The experimenters looked at how many reps guys could do with 70% of their 1 rep max and then correlated this with different measures of the physiological make up of the muscles. Anyway, they found that the fibre type was not that important but that the capilliary density was the important factor.
There are trainers out there that make a lot out of the different fibre types. In this - admittedly specific - case that was not the important factor.
Application? If you want to maximise your reps at 70% of 1 rep max work to increase the capilliary density in the muscles. Now how do you do that?
Fiber Type Composition and Capillary Density in Relation to Submaximal Number of Repetitions in Resistance Exercise.
The purpose of this study was to investigate the relationship between skeletal muscle fiber type composition and the maximum number of repetitions performed during submaximal resistance exercise. Twelve young men performed a maximum repetitions test at 85% of 1 repetition maximum (1RM) in the leg press, which was repeated after 1 week. Seven days after the second 85% 1RM test, they performed a maximum repetitions test at 70% of 1RM in the leg press. This test, at 70% 1RM, was repeated 7 days later. One week before the initiation of the testing sessions, a biopsy sample was obtained from the vastus lateralis muscle and analyzed for fiber type distribution, fiber cross-sectional area, and capillary density (capillaries.mm). A low and nonsignificant relationship was found between the fiber type distribution or percent fiber type area and the number of repetitions performed at either 70% or 85% 1RM. Moreover, the number of repetitions performed at 70% or 85% of 1RM was not related significantly with 1RM strength. In contrast, the number of repetitions performed at 70% 1RM was significantly correlated with the number of capillaries per mm of muscle cross-sectional area (r = 0.70; p = 0.01). These results suggest that fiber type composition is not the major biological variable regulating the number of repetitions performed in submaximal resistance exercise. Rather, it seems that submaximal strength performance depends on muscle capillary density, which is linked with the endurance capacity of the muscle tissue.