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Trigger Point Therapy - Postural and Phasic Muscles

Posted by Jane Ziegler on


Postural Muscles and Trigger Points - Dr. Jonathan Kuttner M.D.


Postural and Phasic Muscles

It's often accepted that muscles that have a stabilizing function (postural) have a tendency to shorten when stressed, and other muscles that play a more active/moving role (phasic) have a tendency to lengthen and become inhibited.

The muscles that tend to shorten have a primary postural role. However, there are some exceptions to the rule that certain muscles follow the pattern of becoming shortened while others become lengthened – some muscles are capable of modifying their structure.

For example, certain authors suggest that the scalenes are postural in nature and some suggest that they are phasic.

We know that, depending on what dysfunction is present within the muscle framework, on specific testing one can find the scalenes to be held in a shortened position and tight, but sometimes when they are tested they can be found to be lengthened and weakened.

There is a distinction between postural and phasic muscles; however, many muscles can display characteristics of both and contain a mixture of Type I and Type II fibres.

The hamstring muscles, for example, have a postural stabilizing function, yet are polyarticular (cross more than one joint) and are notoriously prone to shortening.


  Postural Phasic
 Function   Posture   Movement
 Muscle Type   Type I   Type II
 Fatigue   Late   Early
 Reaction   Shortening   Lengthening


Lengthening and shortening of muscles


Postural Muscles

Also known as tonic muscles, these muscles have an antigravity role and are therefore heavily involved in the maintenance of posture.

Slow-twitch fibres are more suited to maintaining posture; they are capable of sustained contraction and generally become shortened and subsequently tight.

Postural muscles are slow-twitch dominant, due to their resistance to fatigue, and are innervated by a smaller motor neuron.

They therefore have a lower excitability threshold, which means the nerve impulse will reach the postural muscle before the phasic muscle.

With this sequence of innervation, the postural muscle will inhibit the phasic (antagonist) muscle, thus reducing its contractile potential and activation.

When your muscles are placed under faulty or repetitive loading, the postural muscles will shorten and the phasic muscles will weaken.

This consequently alters their length-tension relationship, which will directly affect posture, as the surrounding muscles displace the soft tissues and the skeletal system.


Postural-phasic muscles (anterior view). Purple muscles predominantly postural, and green muscles predominantly phasic.



Postural-phasic muscles (posterior view). Purple muscles predominantly postural, and green muscles predominantly phasic.


Phasic Muscles

Movement is the main function of phasic muscles. These muscles are often relatively more superficial than postural muscles and tend to span several joints (polyarticular).
They are composed of predominantly fast-twitch Type II fibres and are under voluntary reflex control.

A tight muscle often results in inhibition of the phasic muscle, whose function becomes weakened as a result.

The relationship between a tightness-prone muscle and its weakness-prone muscle is one way.

As the tightness-prone muscle becomes tighter and subsequently stronger, this causes an inhibition of the weakness-prone muscle, resulting in its lengthening and consequent weakening.



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