Whether or not you will get bigger muscles (hypertrophy) depends on three basic factors: genetics, gender and training intensity. Genetics is mostly manifested as muscle fiber type; people with predominantly fast-twitch fibers acquire larger muscles more easily than people with predominantly slow-twitch fibers. In relation to gender, males acquire larger muscles than females do, because males have greater amounts of testosterone and other sex hormones that influence protein metabolism (Tipton 2001). Thus, females experience less muscle hypertrophy with strength improvement than males do (Lewis et al. 1986). Training intensity is the only factor you can control.
Hypertrophy results from an increase in the number of contractile proteins (actin and myosin, produced by the body in response to training), which in turn increases the size of the muscle fibers.
If the training goal is hypertrophy, the load lifted should be at least 80 percent of the one-repetition maximum (1 RM), as a general guideline (Zatsiorsky 1995). If your clients are not interested in developing larger muscles, keep the load less than 80 percent of 1 RM. However, hypertrophy can be stimulated any time the training intensity is high enough to overload the muscle. Thus, in an unfit client who has never lifted weights before, 60 percent of 1 RM may be enough to cause slight hypertrophy, especially if the client is predisposed to hypertrophy by having a large proportion of fast-twitch fibers.