronger, we think of doing more shortening exercises, when we think of getting more flexible we think of doing more stretching. But it doesn’t completely work that way.

Let’s start by using slightly different words.

  1. Contraction. The fibers of the muscle cross each other, which is called contraction. This happens in 3 ways.
    1. Concentric contraction. Concentric means towards the centre. So this kind of contraction is what what we are used to thinking of when we think tighten or shorten: the two ends of the muscle (origin and insertion) come closer together. You can see it happening in your upper arm if you pick up a glass of water and bring it to your mouth. The Biceps muscle goes from a long position to a shorter position. It would happen without the glass of water if you just bend your arm, but it is easier to see with a little resistance (weight). Also, during concentric contraction the tension in the muscle rises to meet the resistance.
    2. Isometric contraction. Isometric means same length. So if you pick up the glass of water halfway and hold it there, your muscle is in isometric contraction. Isometric contractions happen a lot in yoga practice, when you hold a pose for any length of time, the muscles are working isometrically.
    3. Eccentric contraction. This is the one we often miss out and don’t think of. Eccentric means away from the centre, and so in fact is the muscle having tension while lengthening. This is what happens when you put your glass of water back on the table. You need to control the descent of the glass and the speed at which it hits the table, but the biceps muscle is moving from a short position to a longer position. This is not the same as relaxing the muscle, as if the muscle spontaneously relaxed, you would drop the glass of water or it would slam down into the table.
  2. Relaxation. Muscle tissues can relax or do nothing voluntarily. However, this happens a lot less often than we think, because the body is constantly at play with physical forces, most especially gravity, and ground reaction force. It is not possible (and definitely not helpful) to try to think of relaxing a muscle that is doing a job, because like in the example above, if you tell a muscle that is holding a glass of water to “relax” the glass would just drop.

    Similarly, often we tell a muscle to relax that is already doing a job – holding the body or posture in place. It isn’t possible to do both at the same time. This is the general theory behind restorative yoga, or supporting the body in relaxation: that when you support the structure of the body, the muscles no longer have to do the job of holding the bones in place (because the props and supports are doing that job) and so the muscle tissue can really relax.

  3. Passive muscle lengthening. This is what we generally call stretching, and what happens when we do not ask a muscle to hold any tension while doing a movement. If you put your arm against a wall and turned the chest away from it, the bicep muscle is being passively pulled and lengthened giving the sensation of stretching. The fibres of the muscle are being pulled away from each other.

Every muscle group in the body works in concert with other muscle groups, in a relationship often referred to as the agonist/antagonist relationship. What this means is that when one muscle group is doing a concentric contraction, the opposing group is doing an eccentric contraction in response to the forces at play. in our glass of water analogy, when you pick the glass up, the bicep muscles are in concentric contraction, while the triceps are controlling the movement eccentricly, and vice verse when you put the glass on the table.

I hope that is helpful and clear.

You can read some similar content from the University of California at San Diego here.

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