A decade of aerobic endurance training: Histological evidence for fibre type transformation

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.