Comparison of Strength Adaptation Responses Among Individuals with Different Muscle Fiber Type Profiles

Background and Objectives: Muscle hypertrophy and strength increase are significantly influenced by the distribution of muscle fiber types in individuals. Individuals exposed to similar training intensity in fitness and bodybuilding exhibit different levels of adaptation suggesting that muscle fiber type may influence training outcomes. The aim of this study is to comparatively examine the strength development that occurs in fitness and bodybuilding athletes with different muscle fiber types (Type I, Type II, and Mixed) following a standardized resistance training program. Materials and Methods: The study was conducted using a quasi-experimental design based on a single-group pre-test-post-test model, with no control group. A total of 48 male athletes aged 19-26 years (22.75 ± 1.68) who had been regularly training in fitness and bodybuilding for at least two years voluntarily participated in the study. Muscle fiber types of the participants were indirectly estimated based on the number of repetitions performed at 80% of one-repetition maximum (1RM) in the bench press exercise, and individuals were divided into three groups: Type I, Type II, and mixed. All participants underwent a resistance training program for 6 weeks, 3 days a week, at 80% 1RM intensity and in the range of 8-12 repetitions. Data were analyzed using Shapiro-Wilk normality test, Wilcoxon Signed-Rank Test, and Mann-Whitney U. Results: The findings showed statistically significant increases between pre-test and post-test measurements in all groups. When percentage improvements were examined, the highest increase was observed in individuals with Type II muscle fiber type, and the lowest increase was observed in individuals with Type I muscle fiber type. Conclusions: In conclusion, the findings indicate that resistance training increases strength development in all muscle fiber types, but the level of development differs depending on the muscle fiber type. These findings highlight the importance of training programs based on individual muscle fiber type characteristics.