Skeletal Muscle Fiber Adaptations Following Resistance Training

Abstract

• Study compared skeletal muscle responses to resistance training (RT) using repetition maximum (RM) or relative intensity (RISR).
• Well-trained males underwent RT 3 days/week for 10 weeks in either RM (n=8) or RISR (n=7) groups.
• RM group trained to muscular failure; RISR group trained based on percentages without failure.
• Muscle biopsies and ultrasonography were performed pre- and post-intervention.
• Variables measured: Fiber type-specific cross-sectional area (CSA), anatomical CSA (ACSA), muscle thickness (MT), mammalian target of rapamycin (mTOR), adenosine monophosphate protein kinase (AMPK), and myosin heavy chains (MHC).
• RISR increased type I CSA ($p = 0.018$, $g = 0.56$), type II CSA ($p = 0.012$, $g = 0.81$), ACSA ($p = 0.002$, $g = 0.53$), and MT (p < 0.001, $g = 1.47$).
• RISR also significantly reduced mTOR ($p = 0.031$, $g = -1.40$).
• RM only increased MT ($p = 0.003$, $g = 0.80$).
• Between-group effect sizes favored RISR for most variables.
• RISR led to greater adaptations in fiber size, whole-muscle size, and contractile proteins compared to RM.

Introduction

• Recent evidence suggests performance outcomes favor resistance training (RT) using relative intensity (RISR) over repetition maximum (RM) training.
• Hypothesis: RISR benefits are due to superior fatigue management from heavy-and-light days and non-failure training sessions.
• RM training involves high-intensity training every session, potentially impacting recovery and adaptation.
• Sarcomeres and protein isoforms are central to muscle activity and plasticity.
• Myosin heavy-chain (MHC) isoforms relate to muscle fiber type and shortening velocity.
• Myofibrillar protein synthesis is controlled by Akt-mTOR and AMPK-PGC1α pathways.
• Akt-mTOR increases following RT, aiding myofibrillar protein synthesis.
• AMPK-PGC1α inhibits Akt-mTOR.
• Study Purpose: To compare the physiological responses of skeletal muscle between RM and RISR resistance training programs.
• Hypothesis: RISR would result in superior gains in muscle size and contractile protein content due to better fatigue management.

Materials and Methods

• Subjects: 15 well-trained males (age = $26.94 \pm 3.95$ years, body mass = $86.21 \pm 12.07$ kg, BMI = $27.07 \pm 3.08$).
• Training: 3 days/week for 10 weeks; sprint training 2 days/week (identical for both groups).
• RISR group used submaximal intensities; RM group used maximal loads to failure.
• Training volume load was equalized between groups.
• Muscle biopsies were taken before and after the training intervention.
• Immunoblot processing was performed with antibodies raised against mTOR and AMPK.
• Ultrasonography assessed anatomical cross-sectional area (ACSA) and muscle thickness (MT).
• Statistical analysis: ANOVA and effect size calculations were used to assess alterations.

Resistance Training Programs

• Both groups followed a block-periodization approach.
• RISR used percentages to guide training; RM used maximal loads within set and repetition prescriptions.
• RISR included heavy and light days.
• RM required subjects to reach muscular failure on the final set.
• Loads adjusted based on performance in the RM group.
• All other training factors (coaching, training time) were controlled.

Muscle Biopsy Sampling and Processing

• Biopsies were taken at rest 72 hours before and after the training.
• Percutaneous needle biopsy of the vastus lateralis (VL) was performed.
• Samples were frozen in liquid nitrogen for analysis.
• Serial sections were analyzed using immunohistochemistry.
• Fiber types were identified and sized based on staining color.

Ultrasonography

• ACSA and MT of the right leg VL were assessed using ultrasonography.
• Measurements were taken 48–72 hours post-training.
• Femur length was measured to determine measurement site.
• A 16 Hz probe was used to obtain ACSA and MT images.
• Three images were collected and averaged for statistical analysis.

Results

• Type I CSA, type II CSA, and MT showed significant main effects for time (p < 0.001).
• Significant interaction effect for ACSA ($p = 0.046$).
• RISR group showed significant increases in type I CSA ($p = 0.018$), type II CSA ($p = 0.012$), ACSA ($p = 0.002$), and MT (p < 0.001).
• RM group only showed significant increase in MT ($p = 0.003$).
• Between-group effect sizes favored RISR.
• Basal levels of total mTOR decreased from pre to post interventions ($p = 0.007$).
• Significant decrease in mTOR for the RISR group ($p = 0.031$).
• No significant main effects were observed for AMPK, MHC2X, MHC2A, or MHC1.

Discussion

• Adaptations to whole muscle size, fiber size, and myofibrillar proteins favored RISR training over RM training.
• Superior fatigue management in the RISR group may have contributed to the results.
• Hypertrophic adaptations favored RISR, with small-to-moderate between-group effect magnitudes (g = 0.48–1.03).
• The greater hypertrophy in the RISR group supports the use of a broader loading spectrum.
• The results suggest that RISR yields more optimal adaptations compared to RM for muscle hypertrophy in strength-trained subjects.
• Failure training may induce greater levels of fatigue, impacting the ability for meaningful accretion of myofibrillar proteins.
• Decreases in basal mTOR may have been a result of molecular adaptation.

Conclusion

• RISR training resulted in a greater effect on fiber and whole-muscle CSA compared to RM training in well-trained males.
• RISR group increased the content of several key MHC isoforms to a greater extent than the RM group.
• These results support the use of RISR training in well-trained populations over that of RM training.