Can muscle typology explain the inter-individual variability in resistance training adaptations?

Considerable inter-individual heterogeneity exists in the muscular adaptations to resistance training. It has been proposed that fast-twitch fibers are more sensitive to hypertrophic stimuli and thus that variation in muscle fiber type composition is a contributing factor to the magnitude of training response. This study investigated if the inter-individual variability in resistance training adaptations is determined by muscle typology and if the most appropriate weekly training frequency depends on muscle typology. In strength-training novices, 11 slow (ST) and 10 fast typology (FT) individuals were selected by measuring muscle carnosine with proton magnetic resonance spectroscopy. Participants trained both upper arm and leg muscles to failure at 60% 1RM for 10 weeks, whereby one arm and leg trained 3x/week, the contralateral arm and leg 2x/week. Muscle volume (MRI-based 3D segmentation), maximal dynamic strength (one-repetition maximum, 1RM) and fiber-type specific cross-sectional area (vastus lateralis biopsies) were evaluated. The training response for total muscle volume (+3 to +14%), fiber size (-19 to +22%) and strength (+17 to +47%) showed considerable inter-individual variability, but these could not be attributed to differences in muscle typology. However, ST individuals performed a significantly higher training volume to gain these similar adaptations as FT individuals. The limb that trained 3x/week had generally more pronounced hypertrophy than the limb that trained 2x/week, and there was no interaction with muscle typology. In conclusion, muscle typology cannot explain the high variability in resistance training adaptations when training is performed to failure at 60% of 1RM. KEY POINTS: This study investigated the influence of muscle typology ( = muscle fiber type composition) on the variability in resistance training adaptations and on its role in the individualization of resistance training frequency. We demonstrate that an individual's muscle typology cannot explain the inter-individual variability in resistance training induced increases in muscle volume, maximal dynamic strength and fiber cross-sectional area when repetitions are performed to failure. Importantly, slow typology individuals performed a significantly higher training volume to obtain similar adaptations compared to fast typology individuals. Muscle typology does not determine the most appropriate resistance training frequency. However, regardless of muscle typology, an additional weekly training (3x/week vs 2x/week) increases muscle hypertrophy but not maximal dynamic strength. These findings expand on our understanding of the underlying mechanisms for the large inter-individual variability in resistance training adaptations. Abstract figure legend This study investigated if muscle typology can explain the high variability in resistance training adaptations. Slow and fast typology resistance training novices were selected to participate in this study by the non-invasive measurement of muscle carnosine with proton magnetic resonance spectroscopy. After the chronic training period a high inter-individual variability was observed in changes in muscle volume, maximal dynamic strength and fiber cross-sectional area. However, this high inter-individual variability could not be explained by muscle typology for any of the outcomes. Visual abstract created with BioRender. This article is protected by copyright. All rights reserved.