The quadriceps is a large muscle group composed of the vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris and controls knee extension. The quadriceps is highly active in endurance activities such as cycling (Gollnick et al., 1972) and running (Widrick et al., 1996a) and power activities such as jumping (Häkkinen and Komi, 1986). The vastus lateralis in untrained individuals is characterized by a large proportion of type I fibers (45-59%), a moderate type IIa proportion (30-39%), and a small type IIb proportion (11-15%) (Esbjörnsson et al., 1999; Simoneau and Bouchard, 1989).
However, there is a wide variation between individuals and type I, type IIa, and type IIb proportions have been observed to range between 15-79%, 13-77%, and 0-44%, respectively (Simoneau and Bouchard, 1989). The vastus medialis and the vastus lateralis contain similar percentages of type I fibers, 52.1% (Gollnick et al, 1972) and 51.4% (Kuzon et al., 1990) respectively.
However, as many as 79% of the gastrocnemius fibers (Costill et al., 1976b) and 88% of the soleus fibers (Johnson et al, 1972) may have type I characteristics. The high proportion of type I fibers in the quadriceps, gastrocnemius and soleus is probably due to the fact that these are postural and locomotive muscles that are active for a long duration, but at low contractile forces. In contrast, muscles of the upper body, such as the triceps and biceps, are used for brief forceful contractions and composed of high proportions (60-70%) of type II fibers (Johnson et al, 1972; MacDougall et al, 1982; 1984).