Postural and phasic muscles (Chek, 2005)

Postural Muscles		Phasic Muscles
Shoulder Girdle - Arm
Pectoral Muscles Levator Scapulae Trapezius (upper) Biceps Brachii Scalenes Subscapularis Sternocleidomastoid Suboccipitals Masseter Temporalis Wrist & Finger Flexors	Trapezius (middle) Trapezius (lower) Serratus Anterior Triceps Brachii Deep Cervical Flexors Supraspinatus Infraspinatus Deltoid Wrist & Finger Extensors
Trunk
Lumbar Erector Spinae Cervical Erector Spinae Quadratus Lumborum		Thoracic Erector Spinae Rectus Abdominis Transversus Abdominus
Pelvis - Thigh
Hamstrings Iliopsoas Rectus Femoris Adductors Piriformis Tensor Fasciae Latae		Vastus Lateralis Vastus Medialis Gluteal Muscles
Lower Leg - Foot
Gastrocnemius Soleus		Tibialis Anterior Peroneals Toe Extensors

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.