The purpose of this study was to evaluate gains in muscular endurance and physical capacity to perform work in 22 adolescents and young adults (ages 13-22 years) with MR. The participants were tested before and after two consecutive 3-week sessions of supervised resistance training. Specific muscle strength was evaluated using a three repetition maximum (3RM) test, and muscular endurance was assessed using a repetition to failure (RF) test at 60% of the 3RM. The chest press, leg extension, and torso arm exercises were tested. Participants trained twice per week during the training intervals. The data were analyzed using a one-way ANOVA for repeated measures. Significant increases (p ≤ .05) in 3RM, RF, and total work performed during the RF test were found for the leg extension and torso arm exercises. Significant increases (p ≤ .05) in RF performance and total work performed during the RF test were found in the chest press. These results demonstrate that adolescents and young adults with MR can experience significant gains in muscular strength and endurance through a supervised resistance training program.