Muscle Fiber Hypertrophy and Myonuclei Addition: A Systematic Review and Meta-analysis

Area of Science:

• Exercise physiology
• Muscle biology
• Cellular adaptation

Background:

• The myonuclear domain theory suggests new myonuclei are added only with substantial muscle fiber hypertrophy (≥26%).
• Recent research indicates myonuclear content can increase with lower hypertrophy levels (e.g., 12%).

Purpose of the Study:

• To investigate if a specific muscle fiber hypertrophy threshold is necessary for adding myonuclei.
• To challenge the existing myonuclear domain theory regarding hypertrophy thresholds.

Main Methods:

• A systematic review and meta-analysis of 27 studies involving 903 subjects.
• Included studies examined resistance training, endurance training, nutrient supplementation, and steroid administration.
• Primary and secondary outcomes included muscle fiber hypertrophy and myonuclei number.

Main Results:

• Muscle fiber hypertrophy as low as ≤10% leads to increased myonuclear content.
• Significantly more myonuclei are observed with approximately 22% muscle hypertrophy.
• Age, sex, and muscle fiber type did not affect hypertrophy or myonuclei addition.

Conclusions:

• The concept of a strict muscle hypertrophy threshold for myonuclear addition is challenged.
• Significant myonuclear addition occurs even with less than 10% muscle fiber hypertrophy.
• While higher hypertrophy levels (>22%) show more consistent myonuclei addition, it's not an absolute requirement.