NUTRITION IN SPORT

Muscle fibre type responses

The conclusions presented so far have been
based on metabolite changes measured in biopsy
samples obtained from the quadriceps femoris
muscle group. However, it is known that human
skeletal muscle is composed of at least two func-
tionally and metabolically different fibre types.
Type I fibres are characterized as being slow con-
tracting, fatigue resistant, having a low peak
power output and favouring aerobic metabolism
for ATP resynthesis during contraction. Con-
versely, in comparison, type II fibres are fast con-
tracting, fatigue rapidly, have a high peak power
output and favour mainly anaerobic metabolism
for ATP resynthesis (Burke & Edgerton 1975).

Maximal exercise

Evidence from animal studies performed on
muscles composed of predominantly type I or
type II fibres and from one study performed

92 nutrition and exercise

using bundles of similar human muscle fibre

types, suggest that the rapid and marked rise and

subsequent decline in maximal power output

observed during intense muscle contraction in

man may be closely related to activation and

rapid fatigue of type II fibres during contraction

(Faulkneret al. 1986).

Figure 6.4 demonstrates glycogen degradation

in type I and type II muscle fibres during

maximal exercise under four different experi-

mental conditions. Notice that during intense

contraction the rates of glycogenolysis are higher

in type II than in type I fibres. This is true for both

dynamic exercise (Greenhaff et al. 1994; treadmill

sprinting) and electrically induced isometric

contractions (Greenhaff et al. 1991, 1993). The

rates of glycogenolysis observed in both fibre

types during treadmill sprinting and intermit-

tent isometric contraction with circulation

occluded, are in good agreement with the V

.

max.of

phosphorylase measured in both fibre types

(Harriset al. 1976), suggesting that glycogenoly-

5

0

4

3

2

Open

circ.

Electrical stimulation

1

Glycogenolytic rate (glucose units, mmol

–1.s

.kg

–1

)

Open circ.

and

adrenaline

Occluded

circ.

Sprinting

Fig. 6.4Glycogenolytic rates in

type I ( ) and type II (
) human

muscle fibres during 30 s of

intermittent electrically evoked

maximal isometric contraction

with intact circulation (circ.), intact

circulation with adrenaline

infusion, occluded circulation and

during 30 s of maximal sprint

running. Adapted from Greenhaff

et al. (1991, 1993, 1994).