This document describes the different types of muscle fibers and their arrangements within muscles. It discusses three main types of muscle fibers: type I slow fibers that are resistant to fatigue; type II fast fibers that fatigue more easily; and intermediate fibers that are less fatigue-resistant than type I but more than type II. It also outlines different fascicular architectures that muscles can have depending on their function, including parallel, oblique, and spiral arrangements of fibers that determine the muscle's power and range of motion. The architectures include quadrilateral, strap-like, fusiform, triangular, pennate, and cruciate patterns.

1. MUSCLES

2. Content
Types of fibers
Fascicular Architecture of Muscles

3. 1. Type I (slow) fibres
 Show a slow 'tonic' contraction characteristic of postural
muscles.
 These are red in colour because of large amounts of
myoglobin.
 The fibres are rich in mitochondria and oxidative
enzymes, but poor in phosphorylases.
 Because of a well-developed aerobic metabolism, slow
fibres are highly resistant to fatigue.

4. 2. Type II (fast) fibres
 Show a fast 'phasic' contraction required for large-scale
movements of body segments.
 These are paler (white) in colour because of small amounts of
myoglobin.
 The fibres are rich in glycogen and phosphorylases, but poor
in mitochondria and oxidative enzymes.
 Because of a glycolytic respiration, the fast fibres are quite
easily fatigued.

6. 3. Intermediate fibres
Represent a variant of type II (fast) fibres which are
relatively resistant to fatigue, although less than type
type I (slow) fibres (Burke etal, 1973).
 In man, most of the skeletal muscles show a mixture
of fibre types, but any one type may predominate.

7. Fascicular Architecture of Muscles
 The arrangement of muscle fibres 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 fibres, and the range of
movement is proportional to the length of fibres.
 The muscles can be classified according to the
arrangement of their fasciculi

8. DIAGRAM

9. A. Parallel Fasciculi
 When the fasciculi are parallel to the line of pull, the muscle
may be :
 1. Quadrilateral (thyrohyoid),
 2. Strap-like (sternohyoid and sartorius).
 3. Strap-like with tendinous intersections (rectus abdominis).
 4. Fusiform (biceps brachii, digastric, etc.). The range of
movement in such muscles is maximum

11. B. Oblique Fasciculi
 When the fasciculi are oblique to the line of pull, the muscle may be
triangular, or pennate (feather-like) in the construction. This arrangement
makes the muscle more powerful, although the range of movement is
reduced.
 . Triangular, e.g. temporalis, adductor longus.
 2. Unipennate, e.g. flexor pollicis longus, extensor digitorum longus, peroneus
tertius, palmar interossei
 3. Bipennate, e.g. rectus femoris, dorsal interossei , peroneus longus, flexor
hallucis longus.
 4. Multipennate, e.g. subscapularis, deltoid (acromial fibres)
 5. Circumpennate, e.g. tibialis anterior

14. C. Spiral or Twisted Fasciculi
 Spiral or twisted fibres are found in trapezius,
pectoralis major, latissimus dorsi, supinator, etc.
 In certain muscles the fasciculi are crossed. These
are called cruciate muscles, e.g. sternocleido-
mastoid, masseter, and adductor magnus.