Introduction: Instability resistance training (IRT) is a combination of unique training movements aimed at improving strength, endurance, coordination, flexibility, power, and core stability within a single workout. The aim of present study was to comparison the effect of instability vs. high-volume resistance training (HVRT) on thigh muscles hypertrophy and hormonal adaptations. Material & Methods: Based on physical health screening, twenty middle-aged men (aged: 26.5 ± 3.6 years; ± SD) selected as the subject. The subjects divided into two groups randomly: HVRT group (n=10) or IRT group (n=10). The subjects in the HVRT group were performed biceps brachii, triceps, chest press, lat pull down, side lateral raise, leg press, dead lift, squat, hamstring and calf sitting with 6 to 10 RMs in 4 sets and 1 min rest between sets for 8 weeks. The subjects in the IRT group were performed total resistance exercise (TRX) suspension training with 10 rep in 2 sets and 20 second rest between the sets for 8 weeks. Paired-sample t-test, independent- sample t-test, wilcoxon and Mann-witney U teases were use for data analyzing. Results: The results indicated that quadriceps and total thigh muscles cross-sectional area (CSA) were increased after two types of resistance training (P<0.05) and hamstrings CSA was increased only after HVRT method. The data indicated that growth hormone (GH) and testosterone concentration and testosterone/cortisol ratio were increased after HVRT and IRT methods (P<0.05), however for cortisol levels no significant changes were observed. Conclusions: In conclusion, it seems that instability and high volume resistance training-induced changes in anabolic hormones contribute in thigh muscles hypertrophy in untrained men. تفاصيل المقالة