Postural and Phasic Muscles
It's often accepted that muscles that have a stabilising 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 stabilising function, yet are polyarticular (cross more than one joint) and are notoriously prone to shortening.
|Muscle Type||Type I||Type II|
Lengthening and shortening of 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 MusclesMovement 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.
About the Author
John Gibbons is a qualified and registered osteopath with the General Osteopathic Council, specialising in the assessment, treatment, and rehabilitation of sport-related injuries. Having lectured in the field of sports medicine and physical therapy for over 12 years, John delivers advanced therapy training to qualified professionals within a variety of sports. He has also published numerous articles on various aspects of manual therapy.
This trigger point therapy blog is intended to be used for information purposes only and is not intended to be used for medical diagnosis or treatment or to substitute for a medical diagnosis and/or treatment rendered or prescribed by a physician or competent healthcare professional. This information is designed as educational material, but should not be taken as a recommendation for treatment of any particular person or patient. Always consult your physician if you think you need treatment or if you feel unwell.
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