The main goal of the dissertation was to compare the effects of equal resistance training programs with different durations of rest intervals on muscle thickness, strength, and muscle architecture. Related to this general goal, a public research hypothesis was made: equal resistance training programs with different durations of rest intervals will affect muscle thickness, strength, and architecture differently. The general goal set is broken down into three specific objectives: to evaluate the particular effects of equal training programs with a resistance of different durations of rest intervals on 1) muscle thickness; 2) maximum mechanical strength output (peak moment of force) of the lower and upper extremities during slow (60 °/s) and fast (120 °/s) muscle actions; and 3) muscle architecture (length and angle of extension of bundles of muscle fibers). The appropriate sample of subjects were young, healthy, and physically active men, students of the Faculty of Kinesiology, University of Zagreb (n = 26) who were randomly divided into: 1) an experimental group of resistance training with a one-minute break between sets, and 2) an experimental group of resistance training with a three-minute break between sets. Each experimental group underwent programmed progressive resistance training three times a week for eight weeks, including seven exercises. The intensity corresponded to a load of 70% 1RM in each exercise, which allowed 12 repetitions per set performed to the point of instantaneous concentric muscle failure. Total relative training volume concerning 1RM increased progressively and was equalized between groups. The only acute program variable that differed between experimental groups was the rest interval duration between sets. The results showed that eight-week training programs equally increased peak and normalized peak moment of force, muscle thickness, and pennation angle, regardless of the rest interval length. Only the group with a longer rest interval significantly increased the length of muscle fiber bundles. Overall, the results of this dissertation do not support the stated general hypothesis but rather suggest that when the goal of training is to maximize muscle strength, the rest intervals should be "long enough" to allow high load intensities to be maintained during each set. When training aims to achieve muscle hypertrophy, long and short rest intervals are effective for increasing muscle hypertrophy, provided the resistance training programs performed are of high training volume. Finally, regardless of the rest interval duration, both training processes increase the pennation angle. However, only longer rest intervals lead to a significant increase in the length of muscle fiber bundles.