Skeletal Muscle Fiber Types

Another important finding of Janda's is the fact that the behavior of weakened and shortened (contracted) muscle groups is not subject to coincidence but to certain laws.

Microscopic and electrophysiological investigations have demonstrated the existence of two different types of cross-striped muscle fibers from a functional perspective: red and white. Both types of muscle fibers are found in all muscles, but in varying quantities. The behavior of muscles is influenced by the amount of muscle fibers of a particular type. First, we describe the characteristics of both types of muscle fibers.

Postural Muscle Fibers (Red Fibers): Type I Fibers (Slow-twitch Fibers)

• High content of myoglobin (red color)

• High number of mitochondria

• High amount of neutral fat

• Oxidative metabolism predominates

• Low glycogenolytic and glycolytic activity

• High mitochondrial enzyme activity

• Slow contraction speed

• Suited for endurance and support functions

• Tendency to shorten

• Treatment: stretching

Phasic Muscle Fibers (White Fibers): ltype II Fibers (Fast-twitch Fibers)

• Strongly developed sarcoplasmic reticulum

• Contain less mitochondria, lipids, and glycogen

• High myosin and actomyosin ATPase activity

• Anaerobic metabolism dominates

• High glycogen consumption

• Serves for quick short efforts

• Additional strength is caused by increased impulse frequency

• Tendency to weaken

• Treatment: strengthening

Muscles that contain primarily red muscle fibers tend towards hyperactivity, tension, shortening, and hyper-tonicity. Muscles that contain more white fibers tend to weaken and slacken instead.

There are controversial names for the two muscle types. We use Janda's terminology, calling muscles that contain primarily red fibers postural muscles and those that contain primarily white fibers phasic muscles (Fig. 6.1).

In his research, Janda was able to find that in most people, certain muscles always have a tendency to shorten and others have a tendency to weaken:

Muscles with a Tendency to Shorten

• Short extensors of the head joints

• Levator scapula

• Middle and upper section of the trapezius

• Lumbar section of the erector spinae

• Quadratus lumborum

• Muscles of mastication

• Sternocleidomastoideus (SCM)

• Scalene muscles

• Subscapularis

• Pectoralis major and minor

• Oblique abdominal muscles

• Hamstrings

• Rectus femoris

• Tensor fasciae latae (TFL)

• Short hip adductors

• Triceps surae

• Flexors of the upper extremity

Muscles with a Tendency to Weaken

• Lower section of the trapezius

• Serratus anterior

• Gluteal muscles

• Rectus abdominis

• Deep neck benders

• Muscles at the floor of the mouth

• Vastus muscles

• Anterior tibial

• Toe extensors

• Peroneus muscles

• Extensors of the upper extremity

The function of muscle fibers, whether postural or phasic, appears not to be genetically conditioned but depends on the activity that the muscle has to execute. Chris Norris (in 41), a British physiotherapist, writes that appropriate training conditions the amount of phasic or postural muscle fibers. Lin et al. (in 41) were able to demonstrate that the postural or phasic property of a muscle depends on its innervation (or on the impulses that it receives). They verified this by transplanting the nerves of a phasic muscle into a postural muscle. Most likely, this also explains why we find different muscle properties in the case of malpositions (such as due to leg length differences) or ex

Sternocleido mastoid

Pectoralis major

Obliquus -abdominis

Flexors

Rectus -femoris

Tensor -

fascia lata

Sternocleido mastoid

Pectoralis major

Obliquus -abdominis

Flexors

Rectus -femoris

Tensor -

fascia lata

- Deltoid

- Adductor longus

- Quadratus lumborum

-Iliopsoas

- Deltoid

- Adductor longus

Levator -scapulae

Erector Spinae

- Quadratus lumborum

-Iliopsoas

Piriformis ■

Adductor — magnus

Semimem- -branosus

Soleus -

Levator -scapulae

Erector Spinae

Piriformis ■

Adductor — magnus

Semimem- -branosus

Trapezius Muscle Descending Fibers

Descending part of the trapezius

Latissimus dorsi

Quadratus

— Biceps femoris

- Semitendinosus

Gastrocnemius

Posterior tibial

Descending part of the trapezius

Latissimus dorsi

Quadratus

— Biceps femoris

- Semitendinosus

Gastrocnemius

Posterior tibial

Flg. 6.1a, b Postural and phasic muscles according to Janda.

cessive strain of certain muscle groups (such as in monotonous motion patterns during work).

For some muscles, classification into postural or phasic muscles is questionable. This applies to the scalene muscles, the oblique abdominal muscles, the gluteus muscles, and the deep neck muscles, as well as the peroneus muscles.

It is also remarkable that postural muscles are also found in the concavities of the spine and the extremities. Thus, from cranial to caudal:

• Neck stretchers

• Pectoralis major and minor

• Lumbar erector spinae

• Iliopsoas for the hip

• Hamstrings for the knee

• Peroneus muscle for the foot

• Flexors of the upper extremity

Janda's explanation for the formation of motion patterns is conditioned by evolution. It refers primarily to muscles with a stabilizing function in the gait.

For Waddell (in ""), postural muscles are those with a stabilizing function, that is, static muscles. These are muscles that are able to tense continuously. Phasic muscles, on the other hand, are dynamic, responsible for movements. Postural and phasic muscles are antagonists in Waddell's view (see above).

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