In this rather lengthy post, I’d like to go over muscle contractions and their types; isotonic (concentric and eccentric) and isometric. However, before I delve into each contraction specifically, I’m going to start off by providing a very watered-down version of an anatomy and physiology lesson in regards to our muscular system from a cellular level. For the record, I am not an A&P professional… Just thought I would use this opportunity to brush-up on some facts that I learned back in ’06. In fact, I probably learned this sometime between ’90 and ’93, but who remembers such detail from almost 20-years ago and while in high school. Hahaha! 🙂
Skeletal muscle tissue—which produce body movements, stabilize body positions, store and move substances within the body, and generate heat—is made up of hundreds to thousands of cells, also called muscle fibers. Muscle fibers consist of sarcomeres, which are made up of thick filaments, thin filaments, and Z discs; among other things. In order for muscle fibers to contract, certain proteins must be present; two of those being actin and myosin. Check out this link for visual aids and here is where you can read more about this in detail.
Okay, enough A&P for now! I’m a layman so onward with layman terms regarding the functions of skeletal muscles, that being muscle contractions. While the muscle is under tension, it may shorten, lengthen, or remain the same…
Muscle contractions are classified as being either isotonic or isometric. An isotonic contraction (iso- = equal; -tonic = tension) happens when tension developed by the muscle remains almost constant while the muscle changes its length. These contractions are used for body movements and for moving objects. The two types are known as concentric and eccentric.
Concentric isotonic contractions happen when two ends of a muscle are brought closer together during the contraction. The muscle shortens and pulls on another structure, such as a tendon, to produce movement and to reduce the angle at the joint. An example of a concentric isotonic contraction is when one does a bicep curl. As the dumbbell is lifted up, visualize how the two ends of the bicep move closer together during the contraction. This is the simplest form of contraction and is usually the least problematic.
Now imagine one lowering the dumbbell after the bicep curl. It is with this movement that the previously shortened bicep now lengthens during the contraction, which is known as an eccentric isotonic contraction. The bicep in this example contracts so it can control the speed at which the dumbbell is lowered. Per Tortora & Derrickson, 11th edition, repeated eccentric isotonic contractions produce more muscle damage and more DOMS (delayed-onset muscle soreness) than concentric isotonic contractions.
Isometric contractions happen when a muscle contracts but the ends do not move at all, and even though they do not result in body movement, energy is still expended. Isometric contractions are vital because they stabilize some joints as others are being moved. They maintain posture and support objects in fixed positions. The perfect example would be me sitting at the computer typing this right now. I’m sitting erect trying to use good posture and my mid-back is starting to hurt… Ugh! My lower trapezius is sustaining a contraction—to hold me upright—and it is becoming too much. At no time have the ends of the muscle come closer together. Make sense? I bet all of you have experienced this at one time or another.
There is one more category that should be mentioned, that of approximation; the length-tension relationship. If two ends of a muscle are brought closer together, over time, the muscle will adjust its length to keep the same tension. While not a true contraction, it acts like one. This is the concept of Davis’ Law—When two ends of a muscle are brought together, the pull of tonus is increased. When two ends of a muscle are separated, tonus is lessened or lost, thereby weakening the muscle.
The first part of Davis’ Law has to do with adaptive-shortening. Unless something acts upon the shortened muscle, it will remain in the shortened position and adapt that position as the new “normal”. It will then resist attempts to return to the old “normal” as it has forgotten what that position was.
The second part of Davis’ Law has to do with stretch-weakeness. Muscles are affected by either duration or magnitude. Stretch-weakness is definitely a factor of duration, not magnitude. Having a shoulder much lower than the other for a short period of time is not as powerful as having a small difference for a long period of time.
It is through this length-tension relationship that PNMT comes in handy. These discrepancies within the muscles can cause pain and dysfunction. That being said, proper measurement from a Certified Precision Neuromuscular Therapist can help verify whether your pain and/or dysfunction is caused by misalignment within the muscular-skeletal system.
I hope this information has been informative! 🙂
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“Neutral balance alignment is key to becoming pain-free” ~ Me
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