Everything you never wanted to know about muscles, how they contract and how they're supposed to lengthen and their effect posture.

Once you have a basic understanding of these aspects of your own physiology and pain symptoms you will better appreciate the work we do- combining orthopedic acupuncture and (ART) Active Release Technique. 

So be sure to view those information pages if you have not already.

The Length-Tension Relationship


Actin and myosin filaments are found within the sarcomere (a contractile unit of skeletal muscle). The myosin cross bridges attach to the actin filaments, pulling them inward and leading to an overall shortening of the muscle fiber. When a bunch of fibers do this at once, we get a concentric muscle action (contraction or shortening).

With the sliding filament theory in mind, you can imagine that changes in the length of a muscle fiber can affect the ability of the muscle to contract optimally. For example, when a sarcomere is too short, it can't generate peak force because of the preexisting overlap of actin filaments. This overlap takes up valuable space that could otherwise be used for the myosin cross bridges to attach. Conversely, when the sarcomere is excessively lengthened, the actin filaments are too spread out for all of the myosin cross bridges to reach them for attachment.

So, we know that a muscle fiber (and, in turn, the entire muscle) is strongest when the sarcomeres are at their ideal resting length (usually resting position or slightly more lengthened). In all other positions, the sarcomere is outside of this ideal length zone and can't generate maximal force. Just consider how your strength varies in certain positions of any activity it is that you perform.

Posture and the Length-Tension Relationship


The length-tension relationship isn't only important at the cellular level; training — or lack thereof — can alter a muscle's normal resting length. Simply put, the more you train a muscle, the shorter it wants to get.
Meanwhile, the response of the antagonist is to lengthen more and more over time to allow the agonist to shorten. If you need a visual, wrap an elastic band around your wrist. Pull on one side to loosen it (the antagonist) and note that the other side tightens (the agonist). This is how concentric muscle actions normally occur; the antagonist must relax to permit the agonist to shorten.
The problem herein lies when the agonists become chronically shortened due to poor training and/or lifestyle behaviors. Summarily, we get shortened (hypertonic or overactive) muscles and lengthened (hypotonic or inhibited) muscles opposing each another. Now, toss the length-tension considerations into the mix; do you think muscles (and their individual fibers) that are always outside of the optimal length zone will be able to generate maximal force? Is the Pope Hindu?
When discussing length and tension, you must also be aware that they're not one and the same. A muscle can have excellent length but still be excessively tight and vice versa (although it’s not as common). It's generally accepted that with length, more is better unless you have the flexibility of a circus sideshow freak. Muscle length is usually improved via stretching (static, dynamic, PNF, etc.)
On the flip side, tension is more of a bell-shaped curve. On one hand, excessive tension is problematic as stated above, but excessive laxity isn’t beneficial either. Tension is a true tight rope and something that should be evaluated frequently. Tension is best improved using modalities like acupuncture, massage, active release techniques, heat, muscle stimulation (e-stim), or myofascial release.

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Potential kyphosis/rounded shoulders manifestations:

bicipital tendonitis, injuries to the glenoid labrum, subacromial impingement and resulting rotator cuff tears, injuries to teres major, scapular winging, decreased thoracic outlet space, degeneration of vertebral facets/acromioclavicular joints/sternoclavicular joints, and various elbow pathologies (due to compensatory overload).

Potential head forward posture manifestations:

headaches, breathing trouble, difficulty swallowing, anterior and posterior neck tightness, and irritation along the medial scapular border.

Potential lower body manifestations:
low back pain, disc injuries, sciatica/radiating pain from the low back into the legs/feet, decreased low body power and strength production, lateral knee pain, medial collateral ligament tears/sprains, anterior cruciate ligament tears/sprains, excessive pronation of the foot (flat feet), ankle sprains, hamstring/lower back strains, sacroiliac joint dysfunction, piriformis syndrome, pain in the forefoot (metatarsalgia), bunions, and plantar fasciitis, reproductive and bowel conditions.

Each persons posture is the result of balanced or imbalanced muscles making up the traditional "S" (spinal curve) from head to toe.
This is determined by one's activities, lifestyle (work- habits) and diet. The posture determines how hard muscles must work to create motion and balance while influencing the function of one's internal viscera in position, weight distribution, nerve flow and vascular function-thats a conversation for another article.