age body fat. Therefore, each muscle is made up
of cells representing the four basic tissue groups:
muscle, connective, nervous and epithelial.
Extrafusal fibres can be further divided among
groups based upon the interrelated twitch char-
acteristics and metabolic capabilities. Fibres of
a particular motor unit (defined as a motor
neurone together with the extrafusal fibres it
innervates) that attain peak force development
relatively slowly are routinely termed ‘slow
twitch’ or type I fibres. Fibres that attain peak
force relatively more rapidly are termed ‘fast
twitch’ or type II fibres and further subdivided
into type IIa, type IIab and type IIb groups, as
based on myosin ATPase staining (Fig. 1.4).
Type I fibres are characterized by high mito-
chondrial density, high myoglobin content, high
aerobic metabolism and modest glycolytic capac-
ity. Early anatomists described muscles with
what we now identify as high type I fibre popula-
tion as ‘red muscle’ because of the darker colour
caused by the high myoglobin content. Type II
fibres have high glycolytic capability, low mito-
chondrial density and low capacity for aerobic
metabolism. Types IIa, IIab and IIb fibres are low
in myoglobin content, the reason for their being
identified many decades ago as white muscle
fibres.
The total number of muscle fibres in a particu-
lar muscle and the proportion identified as type I
and type II appear to be genetically dominated,
with small changes occurring through condition-
ing, injury, ageing, etc. It should also be men-
tioned that, while type II motor units are termed
fast twitch and type I motor units are termed
slow twitch, the comparison is on relative terms
and all extrafusal muscle fibres attain peak force
and shorten extremely fast. The differences
among them are great, however, and the shorten-
ing velocity is generally considered to be 4–10
times faster for the type II fibres than for type I
fibres.
The maximum force that can be developed by
an activated muscle is directly related to the
physiological cross-section of the muscle, a term
that describes the collective cross-sectional area
of the muscle cells, excluding the connective
tissue (including fat), nervous tissue and blood
vessels. The larger the physiological cross-
section of muscle, the greater is the muscle’s
ability to generate peak force (strength). Consid-
erable evidence exists to confirm the importance
of the type II fibre population of a muscle to its
ability to develop high force and power.
A high type I fibre population and the accom-
panying increased capillarization to supply
oxygen has been shown to be important for sus-
tained, rhythmic exercise which depends onbasic exercise physiology 9
Fig. 1.4Cross-section of human
muscle showing the mosaic
pattern of fibres: darkest stain=
type I; lightest stain=type IIa;
medium light stain=type IIab;
medium dark stain=type IIb.
Photo courtesy of William J.
Kraemer.