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Muscle Fiber Hypertrophy and Myonuclei Addition

A Systematic Review and Meta-analysis

CONCEIÇÃO, MIGUEL S.1; VECHIN, FELIPE C.1; LIXANDRÃO, MANOEL1; DAMAS, FELIPE1; LIBARDI, CLEITON A.2; TRICOLI, VALMOR1; ROSCHEL, HAMILTON1; CAMERA, DONNY3; UGRINOWITSCH, CARLOS1

Medicine & Science in Sports & Exercise: July 2018 - Volume 50 - Issue 7 - p 1385–1393
doi: 10.1249/MSS.0000000000001593
BASIC SCIENCES
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Introduction The myonuclear domain theory postulates that myonuclei are added to muscle fibers when increases in fiber cross-sectional area (i.e., hypertrophy) are ≥26%. However, recent studies have reported increased myonuclear content with lower levels (e.g., 12%) of muscle fiber hypertrophy.

Purpose This study aimed to determine whether a muscle fiber hypertrophy “threshold” is required to drive the addition of new myonuclei to existing muscle fibers.

Methods Studies of resistance training endurance training with or without nutrient (i.e., protein) supplementation and steroid administration with measures of muscle fiber 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.

Results Muscle fiber hypertrophy of ≤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 fiber type do not influence muscle fiber hypertrophy or myonuclei addition.

Conclusions Although a more consistent myonuclei addition occurs when muscle fiber hypertrophy is >22%, our results challenge the concept of a muscle hypertrophy threshold as significant myonuclei addition occurs with lower muscle hypertrophy (i.e., <10%).

1School of Physical Education and Sport, University of São Paulo, São Paulo, BRAZIL;

2Laboratory of Neuromuscular Adaptations to Resistance Training, Department of Physical Education, Federal University of São Carlos, São Carlos, BRAZIL; and

3Mary MacKillop Institute for Health Research, Centre for Exercise and Nutrition, Australian Catholic University, Melbourne, AUSTRALIA

Address for correspondence: Hamilton Roschel, Ph.D., School of Physical Education and Sport, University of São Paulo, Av. Prof. Mello Moraes, 65, São Paulo, SP, Brazil; E-mail: hars@usp.br.

Submitted for publication October 2017.

Accepted for publication February 2018.

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© 2018 American College of Sports Medicine