Influence of intramuscular steroid receptor content and fiber capillarization on skeletal muscle hypertrophy.

Multiple intramuscular variables have been proposed to explain the high variability in resistance training induced muscle hypertrophy across humans. This study investigated if muscular androgen receptor (AR), estrogen receptor α (ERα) and ß (ERß) content and fiber capillarization are associated with fiber and whole-muscle hypertrophy after chronic resistance training. Male (n = 11) and female (n = 10) resistance training novices (22.1 ± 2.2 years) trained their knee extensors 3×/week for 10 weeks. Vastus lateralis biopsies were taken at baseline and post the training period to determine changes in fiber type specific cross-sectional area (CSA) and fiber capillarization by immunohistochemistry and, intramuscular AR, ERα and ERß content by Western blotting. Vastus lateralis volume was quantified by MRI-based 3D segmentation. Vastus lateralis muscle volume significantly increased over the training period (+7.22%; range: -1.82 to +18.8%, p < 0.0001) but no changes occurred in all fiber (+1.64%; range: -21 to +34%, p = 0.869), type I fiber (+1.33%; range: -24 to +41%, p = 0.952) and type II fiber CSA (+2.19%; range: -23 to +29%, p = 0.838). However, wide inter-individual ranges were found. Resistance training increased the protein expression of ERα but not ERß and AR, and the increase in ERα content was positively related to changes in fiber CSA. Only for the type II fibers, the baseline capillary-to-fiber-perimeter index was positively related to type II fiber hypertrophy but not to whole muscle responsiveness. In conclusion, an upregulation of ERα content and an adequate initial fiber capillarization may be contributing factors implicated in muscle fiber hypertrophy responsiveness after chronic resistance training.

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 fibres are more sensitive to hypertrophic stimuli and thus that variation in muscle fibre 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% of one-repetition maximum (1RM) for 10 weeks, whereby one arm and leg trained 3×/week and the contralateral arm and leg 2×/week. Muscle volume (MRI-based 3D segmentation), maximal dynamic strength (1RM) and fibre type-specific cross-sectional area (vastus lateralis biopsies) were evaluated. The training response for total muscle volume (+3 to +14%), fibre 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 than FT individuals. The limb that trained 3×/week had generally more pronounced hypertrophy than the limb that trained 2×/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 fibre 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 fibre 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 (3×/week vs. 2×/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.

Evidence for Simultaneous Muscle Atrophy and Hypertrophy in Response to Resistance Training in Humans.

PURPOSE: Human skeletal muscle has the profound ability to hypertrophy in response to resistance training (RT). Yet, this has a high energy and protein cost and is presumably mainly restricted to recruited muscles. It remains largely unknown what happens with non-recruited muscles during RT. This study investigated the volume changes of 17 recruited and 13 non-recruited muscles during a 10-week single-joint RT program targeting upper arm and upper leg musculature. METHODS: Muscle volume changes were measured by manual or automatic 3D segmentation in 21 RT novices. Subjects ate ad libitum during the study and energy and protein intake were assessed by self-reported diaries. RESULTS: Post-training, all recruited muscles increased in volume (range: +2.2% to +17.7%, p < 0.05) while the non-recruited adductor magnus (mean: -1.5 ± 3.1%, p = 0.038) and soleus (-2.4 ± 2.3%, p = 0.0004) decreased in volume. Net muscle growth (r = 0.453, p = 0.045) and changes in adductor magnus volume (r = 0.450, p = 0.047) were positively associated with protein intake. Changes in total non-recruited muscle volume (r = 0.469, p = 0.037), adductor magnus (r = 0.640, p = 0.002), adductor longus (r = 0.465, p = 0.039) and soleus muscle volume (r = 0.481, p = 0.032) were positively related to energy intake (p < 0.05). When subjects were divided into a HIGH or LOW energy intake group, overall non-recruited muscle volume (-1.7 ± 2.0%), adductor longus (-5.6 ± 3.7%), adductor magnus (-2.8 ± 2.4%) and soleus volume (-3.7 ± 1.8%) decreased significantly (p < 0.05) in the LOW but not the HIGH group. CONCLUSIONS: To our knowledge, this is the first study documenting that some non-recruited muscles significantly atrophy during a period of resistance training. Our data therefore suggest muscle mass reallocation, i.e., that hypertrophy in recruited muscles takes place at the expense of atrophy in non-recruited muscles, especially when energy and protein availability are limited.

Muscle typology influences the number of repetitions to failure during resistance training.

This study examined whether muscle typology (muscle fibre type composition) is related to maximal strength and whether it can explain the high inter-individual variability in number of repetitions to failure during resistance training. Ninety-five resistance training novices (57 males) were assessed for their maximal isometric knee extension strength and muscle typology. Muscle typology was estimated by measuring carnosine in the soleus, gastrocnemius and/or vastus lateralis using proton magnetic resonance spectroscopy. Forty-four subjects (22 males) performed dynamic strength tests (1RM) and 3 sets of leg extensions and curls to failure (60%1RM) to determine the association between muscle typology and (total) number of repetitions. Twenty-one subjects performed additional biceps curls and triceps extensions (60%1RM) to assess influence of exercise, 23 subjects performed additional leg extensions and curls at 80% and 40%1RM to evaluate influence of training load. There was a weak but significant relationship between muscle typology and maximal isometric strength (r = 0.22, p = 0.03) favouring the fast typology individuals. Slow and fast typology individuals did not differ in upper arm and upper leg 1RM. Total number of repetitions was related to muscle typology at 80% (r = -0.42; p = 0.04) and 60% (p = -0.44; p = 0.003) but not at 40%1RM. Slow typology individuals performed more repetitions to failure at 60%1RM in the leg extension (p = 0.03), leg curl (p = 0.01) and biceps curl (p = 0.02). In conclusion, muscle typology has a small contribution to maximal isometric strength but not dynamic strength and partly determines the number of repetitions to failure during resistance training. This insight can help individualizing resistance training prescriptions.

Having a fast muscle typology is positively associated with maximal isometric strength delivery in resistance training novices.The muscle typology seems to be a determining characteristic in the number of repetitions that can be performed during resistance training as slow typology individuals perform significantly more repetitions to failure compared to fast typology individuals.This study indicates the importance for coaches to shift from using traditional load-repetition tables and 1RM prediction equations to individualized 1RM testing and training volume prescriptions.