The influence of variations in muscle fibre composition on muscle strength and cross-sectional area in untrained males

The force produced by a maximum voluntary isometric contraction of the knee-extensor muscles was measured in a group of fifteen healthy young male volunteers. All subjects were untrained at the time of the study. The cross-sectional area of the knee-extensor muscles was measured at the mid-thigh level using computed tomography. Skeletal muscle samples were obtained by needle biopsy from the mid-point of m. vastus lateralis of the stronger leg of each subject. Samples were mounted, frozen and sectioned for histochemical analysis. On the basis of the pH dependent lability of the myosin ATP-ase reaction, fibres were classified as Type I, Type IIA or Type II B. Using computerized planimetry, muscle fibre cross-sectional areas were measured on serial sections stained for succinate dehydrogenase activity. As previously described, muscle strength (maximum voluntary contraction) was correlated with the muscle cross-sectional area (r = 0.70, P less than 0.01). The ratio of strength (in N) to cross-sectional area (in cm2) was 8.92 +/- 1.01 (mean +/- S.D.) with a wide range of values, from 7.09 to 10.85. Muscle fibre composition of m. vastus lateralis in these subjects was 46.1 +/- 10.5% Type I, 42.8 +/- 11.4% Type IIA and 11.1 +/- 9.7% Type IIB. After correction for differences in the cross-sectional areas of the different fibre types, the proportions of total area occupied by the different fibre types were: 43.6 +/- 11.9% Type I, 46.4 +/- 13.1% Type IIA and 10.0 +/- 9.1% Type IIB. No relationship was observed to exist between muscle strength and muscle fibre composition. Similarly, the muscle strength/cross-sectional area ratio was not related to the proportions of the different fibre types present or to the fraction of the total cross-sectional area occupied by the different fibre types. From the results it can be concluded that there is no difference in the force per unit area which can be generated by the different muscle fibre types present in human skeletal muscle. Variations in muscle fibre composition between individuals cannot, therefore, account for the large variations observed in the ratio of strength to muscle cross-sectional area.