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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.

In the women, the relationship between strength and the percentage of type II fibres changed with age (from 16 to 27 years of age) from a positive correlation (only Sargent jump) to negative correlations for all the strength tests, i.e. the more type I fibres the stronger the subject. A positive correlation between strength and the level of physical activity during leisure time was revealed in the women at both ages. The positive correlation between strength and type II fibres in the 16-year-old men had disappeared at age 27.

The relationship between maximum voluntary concentric strength, muscle fibre type distribution and muscle cross-sectional areas were examined in 23 subjects (7 female and 11 male phys. ed. students as well as 5 male bodybuilders). Maximal knee and elbow extension as well as elbow flexion torque at the angular velocities 30, 90 and 180 degrees per second was measured. Muscle biopsies were taken from vastus lateralis and m. triceps brachii. The muscle cross-sectional area of the thigh and upper arm was measured with computed tomography scanning. The maximal torque correlated strongly to the muscle cross-sectional area times an approximative measure on the lever arm (body height). Maximal tension developed per unit of muscle cross-sectional area did not correlate significantly with per cent type I fibre area and did not differ between the female and male students or bodybuilders.

Although there was a high correlation between average cross-sectional fiber area and total muscle cross-sectional area within each group, many of the subjects with the largest muscles also tended to have a large number of fibers.

Muscle biopsy samples were obtained from m. vastus lateralis and m. deltoideus of three high caliber bodybuilders. Tissue specimens were analysed with respect to relative distribution of fast twitch (FT) and slow twitch (ST) fiber types and different indices of fiber area. In comparison to a reference group of competitive power/weight-lifters the following tendencies were observed: the percentage of FT fibers was less, mean fiber area was smaller and selective FT fiber hypertrophy was not evident.

Subjects with predominantly FT fibers were able to generate 11, 16, 23 and 47 percent greater relative peak torque than could predominantly ST subjects at lever arm velocities of 115, 200, 287 and 400 degrees/second respectively. Likewise the correlation between relative torque production and % FT were significant (p less than .05) and increased from r = 0.44 to r = 0.75 as velocity increased from 115 to 400 degrees/second respectively. These data suggest that muscle fiber composition becomes increasingly more related to power performance as the velocity of movement increases

Muscle biopsies from m. vastus lateralis of two world class shot putters (shot putter 1 and 2) and the untrained brother of shot putter 1 were analyzed for fiber type distribution with ATPase staining and in situ hybridization for the expression of alkali myosin light chain (MLC) isoforms. Shot putter 2 had a predominance of type II fibers (67 X) and distinct hypertrophy of type I as well as type II fibers (fiber areas of 5939 and 8531 microm2). In shot putter 1, type II fibers amounted to only 40%, due to their selective hypertrophy, however type II fibers (10265 microm2) accounted for 67 2% of the total cross-sectional area. We suggest that the ability to selectively increase the relative area of his type II fibers in the 15 years of strength training was a key element in his success as a shot putter.

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.

The sex differences in strength increased from age 16 to 17 years. Body dimension, sex, percentage of type II, mean fibre area and the activity index contributed to explaining 50-75% of the strength at both ages. Different changes in relationship between fibre type composition and strength in women and men was seen with increasing age. In the women, the relationship between strength and the percentage of type II fibres changed with age (from 16 to 27 years of age) from a positive correlation (only Sargent jump) to negative correlations for all the strength tests, i.e. the more type I fibres the stronger the subject.

The maximal torque correlated strongly to the muscle cross-sectional area times an approximative measure on the lever arm (body height). Maximal tension developed per unit of muscle cross-sectional area did not correlate significantly with per cent type I fibre area and did not differ between the female and male students or bodybuilders. Neither did the relative decrease in torque with increasing contraction velocity show any significant relationship to the per cent type I fibre area.

We conclude that in humans, heavy resistance training directed toward achieving maximum size in skeletal muscle does not result in an increase in fiber numbers.

Values for fiber type composition and fiber size were more similar to values reported for physical education students and non-strength trained individuals. The results suggest that weight training induced muscle hypertrophy may be regulated by different mechanisms depending upon the volume and intensity of exercise

These data suggest that muscle fiber composition becomes increasingly more related to power performance as the velocity of movement increases.

The large difference between type I and type II fiber size, even in the untrained state, in both shot putter 1 and his brother is not usually seen in humans and maybe a genetic characteristic. We suggest that the ability to selectively increase the relative area of his type II fibers in the 15 years of strength training was a key element in his success as a shot putter. The observed increase in the expression of fast myosin light chain mRNAs in both fiber types is indicative of further adjustment of the myofibrillar apparatus towards the generation of very high peak power.