[Purpose] The aim of this study was to examine the effects of muscle activity and the number of resistance exercise repetitions on perceived exertion in tonic and phasic muscles in young Korean adults. [Subjects] Janda’s classification system was used to divide 40 Korean males and females in their 20s into a tonic muscle group (10 males, 10 females) and phasic muscle group (10 males, 10 females). [Methods] Each participant performed resistance exercise at 70% of maximum exertion for a single repetition. Muscle activity and number of repetitions were measured according to the Borg Rating of Perceived Exertion scale, with fairly light, hard, and very hard rated as 11, 15, and 19, respectively. Multiple regression analysis was performed. [Results] As the number of tonic and phasic muscle repetitions for males and females and female phasic muscle activity increased, the perceived exertion increased. Perceived exertion increased as the number of tonic muscle repetitions and activity of gastrocnemius muscles in males and females and the hamstring in males increased. Increased activity of phasic muscles in males and females and rhomboid muscle activity in males was associated with significantly increased perceived exertion. [Conclusion] Muscle activity and number of repetitions affect perceived exertion. The perception of exertion differs by muscle type and can differ by gender. The influence of the number of repetitions exceeds that of muscle activity.