the myosin head changes from its activated con-
figuration to its bent shape, which causes the
head to pull on the thin filament, sliding it
towards the centre of the sarcomere. This action
represents the power stroke of the cross bridge
cycle, and simultaneously adenosine diphos-
phate (ADP) and inorganic phosphate (Pi) are
released from the myosin head. As a new ATP
molecule binds to the myosin head at the ATPase
active site, the myosin cross bridge detaches
from the actin. Hydrolysis of the ATP to ADP and
Piby the ATPase provides the energy required to
return the myosin to its activated ‘cocked’ state,
empowering it with the potential energy needed
for the next cross bridge attachment–power
stroke sequence. While the myosin is in the acti-
vated state, the ADP and Piremain attached to
the myosin head. Now the myosin head can
attach to another actin unit farther along the thin
filament, and the cycle of attachment, power
stroke, detachment and activation of myosin is
repeated. Sliding of the filaments in this manner
continues as long as calcium is present (at a con-
centration in excess of 10mmol · l–1) in the sar-
coplasm. Removal and sequestration of the
calcium by the ATP-dependent calcium pump
(ATPase) of the sarcoplasmic reticulum restores
the tropomyosin inhibition of cross bridge for-
mation and the muscle fibre relaxes.
Fibre types
The existence of different fibre types in skeletal
muscle is readily apparent and has long been rec-
ognized; the detailed physiological and bio-
chemical bases for these differences and their
functional significance have, however, only more
recently been established. Much of the impetus
for these investigations has come from the reali-
zation that success in athletic events which
require either the ability to generate a high
power output or great endurance is dependent in
large part on the proportions of the different fibre
types which are present in the muscles. The
muscle fibres are, however, extremely plastic,
and although the fibre type distribution is geneti-
cally determined, and not easily altered, an
appropriate training programme will have a
major effect on the metabolic potential of the
muscle, irrespective of the fibre types present.
Fibre type classification is usually based on
histochemical staining of serial cross-sections.
On this basis, human muscle fibres are com-
monly divided into three major kinds: types I, IIa
and IIb. These are analogous to the muscle fibres
from animals that have been classified on the
basis of their directly determined functional
properties as (i) slow twitch fibres, (ii) fast twitch,
fatigue resistant fibres, and (iii) fast twitch,
fatiguable fibres, respectively. The myosin of the
different fibre types exists in different molecular
forms (isoforms), and the myofibrillar ATPase
activity of the different fibre types displays dif-
ferential pH sensitivity; this provides the basis
for the differential histochemical staining of the
fibre types (Åstrand & Rodahl 1986). The bio-
chemical characteristics of the three major fibre
types are summarized in Table 2.1.
Type I fibres are small-diameter red cells that
contain relatively slow acting myosin ATPases
and hence contract slowly. The red colour is due
to the presence of myoglobin, an intracellular
respiratory pigment, capable of binding oxygen
and only releasing it at very low partial pressures
(as are found in the proximity of the mitochon-
dria). Type I fibres have numerous mitochondria,
mostly located close to the periphery of the fibre,
near to the blood capillaries which provide a rich
supply of oxygen and nutrients. These fibres
possess a high capacity for oxidative metabo-
lism, are extremely fatigue resistant, and are
specialized for the performance of repeated con-
tractions over prolonged periods.
Type IIb fibres are much paler, because they
contain little myoglobin. They possess rapidly
acting myosin ATPases and so their contraction
(and relaxation) time is relatively fast. They have
fewer mitochondria and a poorer capillary
supply, but greater glycogen and PCr stores than
the type I fibres. A high activity of glycogenolytic
and glycolytic enzymes endows type IIb fibres
with a high capacity for rapid (but relatively
short-lived) ATP production in the absence of
oxygen (anaerobic capacity). As a result, lactic