Background. Researchers employing a variety of training methods have demons
trated a fast-to-slow fibre transformation in animal skeletal muscle. The o
bservation as to whether this occurs in exercise trained humans is limited
and equivocal.
Methods. Experimental design: to examine this issue, skeletal muscle from s
even subjects who had participated in a decade or more of high intensity ae
robic training (DT) and six nontrained (NT) subjects was obtained by muscle
biopsy from the vastus lateralis muscle (VL) and subjected to a modified m
yofibrillar ATPase technique to identify muscle fibre types. Muscle tissue
was histochemically treated by exposure to an alkaline preincubation (pH 9.
9), an acid preincubation (pH 4.3 or 4.6) and the formate-KCl preincubation
buffer (pH 4.54), previously employed in animal studies.
Results. The formate-KCl preincubation medium identified all major fibre ty
pes at a single pH in human subjects. The percentage of type I fibres in DT
was 70.9% vs 37.7% in NT (p<0.01), while the type IIa fibres in DT (25.3%)
was much lower (p<0.01) than NT (51.8%). Surprisingly, type IIa fibres in
the DT group displayed lesser oxidative staining intensity (p<0.01) than ty
pe IIa fibres from the NT group. Mean cross-sectional area of type I fibres
for DT (6233.9+/-1421.7 <mu>m(2)) was greater (p<0.05) than either type I
(5746.8+/-1135.2 <mu>m(2)) or II (5693.5+/-1214.6 mum(2)) from NT.
Conclusions. The results revealed that endurance training may promote a tra
nsition from type II to type I muscle fibre types and occurs at the expense
of the type II fibre population.