Systematic Review and Meta-Analysis Examining If Hyperplasia Occurs in Humans in Response to Resistance Exercise

Background: It is commonly believed that resistance exercise in humans induces muscle growth primarily via muscle hypertrophy, but there is some debate as to whether muscle hyperplasia occurs. The purpose of this meta-analysis was to quantitively assess whether the presence of muscle hyperplasia occurs in humans performing resistance training. Methods: Medline, Scopus, and Web of Science were searched on May 17, 2024. Studies that assessed muscle fiber number pre and post resistance training were included in a three-level random effects model to assess changes in muscle fiber number. Results: A total of 11 studies met the search criteria, with all the included studies assessing changes in fiber number of the biceps brachii or vastus lateralis. The results of the quantitative meta-analysis demonstrated that resistance exercise did not alter the number of muscle fiber numbers present [ES: 4086; (95% CI: -6,430, 14,604; p=0.419; Figure 2]. Furthermore, the presence of muscle hyperplasia was not moderated by prior training status, training duration, or the muscle group assessed (all p>0.05). Conclusion: The results of the present study suggest that muscle hyperplasia is unlikely to occur to a meaningful extent in response to humans performing resistance training. While muscle hyperplasia was not evident, it may be difficult to detect due to the relatively short duration of the included training interventions and/or difficulties in estimating muscle fiber number, which resulted in a wide range of estimates across studies.