Our body comprises three main types of muscle tissues, including the cardiac muscle tissue, the skeletal muscle, and smooth muscle tissue, often found in the walls of internal organs or places requiring involuntary movement. The practice of yoga concerns the skeletal muscles.
In anatomical language, when someone refers to the skeletal muscles contracting, it implies the muscle is 'fired up' or activated. It's important to understand that this doesn't necessarily mean the entire muscle shortens.
Skeletal muscles could contract to resist movement, create movement, or stabilize the bones. These muscles can alter their length and tone to accomplish these functions. Let's dive deeper to understand the anatomical terms that better describe these functions.
Isometric Contraction
The tone of the muscles changes anatomically in this case, but the length remains unchanged. This type of contraction is functionally used to stabilize things in the body. Think of a high plank during yoga. The pose requires muscular effort, but no movement is involved. Muscles in our body undergo isometric contractions for the entire team even when we think we aren't doing anything.
Isotonic Concentric Contraction
The muscle length essentially changes in isotonic concentration contractions. Specifically, the muscle's overall length becomes shorter during such contractions. This kind of muscle contraction creates movement by moving bones at joints. Think of your arms reaching up at the beginning of sun salutations. Muscles have to shorten and contract to move the humerus at the shoulder joint.
Isotonic Eccentric Contraction
The length of the muscle also changes during an isotonic eccentric contraction. However, the overall muscle length gets elongated. Resisting movement is the prime function of this kind of contraction. Think of the hamstrings resisting the gravity pull during slow forward folds. You might move into the forward fold rapidly without isotonic eccentric contraction.
Unpacking Eccentric and Concentric Contraction
Muscles typically activate whenever they have a resistance to work against, the base level of which is gravity—the thing that causes the body and its part to weigh something. When you stand upright with the arms out to your side, the arm, in anatomical language, is in the abducted position at the shoulder's joint.
Now, if you lower the arm back to your side, the action would be called adduction. It's easy to assume that the muscles adducting the shoulder joint are helping the arms return to the side from the abducted position. But that's not true.
No muscle needs to be contracted to return the arm to the side. The muscles need to be relaxed as gravity brings the arm down back relatively quickly. The isotonic eccentric contraction would thus help benefit from the muscle lengthening and contracting simultaneously.
Surprisingly, the abductor muscle on the shoulder joint controls adduction in the above example. Even though the motion isn't created by any muscle or the adductor, in this case, the movement of letting the arms fall back to the sides from the abducted position would still be called adduction.
Understanding this is important because our position impacts how gravity affects our body parts.
Since muscles are the dynamic structures that prevent or create movement in our body, understanding these basics is especially helpful for puzzling through the experiences during yoga. Here's how the concepts apply to yoga. Let's try to gain a better understanding by using the triangle pose.
Moving to the Triangle Pose (Trikonasana)
Holding the Pose
Once you're in the pose, you're not moving. The muscles contract to help the body stabilize and remain in the posture. The hamstrings no longer change in length at this point, but the tension remains due to isometric contraction.
Moving Out of The Pose
When you're ready to release out of the pose, the hamstring would again start changing the length to get shorter to help you come up. As they get shorter and contract, the pelvis gets pulled back to the original position, anatomically known as isotonic concentric contraction. 
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