Skeletal muscle and hormonal adaptations to circuit weight training in untrained men.

Twelve men either performed 10 weeks of timed circuit weight training 3 days week(-1) (CWT; n=8; X+/-SE; age=23.6+/-1.8 years), or were part of a sedentary control group (n=4; age=20.5+/-1.0 years). Significance was P<0.05 for all analyses. The CWT program significantly increased 1 repetition maximum (1 RM) strength for nine of 10 exercises (15-42%). Although no body composition measure significantly changed for the CWT group, low-to-moderate effect sizes were evident for body weight, lean body mass, and relative fat. CWT did not alter percent fiber type, but did increase cross-sectional areas for type IIA fibers (microm(2); pre=5988+/-323, post=7259+/-669). Relative (%) myosin heavy-chain (MHC) expression increased for MHC IIa (pre=42.5+/-2.7, post=50.1+/-2.6), and decreased for MHC IIb (pre=21.8+/-2.8, post=15.4+/-2.4) for the CWT group. Serum testosterone, cortisol, and the testosterone/cortisol ratio did not change at any time for the CWT group. None of the measured variables changed for the control group. These data indicate that for untrained subjects, CWT of the type used resulted in improved muscular strength and a tendency toward increased lean mass. Compared with other types of weight training, fewer adaptations of the muscle fibers were evident. This is likely due in part to the relatively low loads used with this type of resistance exercise.