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Pennate muscles

Synonym: pinnate muscle, penniform muscle
German: Fiederung

1 Definition

The characteristic of the pennate muscle is the alignment of the muscular fascicles in relation to its tendon. The fibres are attached obliquely, which allows a higher force at the expense of a smaller range of motion.

2 Classification

The pennate muscles can be classified by its alignment of fibres into the following

  • unipennate muscle (Musculus unipennatus)
  • bipennate muscle (Musculus bipennatus)
  • multipennate muscle (Musculus multipennatus)

Muscles which do not possess any penniform aligned fibres are called muscles with parallel fibres.

3 Beispiele

4 Physiology

The force of a muscle Fm is calculated by its physiological cross-sectional area and amounts to approximately 40 N/cm². In relation to muscles with parallel fibres, the force which is transmitted to the tendon Ft is directly proportional to the anatomical cross-sectional area. Ft = Fm

With regard to pennate muscles, the force which is transmitted to the tendon is modified by the factor of the cosine of the pennation-angle: Ft = Fm x cos α

Through the pennation of the muscular fibres, a greater physiological cross-sectional area gets created. The force of the muscle amounts to approximately 40 N/cm². If the anatomical cross-sectional area for example gets tripled, the force of the muscle equally gets tripled. Through the pennation of the muscles the force of the muscle generally increases, likewise does the force which is transmitted to the tendon Ft .

4.1 Calculation example

Let's assume a physiological cross-sectional area of the muscle gets tripled by the pennation of the muscle fibres. The anatomical cross-sectional area amounts to 10 cm². Through pennation of the fibres, the physiological cross-sectional area now amounts to 30 cm². With an angle of pennation of 45°, the force transmitted to the tendon would only amount to 70% of the force of a parallel-fibred muscle of the same length, because cos(45°) = 0.7. However, through the tripling of the physiological cross-sectional area the muscular force gets tripled as well.

Ft = 3 x Fm x 0.7

so

Ft = (10 cm3 x 3) x 40 N x 0.7

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