The arrangement of muscle fibers varies according to the direction, force and range of habitual movement at a particular joint. The force of movement is directly proportional to the number and size of muscle fibers, and the range of movement is proportional to the length of fibers. The muscles can be classified according to the arrangement of their fasciculi into the following groups.
Muscles with Parallel Fasciculi:
These are muscles in which the fasciculi are parallel to the line of pull. These muscles may be:
- Quadrilateral, for example thyrohyoid,
- Strap-like, for example sternohyoid and sartorius.
- Strap-like with tendinous intersections, for example rectus abdominis.
- Fusiform, for example biceps brachii, digastric.
The range of movement in such muscles is maximum.
Muscles with Oblique Fasciculi:
When the fasciculi arc oblique to the line of pull, the muscle may be triangular, or pennate (feathcr-like) in the construction. This arrangement makes the muscle more powerful, although the range of movement is reduced. Oblique arrangements are of the following types:
- Triangular, e.g. temporalis, adductor longus.
- Unipennate, e.g. flexor pollicis longus, extensor digitorum longus, pcroncus tcrtius, palmar intcrossci.
- Bipennate, e.g. rectus femoris, dorsal interossei, pcroncus longus, flexor hallucis longus.
- Multipennate, e.g. subscapularis, deltoid (acromial fibers].
- Circumpennate, e.g. tibialis anterior.
Muscles with Spiral or Twisted Fasciculi:
Spiral or twisted fibers are found in trapezius, pectoralis major, latissimus dorsi, supinator, etc. In certain muscles the fasciculi are crossed. These are called cruciate muscles, e.g. sternocleidomastoid, masseter and adductor magnus.