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

ABSTRACTIntroductionThe myonuclear domain theory postulates that myonuclei are added to muscle fibres when increases in fibre cross-sectional area (i.e. hypertrophy) are ≥26%. However, recent studies have reported increased myonuclear content with lower levels (e.g. 12%) of muscle fibre hypertrophy.PurposeTo determine whether a muscle fibre hypertrophy "threshold" is required to drive the addition of new myonuclei to existing muscle fibres.MethodsStudies of resistance training (RT), endurance training (ET), with or without nutrient (i.e., protein) supplementation and steroid administration, with measures of muscle fibre hypertrophy and myonuclei number, as primary or secondary outcomes, were considered. Twenty-seven studies incorporating 62 treatment groups and 903 subjects fulfilled the inclusion criteria and were included in the analyses.ResultsMuscle fibre hypertrophy ≤ 10% induces increases in myonuclear content although a significantly higher number of myonuclei are observed when muscle hypertrophy is ~22%. Additional analyses showed that age, sex and muscle fibre type do not influence muscle fibre hypertrophy nor myonuclei addition.ConclusionEven though a more consistent myonuclei addition occurs when muscle fibre hypertrophy is >22%, our results challenge the concept of a muscle hypertrophy threshold as significant myonuclei addition occurs with lower muscle hypertrophy (i.e. 22%, our results challenge the concept of a muscle hypertrophy threshold as significant myonuclei addition occurs with lower muscle hypertrophy (i.e.

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