NUTRITION IN SPORT

acid accumulates quickly in these fibres and they
fatigue rapidly. Hence, these fibres are best
suited for delivering rapid, powerful contrac-
tions for brief periods. The metabolic characteris-
tics of type IIa fibres lie between the extreme
properties of the other two fibre types. They
contain fast-acting myosin ATPases like the type
IIb fibres, but have an oxidative capacity more
akin to that of the type I fibres.
The differences in activation threshold of the
motor neurones supplying the different fibre
types determine the order in which fibres are
recruited during exercise, and this in turn influ-
ences the metabolic response to exercise. During
most forms of movement, there appears to be an
orderly hierarchy of motor unit recruitment,
which roughly corresponds with a progression
from type I to type IIa to type IIb. It follows that
during light exercise, mostly type I fibres will be
recruited; during moderate exercise, both type I
and type IIa will be recruited; and during more
severe exercise, all fibre types will contribute to
force production.
Whole muscles in the body contain a mixture
of these three different fibre types, though the
proportions in which they are found differ
substantially between different muscles and can
also differ between different individuals. For
example, muscles involved in maintaining

20 nutrition and exercise

posture (e.g. soleus in the leg) have a high pro-

portion (usually more than 70%) of type I fibres,

which is in keeping with their function in main-

taining prolonged, chronic, but relatively weak

contractions. Fast type II fibres, however, pre-

dominate in muscles where rapid movements

are required (e.g. in the muscles of the hand and

the eye). Other muscles, such as the quadriceps

group in the leg, contain a variable mixture of

fibre types. The fibre type composition in such

muscles is a genetically determined attribute,

which does not appear to be pliable to a signifi-

cant degree by training. Hence, athletic capabili-

ties are inborn to a large extent (assuming the

genetic potential of the individual is realized

through appropriate nutrition and training). The

vastus lateralis muscle of successful marathon

runners has been shown to have a high percent-

age (about 80%) of type I fibres, while that of elite

sprinters contains a higher percentage (about

60%) of the type II fast twitch fibres (see Komi &

Karlsson 1978).

Sources of energy for muscle

force generation

Energy can be defined as the potential for per-

forming work or producing force. Development

of force by skeletal muscles requires a source of

Table 2.1Biochemical characteristics of human muscle fibre types. Values of metabolic characteristics of type II
fibres are shown relative to those found in type I fibres.

Characteristic Type I Type IIa Type IIb

Nomenclature Slow, red Fast, red Fast, white

Fatigue resistant Fatigue resistant Fatiguable

Oxidative Oxidative/glycolytic Glycolytic

Capillary density 1.0 0.8 0.6

Mitochondrial density 1.0 0.7 0.4

Myoglobin content 1.0 0.6 0.3

Phosphorylase activity 1.0 2.1 3.1

PFK activity 1.0 1.8 2.3

Citrate synthase activity 1.0 0.8 0.6

SDH activity 1.0 0.7 0.4

Glycogen content 1.0 1.3 1.5

Triacylglycerol content 1.0 0.4 0.2

Phosphocreatine content 1.0 1.2 1.2

Myosin ATPase activity 1.0 > 2 > 2

ATP, adenosine triphosphate; PFK, phosphofructokinase; SDH, succinate dehydrogenase.