Postural and phasic muscles (Chek, 2005)
Wrist & Finger Flexors
Deep Cervical Flexors
Wrist & Finger Extensors
Cervical Erector Spinae
Tensor Fasciae Latae
Vladamir Janda discovered that muscles can be categorized as either postural, phasic, or a mixture of the two.
Postural muscles act predominantly to sustain your posture in the gravity field. These muscles contain mostly slow-twitch muscle fibres and have a greater capacity for sustained work. They are prone to hyperactivity.
Phasic muscles contain mostly fast-twitch muscle fibres, and are therefore more suited to movement. They are prone to inhibition. They are also more easily fatiguable.
Postural muscles tend to shorten in response to over-use, under-use or trauma, whereas phasic muscles tend to lengthen and weaken in response to these types of stimuli. These effects can lead to musculo-skeletal imbalance and joint instability when postural and phasic muscles are located on opposing sides of the agonist-antagonist relationship.
These relationships are the key to understanding common patterns of postural imbalance such as the upper-crossed and lower-crossed syndromes.